"""Facilities to analyze entire programs, including imported modules.

Parse and analyze the source files of a program in the correct order
(based on file dependencies), and collect the results.

This module only directs a build, which is performed in multiple passes per
file.  The individual passes are implemented in separate modules.

The function build() is the main interface to this module.
"""
# TODO: More consistent terminology, e.g. path/fnam, module/id, state/file

import binascii
import collections
import contextlib
from distutils.sysconfig import get_python_lib
import gc
import hashlib
import json
import os.path
import re
import site
import stat
import sys
import time
from os.path import dirname, basename
import errno

from typing import (AbstractSet, Any, cast, Dict, Iterable, Iterator, List,
                    Mapping, NamedTuple, Optional, Set, Tuple, Union, Callable)
# Can't use TYPE_CHECKING because it's not in the Python 3.5.1 stdlib
MYPY = False
if MYPY:
    from typing import Deque

from mypy.nodes import (MODULE_REF, MypyFile, Node, ImportBase, Import, ImportFrom, ImportAll)
from mypy.semanal_pass1 import SemanticAnalyzerPass1
from mypy.semanal import SemanticAnalyzerPass2
from mypy.semanal_pass3 import SemanticAnalyzerPass3
from mypy.checker import TypeChecker
from mypy.indirection import TypeIndirectionVisitor
from mypy.errors import Errors, CompileError, DecodeError, report_internal_error
from mypy.report import Reports
from mypy import moduleinfo
from mypy import util
from mypy.fixup import fixup_module_pass_one, fixup_module_pass_two
from mypy.nodes import Expression
from mypy.options import Options
from mypy.parse import parse
from mypy.stats import dump_type_stats
from mypy.types import Type
from mypy.version import __version__
from mypy.plugin import Plugin, DefaultPlugin, ChainedPlugin
from mypy.defaults import PYTHON3_VERSION_MIN
from mypy.server.deps import get_dependencies


# Switch to True to produce debug output related to fine-grained incremental
# mode only that is useful during development. This produces only a subset of
# output compared to --verbose output. We use a global flag to enable this so
# that it's easy to enable this when running tests.
DEBUG_FINE_GRAINED = False


PYTHON_EXTENSIONS = ['.pyi', '.py']


Graph = Dict[str, 'State']


def getmtime(name: str) -> int:
    return int(os.path.getmtime(name))


# TODO: Get rid of BuildResult.  We might as well return a BuildManager.
class BuildResult:
    """The result of a successful build.

    Attributes:
      manager: The build manager.
      files:   Dictionary from module name to related AST node.
      types:   Dictionary from parse tree node to its inferred type.
      errors:  List of error messages.
    """

    def __init__(self, manager: 'BuildManager', graph: Graph) -> None:
        self.manager = manager
        self.graph = graph
        self.files = manager.modules
        self.types = manager.all_types  # Non-empty for tests only or if dumping deps
        self.errors = []  # type: List[str]  # Filled in by build if desired


class BuildSource:
    def __init__(self, path: Optional[str], module: Optional[str],
            text: Optional[str]) -> None:
        self.path = path
        self.module = module or '__main__'
        self.text = text

    def __repr__(self) -> str:
        return '<BuildSource path=%r module=%r has_text=%s>' % (self.path,
                                                                self.module,
                                                                self.text is not None)


class BuildSourceSet:
    """Efficiently test a file's membership in the set of build sources."""

    def __init__(self, sources: List[BuildSource]) -> None:
        self.source_text_present = False
        self.source_modules = set()  # type: Set[str]
        self.source_paths = set()  # type: Set[str]

        for source in sources:
            if source.text is not None:
                self.source_text_present = True
            elif source.path:
                self.source_paths.add(source.path)
            else:
                self.source_modules.add(source.module)

    def is_source(self, file: MypyFile) -> bool:
        if file.path and file.path in self.source_paths:
            return True
        elif file._fullname in self.source_modules:
            return True
        elif file.path is None and self.source_text_present:
            return True
        else:
            return False


# A dict containing saved cache data from a previous run.  This will
# be updated in place with newly computed cache data.  See dmypy.py.
SavedCache = Dict[str, Tuple['CacheMeta', MypyFile, Dict[Expression, Type]]]


def build(sources: List[BuildSource],
          options: Options,
          alt_lib_path: Optional[str] = None,
          bin_dir: Optional[str] = None,
          saved_cache: Optional[SavedCache] = None,
          flush_errors: Optional[Callable[[List[str], bool], None]] = None,
          ) -> BuildResult:
    """Analyze a program.

    A single call to build performs parsing, semantic analysis and optionally
    type checking for the program *and* all imported modules, recursively.

    Return BuildResult if successful or only non-blocking errors were found;
    otherwise raise CompileError.

    If a flush_errors callback is provided, all error messages will be
    passed to it and the errors and messages fields of BuildResult and
    CompileError (respectively) will be empty. Otherwise those fields will
    report any error messages.

    Args:
      sources: list of sources to build
      options: build options
      alt_lib_path: an additional directory for looking up library modules
        (takes precedence over other directories)
      bin_dir: directory containing the mypy script, used for finding data
        directories; if omitted, use '.' as the data directory
      saved_cache: optional dict with saved cache state for dmypy (read-write!)
      flush_errors: optional function to flush errors after a file is processed

    """
    # If we were not given a flush_errors, we use one that will populate those
    # fields for callers that want the traditional API.
    messages = []

    def default_flush_errors(new_messages: List[str], is_serious: bool) -> None:
        messages.extend(new_messages)

    flush_errors = flush_errors or default_flush_errors

    try:
        result = _build(sources, options, alt_lib_path, bin_dir, saved_cache, flush_errors)
        result.errors = messages
        return result
    except CompileError as e:
        # CompileErrors raised from an errors object carry all of the
        # messages that have not been reported out by error streaming.
        # Patch it up to contain either none or all none of the messages,
        # depending on whether we are flushing errors.
        serious = not e.use_stdout
        flush_errors(e.messages, serious)
        e.messages = messages
        raise


def _build(sources: List[BuildSource],
           options: Options,
           alt_lib_path: Optional[str],
           bin_dir: Optional[str],
           saved_cache: Optional[SavedCache],
           flush_errors: Callable[[List[str], bool], None],
           ) -> BuildResult:
    # This seems the most reasonable place to tune garbage collection.
    gc.set_threshold(50000)

    data_dir = default_data_dir(bin_dir)

    find_module_clear_caches()

    # Determine the default module search path.
    lib_path = default_lib_path(data_dir,
                                options.python_version,
                                custom_typeshed_dir=options.custom_typeshed_dir)

    if options.use_builtins_fixtures:
        # Use stub builtins (to speed up test cases and to make them easier to
        # debug).  This is a test-only feature, so assume our files are laid out
        # as in the source tree.
        root_dir = dirname(dirname(__file__))
        lib_path.insert(0, os.path.join(root_dir, 'test-data', 'unit', 'lib-stub'))
    else:
        for source in sources:
            if source.path:
                # Include directory of the program file in the module search path.
                dir = remove_cwd_prefix_from_path(dirname(source.path))
                if dir not in lib_path:
                    lib_path.insert(0, dir)

        # Do this even if running as a file, for sanity (mainly because with
        # multiple builds, there could be a mix of files/modules, so its easier
        # to just define the semantics that we always add the current director
        # to the lib_path
        # TODO: Don't do this in some cases; for motivation see see
        # https://github.com/python/mypy/issues/4195#issuecomment-341915031
        lib_path.insert(0, os.getcwd())

    # Prepend a config-defined mypy path.
    lib_path[:0] = options.mypy_path

    # Add MYPYPATH environment variable to front of library path, if defined.
    lib_path[:0] = mypy_path()

    # If provided, insert the caller-supplied extra module path to the
    # beginning (highest priority) of the search path.
    if alt_lib_path:
        lib_path.insert(0, alt_lib_path)

    reports = Reports(data_dir, options.report_dirs)
    source_set = BuildSourceSet(sources)
    errors = Errors(options.show_error_context, options.show_column_numbers)
    plugin = load_plugins(options, errors)

    # Construct a build manager object to hold state during the build.
    #
    # Ignore current directory prefix in error messages.
    manager = BuildManager(data_dir, lib_path,
                           ignore_prefix=os.getcwd(),
                           source_set=source_set,
                           reports=reports,
                           options=options,
                           version_id=__version__,
                           plugin=plugin,
                           errors=errors,
                           saved_cache=saved_cache,
                           flush_errors=flush_errors)

    try:
        graph = dispatch(sources, manager)
        return BuildResult(manager, graph)
    finally:
        manager.log("Build finished in %.3f seconds with %d modules, and %d errors" %
                    (time.time() - manager.start_time,
                     len(manager.modules),
                     manager.errors.num_messages()))
        # Finish the HTML or XML reports even if CompileError was raised.
        reports.finish()


def default_data_dir(bin_dir: Optional[str]) -> str:
    """Returns directory containing typeshed directory

    Args:
      bin_dir: directory containing the mypy script
    """
    if not bin_dir:
        if os.name == 'nt':
            prefixes = [os.path.join(sys.prefix, 'Lib')]
            try:
                prefixes.append(os.path.join(site.getuserbase(), 'lib'))
            except AttributeError:
                # getuserbase in not available in virtualenvs
                prefixes.append(os.path.join(get_python_lib(), 'lib'))
            for parent in prefixes:
                    data_dir = os.path.join(parent, 'mypy')
                    if os.path.exists(data_dir):
                        return data_dir
        mypy_package = os.path.dirname(__file__)
        parent = os.path.dirname(mypy_package)
        if (os.path.basename(parent) == 'site-packages' or
                os.path.basename(parent) == 'dist-packages'):
            # Installed in site-packages or dist-packages, but invoked with python3 -m mypy;
            # __file__ is .../blah/lib/python3.N/site-packages/mypy/build.py
            # or .../blah/lib/python3.N/dist-packages/mypy/build.py (Debian)
            # or .../blah/lib64/python3.N/dist-packages/mypy/build.py (Gentoo)
            # or .../blah/lib/site-packages/mypy/build.py (Windows)
            # blah may be a virtualenv or /usr/local.  We want .../blah/lib/mypy.
            lib = parent
            for i in range(2):
                lib = os.path.dirname(lib)
                if os.path.basename(lib) in ('lib', 'lib32', 'lib64'):
                    return os.path.join(os.path.dirname(lib), 'lib/mypy')
        subdir = os.path.join(parent, 'lib', 'mypy')
        if os.path.isdir(subdir):
            # If installed via buildout, the __file__ is
            # somewhere/mypy/__init__.py and what we want is
            # somewhere/lib/mypy.
            return subdir
        # Default to directory containing this file's parent.
        return parent
    base = os.path.basename(bin_dir)
    dir = os.path.dirname(bin_dir)
    if (sys.platform == 'win32' and base.lower() == 'scripts'
            and not os.path.isdir(os.path.join(dir, 'typeshed'))):
        # Installed, on Windows.
        return os.path.join(dir, 'Lib', 'mypy')
    elif base == 'scripts':
        # Assume that we have a repo check out or unpacked source tarball.
        return dir
    elif base == 'bin':
        # Installed to somewhere (can be under /usr/local or anywhere).
        return os.path.join(dir, 'lib', 'mypy')
    elif base == 'python3':
        # Assume we installed python3 with brew on os x
        return os.path.join(os.path.dirname(dir), 'lib', 'mypy')
    elif dir.endswith('python-exec'):
        # Gentoo uses a python wrapper in /usr/lib to which mypy is a symlink.
        return os.path.join(os.path.dirname(dir), 'mypy')
    else:
        # Don't know where to find the data files!
        raise RuntimeError("Broken installation: can't determine base dir")


def mypy_path() -> List[str]:
    path_env = os.getenv('MYPYPATH')
    if not path_env:
        return []
    return path_env.split(os.pathsep)


def default_lib_path(data_dir: str,
                     pyversion: Tuple[int, int],
                     custom_typeshed_dir: Optional[str]) -> List[str]:
    """Return default standard library search paths."""
    # IDEA: Make this more portable.
    path = []  # type: List[str]

    if custom_typeshed_dir:
        typeshed_dir = custom_typeshed_dir
    else:
        auto = os.path.join(data_dir, 'stubs-auto')
        if os.path.isdir(auto):
            data_dir = auto
        typeshed_dir = os.path.join(data_dir, "typeshed")
    if pyversion[0] == 3:
        # We allow a module for e.g. version 3.5 to be in 3.4/. The assumption
        # is that a module added with 3.4 will still be present in Python 3.5.
        versions = ["%d.%d" % (pyversion[0], minor)
                    for minor in reversed(range(PYTHON3_VERSION_MIN[1], pyversion[1] + 1))]
    else:
        # For Python 2, we only have stubs for 2.7
        versions = ["2.7"]
    # E.g. for Python 3.5, try 3.5/, 3.4/, 3.3/, 3/, 2and3/.
    for v in versions + [str(pyversion[0]), '2and3']:
        for lib_type in ['stdlib', 'third_party']:
            stubdir = os.path.join(typeshed_dir, lib_type, v)
            if os.path.isdir(stubdir):
                path.append(stubdir)

    # Add fallback path that can be used if we have a broken installation.
    if sys.platform != 'win32':
        path.append('/usr/local/lib/mypy')
    if not path:
        print("Could not resolve typeshed subdirectories. If you are using mypy\n"
              "from source, you need to run \"git submodule update --init\".\n"
              "Otherwise your mypy install is broken.\nPython executable is located at "
              "{0}.\nMypy located at {1}".format(sys.executable, data_dir), file=sys.stderr)
        sys.exit(1)
    return path


CacheMeta = NamedTuple('CacheMeta',
                       [('id', str),
                        ('path', str),
                        ('memory_only', bool),  # no corresponding json files (fine-grained only)
                        ('mtime', int),
                        ('size', int),
                        ('hash', str),
                        ('dependencies', List[str]),  # names of imported modules
                        ('data_mtime', int),  # mtime of data_json
                        ('data_json', str),  # path of <id>.data.json
                        ('suppressed', List[str]),  # dependencies that weren't imported
                        ('child_modules', List[str]),  # all submodules of the given module
                        ('options', Optional[Dict[str, object]]),  # build options
                        ('dep_prios', List[int]),
                        ('dep_lines', List[int]),
                        ('interface_hash', str),  # hash representing the public interface
                        ('version_id', str),  # mypy version for cache invalidation
                        ('ignore_all', bool),  # if errors were ignored
                        ])
# NOTE: dependencies + suppressed == all reachable imports;
# suppressed contains those reachable imports that were prevented by
# silent mode or simply not found.


def cache_meta_from_dict(meta: Dict[str, Any], data_json: str) -> CacheMeta:
    sentinel = None  # type: Any  # Values to be validated by the caller
    return CacheMeta(
        meta.get('id', sentinel),
        meta.get('path', sentinel),
        meta.get('memory_only', False),
        int(meta['mtime']) if 'mtime' in meta else sentinel,
        meta.get('size', sentinel),
        meta.get('hash', sentinel),
        meta.get('dependencies', []),
        int(meta['data_mtime']) if 'data_mtime' in meta else sentinel,
        data_json,
        meta.get('suppressed', []),
        meta.get('child_modules', []),
        meta.get('options'),
        meta.get('dep_prios', []),
        meta.get('dep_lines', []),
        meta.get('interface_hash', ''),
        meta.get('version_id', sentinel),
        meta.get('ignore_all', True),
    )


# Priorities used for imports.  (Here, top-level includes inside a class.)
# These are used to determine a more predictable order in which the
# nodes in an import cycle are processed.
PRI_HIGH = 5  # top-level "from X import blah"
PRI_MED = 10  # top-level "import X"
PRI_LOW = 20  # either form inside a function
PRI_MYPY = 25  # inside "if MYPY" or "if TYPE_CHECKING"
PRI_INDIRECT = 30  # an indirect dependency
PRI_ALL = 99  # include all priorities


def import_priority(imp: ImportBase, toplevel_priority: int) -> int:
    """Compute import priority from an import node."""
    if not imp.is_top_level:
        # Inside a function
        return PRI_LOW
    if imp.is_mypy_only:
        # Inside "if MYPY" or "if typing.TYPE_CHECKING"
        return max(PRI_MYPY, toplevel_priority)
    # A regular import; priority determined by argument.
    return toplevel_priority


def load_plugins(options: Options, errors: Errors) -> Plugin:
    """Load all configured plugins.

    Return a plugin that encapsulates all plugins chained together. Always
    at least include the default plugin (it's last in the chain).
    """

    default_plugin = DefaultPlugin(options)  # type: Plugin
    if not options.config_file:
        return default_plugin

    line = find_config_file_line_number(options.config_file, 'mypy', 'plugins')
    if line == -1:
        line = 1  # We need to pick some line number that doesn't look too confusing

    def plugin_error(message: str) -> None:
        errors.report(line, 0, message)
        errors.raise_error()

    custom_plugins = []  # type: List[Plugin]
    errors.set_file(options.config_file, None)
    for plugin_path in options.plugins:
        # Plugin paths are relative to the config file location.
        plugin_path = os.path.join(os.path.dirname(options.config_file), plugin_path)

        if not os.path.isfile(plugin_path):
            plugin_error("Can't find plugin '{}'".format(plugin_path))
        plugin_dir = os.path.dirname(plugin_path)
        fnam = os.path.basename(plugin_path)
        if not fnam.endswith('.py'):
            plugin_error("Plugin '{}' does not have a .py extension".format(fnam))
        module_name = fnam[:-3]
        import importlib
        sys.path.insert(0, plugin_dir)
        try:
            m = importlib.import_module(module_name)
        except Exception:
            print('Error importing plugin {}\n'.format(plugin_path))
            raise  # Propagate to display traceback
        finally:
            assert sys.path[0] == plugin_dir
            del sys.path[0]
        if not hasattr(m, 'plugin'):
            plugin_error('Plugin \'{}\' does not define entry point function "plugin"'.format(
                plugin_path))
        try:
            plugin_type = getattr(m, 'plugin')(__version__)
        except Exception:
            print('Error calling the plugin(version) entry point of {}\n'.format(plugin_path))
            raise  # Propagate to display traceback
        if not isinstance(plugin_type, type):
            plugin_error(
                'Type object expected as the return value of "plugin"; got {!r} (in {})'.format(
                    plugin_type, plugin_path))
        if not issubclass(plugin_type, Plugin):
            plugin_error(
                'Return value of "plugin" must be a subclass of "mypy.plugin.Plugin" '
                '(in {})'.format(plugin_path))
        try:
            custom_plugins.append(plugin_type(options))
        except Exception:
            print('Error constructing plugin instance of {}\n'.format(plugin_type.__name__))
            raise  # Propagate to display traceback
    # Custom plugins take precedence over the default plugin.
    return ChainedPlugin(options, custom_plugins + [default_plugin])


def find_config_file_line_number(path: str, section: str, setting_name: str) -> int:
    """Return the approximate location of setting_name within mypy config file.

    Return -1 if can't determine the line unambiguously.
    """
    in_desired_section = False
    try:
        results = []
        with open(path) as f:
            for i, line in enumerate(f):
                line = line.strip()
                if line.startswith('[') and line.endswith(']'):
                    current_section = line[1:-1].strip()
                    in_desired_section = (current_section == section)
                elif in_desired_section and re.match(r'{}\s*='.format(setting_name), line):
                    results.append(i + 1)
        if len(results) == 1:
            return results[0]
    except OSError:
        pass
    return -1


class BuildManager:
    """This class holds shared state for building a mypy program.

    It is used to coordinate parsing, import processing, semantic
    analysis and type checking.  The actual build steps are carried
    out by dispatch().

    Attributes:
      data_dir:        Mypy data directory (contains stubs)
      lib_path:        Library path for looking up modules
      modules:         Mapping of module ID to MypyFile (shared by the passes)
      semantic_analyzer:
                       Semantic analyzer, pass 2
      semantic_analyzer_pass3:
                       Semantic analyzer, pass 3
      all_types:       Map {Expression: Type} collected from all modules (tests only)
      options:         Build options
      missing_modules: Set of modules that could not be imported encountered so far
      stale_modules:   Set of modules that needed to be rechecked (only used by tests)
      version_id:      The current mypy version (based on commit id when possible)
      plugin:          Active mypy plugin(s)
      errors:          Used for reporting all errors
      flush_errors:    A function for processing errors after each SCC
      saved_cache:     Dict with saved cache state for dmypy and fine-grained incremental mode
                       (read-write!)
      stats:           Dict with various instrumentation numbers
    """

    def __init__(self, data_dir: str,
                 lib_path: List[str],
                 ignore_prefix: str,
                 source_set: BuildSourceSet,
                 reports: Reports,
                 options: Options,
                 version_id: str,
                 plugin: Plugin,
                 errors: Errors,
                 flush_errors: Callable[[List[str], bool], None],
                 saved_cache: Optional[SavedCache] = None,
                 ) -> None:
        self.start_time = time.time()
        self.data_dir = data_dir
        self.errors = errors
        self.errors.set_ignore_prefix(ignore_prefix)
        self.lib_path = tuple(lib_path)
        self.source_set = source_set
        self.reports = reports
        self.options = options
        self.version_id = version_id
        self.modules = {}  # type: Dict[str, MypyFile]
        self.missing_modules = set()  # type: Set[str]
        self.plugin = plugin
        self.semantic_analyzer = SemanticAnalyzerPass2(self.modules, self.missing_modules,
                                                  lib_path, self.errors, self.plugin)
        self.semantic_analyzer_pass3 = SemanticAnalyzerPass3(self.modules, self.errors,
                                                             self.semantic_analyzer)
        self.all_types = {}  # type: Dict[Expression, Type]  # Used by tests only
        self.indirection_detector = TypeIndirectionVisitor()
        self.stale_modules = set()  # type: Set[str]
        self.rechecked_modules = set()  # type: Set[str]
        self.plugin = plugin
        self.flush_errors = flush_errors
        self.saved_cache = saved_cache if saved_cache is not None else {}  # type: SavedCache
        self.stats = {}  # type: Dict[str, Any]  # Values are ints or floats

    def maybe_swap_for_shadow_path(self, path: str) -> str:
        if (self.options.shadow_file and
                os.path.samefile(self.options.shadow_file[0], path)):
            path = self.options.shadow_file[1]
        return path

    def get_stat(self, path: str) -> os.stat_result:
        return os.stat(self.maybe_swap_for_shadow_path(path))

    def all_imported_modules_in_file(self,
                                     file: MypyFile) -> List[Tuple[int, str, int]]:
        """Find all reachable import statements in a file.

        Return list of tuples (priority, module id, import line number)
        for all modules imported in file; lower numbers == higher priority.
        """

        def correct_rel_imp(imp: Union[ImportFrom, ImportAll]) -> str:
            """Function to correct for relative imports."""
            file_id = file.fullname()
            rel = imp.relative
            if rel == 0:
                return imp.id
            if os.path.basename(file.path).startswith('__init__.'):
                rel -= 1
            if rel != 0:
                file_id = ".".join(file_id.split(".")[:-rel])
            new_id = file_id + "." + imp.id if imp.id else file_id

            return new_id

        res = []  # type: List[Tuple[int, str, int]]
        for imp in file.imports:
            if not imp.is_unreachable:
                if isinstance(imp, Import):
                    pri = import_priority(imp, PRI_MED)
                    ancestor_pri = import_priority(imp, PRI_LOW)
                    for id, _ in imp.ids:
                        ancestor_parts = id.split(".")[:-1]
                        ancestors = []
                        for part in ancestor_parts:
                            ancestors.append(part)
                            res.append((ancestor_pri, ".".join(ancestors), imp.line))
                        res.append((pri, id, imp.line))
                elif isinstance(imp, ImportFrom):
                    cur_id = correct_rel_imp(imp)
                    pos = len(res)
                    all_are_submodules = True
                    # Also add any imported names that are submodules.
                    pri = import_priority(imp, PRI_MED)
                    for name, __ in imp.names:
                        sub_id = cur_id + '.' + name
                        if self.is_module(sub_id):
                            res.append((pri, sub_id, imp.line))
                        else:
                            all_are_submodules = False
                    # If all imported names are submodules, don't add
                    # cur_id as a dependency.  Otherwise (i.e., if at
                    # least one imported name isn't a submodule)
                    # cur_id is also a dependency, and we should
                    # insert it *before* any submodules.
                    if not all_are_submodules:
                        pri = import_priority(imp, PRI_HIGH)
                        res.insert(pos, ((pri, cur_id, imp.line)))
                elif isinstance(imp, ImportAll):
                    pri = import_priority(imp, PRI_HIGH)
                    res.append((pri, correct_rel_imp(imp), imp.line))

        return res

    def is_module(self, id: str) -> bool:
        """Is there a file in the file system corresponding to module id?"""
        return find_module(id, self.lib_path) is not None

    def parse_file(self, id: str, path: str, source: str, ignore_errors: bool) -> MypyFile:
        """Parse the source of a file with the given name.

        Raise CompileError if there is a parse error.
        """
        num_errs = self.errors.num_messages()
        tree = parse(source, path, id, self.errors, options=self.options)
        tree._fullname = id
        self.add_stats(files_parsed=1,
                       modules_parsed=int(not tree.is_stub),
                       stubs_parsed=int(tree.is_stub))

        if self.errors.num_messages() != num_errs:
            self.log("Bailing due to parse errors")
            self.errors.raise_error()

        self.errors.set_file_ignored_lines(path, tree.ignored_lines, ignore_errors)
        return tree

    def module_not_found(self, path: str, source: str, line: int, target: str) -> None:
        self.errors.set_file(path, source)
        stub_msg = "(Stub files are from https://github.com/python/typeshed)"
        if target == 'builtins':
            self.errors.report(line, 0, "Cannot find 'builtins' module. Typeshed appears broken!",
                               blocker=True)
            self.errors.raise_error()
        elif ((self.options.python_version[0] == 2 and moduleinfo.is_py2_std_lib_module(target))
              or (self.options.python_version[0] >= 3
                  and moduleinfo.is_py3_std_lib_module(target))):
            self.errors.report(
                line, 0, "No library stub file for standard library module '{}'".format(target))
            self.errors.report(line, 0, stub_msg, severity='note', only_once=True)
        elif moduleinfo.is_third_party_module(target):
            self.errors.report(line, 0, "No library stub file for module '{}'".format(target))
            self.errors.report(line, 0, stub_msg, severity='note', only_once=True)
        else:
            self.errors.report(line, 0, "Cannot find module named '{}'".format(target))
            self.errors.report(line, 0, '(Perhaps setting MYPYPATH '
                               'or using the "--ignore-missing-imports" flag would help)',
                               severity='note', only_once=True)

    def report_file(self,
                    file: MypyFile,
                    type_map: Dict[Expression, Type],
                    options: Options) -> None:
        if self.source_set.is_source(file):
            self.reports.file(file, type_map, options)

    def log(self, *message: str) -> None:
        if self.options.verbosity >= 1:
            if message:
                print('LOG: ', *message, file=sys.stderr)
            else:
                print(file=sys.stderr)
            sys.stderr.flush()

    def log_fine_grained(self, *message: str) -> None:
        if self.options.verbosity >= 1:
            self.log('fine-grained:', *message)
        elif DEBUG_FINE_GRAINED:
            # Output log in a simplified format that is quick to browse.
            if message:
                print(*message, file=sys.stderr)
            else:
                print(file=sys.stderr)
            sys.stderr.flush()

    def trace(self, *message: str) -> None:
        if self.options.verbosity >= 2:
            print('TRACE:', *message, file=sys.stderr)
            sys.stderr.flush()

    def add_stats(self, **kwds: Any) -> None:
        for key, value in kwds.items():
            if key in self.stats:
                self.stats[key] += value
            else:
                self.stats[key] = value

    def stats_summary(self) -> Mapping[str, object]:
        return self.stats


def remove_cwd_prefix_from_path(p: str) -> str:
    """Remove current working directory prefix from p, if present.

    Also crawl up until a directory without __init__.py is found.

    If the result would be empty, return '.' instead.
    """
    cur = os.getcwd()
    # Add separator to the end of the path, unless one is already present.
    if basename(cur) != '':
        cur += os.sep
    # Compute root path.
    while (p and
           (os.path.isfile(os.path.join(p, '__init__.py')) or
            os.path.isfile(os.path.join(p, '__init__.pyi')))):
        dir, base = os.path.split(p)
        if not base:
            break
        p = dir
    # Remove current directory prefix from the path, if present.
    if p.startswith(cur):
        p = p[len(cur):]
    # Avoid returning an empty path; replace that with '.'.
    if p == '':
        p = '.'
    return p


# Cache find_module: (id, lib_path) -> result.
find_module_cache = {}  # type: Dict[Tuple[str, Tuple[str, ...]], Optional[str]]

# Cache some repeated work within distinct find_module calls: finding which
# elements of lib_path have even the subdirectory they'd need for the module
# to exist.  This is shared among different module ids when they differ only
# in the last component.
find_module_dir_cache = {}  # type: Dict[Tuple[str, Tuple[str, ...]], List[str]]

# Cache directory listings.  We assume that while one os.listdir()
# call may be more expensive than one os.stat() call, a small number
# of os.stat() calls is quickly more expensive than caching the
# os.listdir() outcome, and the advantage of the latter is that it
# gives us the case-correct filename on Windows and Mac.
find_module_listdir_cache = {}  # type: Dict[str, Optional[List[str]]]

# Cache for is_file()
find_module_is_file_cache = {}  # type: Dict[str, bool]

# Cache for isdir(join(head, tail))
find_module_isdir_cache = {}  # type: Dict[Tuple[str, str], bool]


def find_module_clear_caches() -> None:
    find_module_cache.clear()
    find_module_dir_cache.clear()
    find_module_listdir_cache.clear()
    find_module_is_file_cache.clear()
    find_module_isdir_cache.clear()


def list_dir(path: str) -> Optional[List[str]]:
    """Return a cached directory listing.

    Returns None if the path doesn't exist or isn't a directory.
    """
    res = find_module_listdir_cache.get(path)
    if res is None:
        try:
            res = os.listdir(path)
        except OSError:
            res = None
        find_module_listdir_cache[path] = res
    return res


def is_file(path: str) -> bool:
    """Return whether path exists and is a file.

    On case-insensitive filesystems (like Mac or Windows) this returns
    False if the case of the path's last component does not exactly
    match the case found in the filesystem.
    """
    res = find_module_is_file_cache.get(path)
    if res is None:
        head, tail = os.path.split(path)
        if not tail:
            res = False
        else:
            names = list_dir(head)
            res = names is not None and tail in names and os.path.isfile(path)
        find_module_is_file_cache[path] = res
    return res


def find_module(id: str, lib_path_arg: Iterable[str]) -> Optional[str]:
    """Return the path of the module source file, or None if not found."""
    lib_path = tuple(lib_path_arg)

    def find() -> Optional[str]:
        # If we're looking for a module like 'foo.bar.baz', it's likely that most of the
        # many elements of lib_path don't even have a subdirectory 'foo/bar'.  Discover
        # that only once and cache it for when we look for modules like 'foo.bar.blah'
        # that will require the same subdirectory.
        components = id.split('.')
        dir_chain = os.sep.join(components[:-1])  # e.g., 'foo/bar'
        if (dir_chain, lib_path) not in find_module_dir_cache:
            dirs = []
            for pathitem in lib_path:
                # e.g., '/usr/lib/python3.4/foo/bar'
                isdir = find_module_isdir_cache.get((pathitem, dir_chain))
                if isdir is None:
                    dir = os.path.normpath(os.path.join(pathitem, dir_chain))
                    isdir = os.path.isdir(dir)
                    find_module_isdir_cache[pathitem, dir_chain] = isdir
                if isdir:
                    dirs.append(dir)
            find_module_dir_cache[dir_chain, lib_path] = dirs
        candidate_base_dirs = find_module_dir_cache[dir_chain, lib_path]

        # If we're looking for a module like 'foo.bar.baz', then candidate_base_dirs now
        # contains just the subdirectories 'foo/bar' that actually exist under the
        # elements of lib_path.  This is probably much shorter than lib_path itself.
        # Now just look for 'baz.pyi', 'baz/__init__.py', etc., inside those directories.
        seplast = os.sep + components[-1]  # so e.g. '/baz'
        sepinit = os.sep + '__init__'
        for base_dir in candidate_base_dirs:
            base_path = base_dir + seplast  # so e.g. '/usr/lib/python3.4/foo/bar/baz'
            # Prefer package over module, i.e. baz/__init__.py* over baz.py*.
            for extension in PYTHON_EXTENSIONS:
                path = base_path + sepinit + extension
                if is_file(path) and verify_module(id, path):
                    return path
            # No package, look for module.
            for extension in PYTHON_EXTENSIONS:
                path = base_path + extension
                if is_file(path) and verify_module(id, path):
                    return path
        return None

    key = (id, lib_path)
    if key not in find_module_cache:
        find_module_cache[key] = find()
    return find_module_cache[key]


def find_modules_recursive(module: str, lib_path: List[str]) -> List[BuildSource]:
    module_path = find_module(module, lib_path)
    if not module_path:
        return []
    result = [BuildSource(module_path, module, None)]
    if module_path.endswith(('__init__.py', '__init__.pyi')):
        # Subtle: this code prefers the .pyi over the .py if both
        # exists, and also prefers packages over modules if both x/
        # and x.py* exist.  How?  We sort the directory items, so x
        # comes before x.py and x.pyi.  But the preference for .pyi
        # over .py is encoded in find_module(); even though we see
        # x.py before x.pyi, find_module() will find x.pyi first.  We
        # use hits to avoid adding it a second time when we see x.pyi.
        # This also avoids both x.py and x.pyi when x/ was seen first.
        hits = set()  # type: Set[str]
        for item in sorted(os.listdir(os.path.dirname(module_path))):
            abs_path = os.path.join(os.path.dirname(module_path), item)
            if os.path.isdir(abs_path) and \
                    (os.path.isfile(os.path.join(abs_path, '__init__.py')) or
                    os.path.isfile(os.path.join(abs_path, '__init__.pyi'))):
                hits.add(item)
                result += find_modules_recursive(module + '.' + item, lib_path)
            elif item != '__init__.py' and item != '__init__.pyi' and \
                    item.endswith(('.py', '.pyi')):
                mod = item.split('.')[0]
                if mod not in hits:
                    hits.add(mod)
                    result += find_modules_recursive(
                        module + '.' + mod, lib_path)
    return result


def verify_module(id: str, path: str) -> bool:
    """Check that all packages containing id have a __init__ file."""
    if path.endswith(('__init__.py', '__init__.pyi')):
        path = dirname(path)
    for i in range(id.count('.')):
        path = dirname(path)
        if not any(is_file(os.path.join(path, '__init__{}'.format(extension)))
                   for extension in PYTHON_EXTENSIONS):
            return False
    return True


def read_with_python_encoding(path: str, pyversion: Tuple[int, int]) -> Tuple[str, str]:
    """Read the Python file with while obeying PEP-263 encoding detection.

    Returns:
      A tuple: the source as a string, and the hash calculated from the binary representation.
    """
    source_bytearray = bytearray()
    encoding = 'utf8' if pyversion[0] >= 3 else 'ascii'

    with open(path, 'rb') as f:
        # read first two lines and check if PEP-263 coding is present
        source_bytearray.extend(f.readline())
        source_bytearray.extend(f.readline())
        m = hashlib.md5(source_bytearray)

        # check for BOM UTF-8 encoding and strip it out if present
        if source_bytearray.startswith(b'\xef\xbb\xbf'):
            encoding = 'utf8'
            source_bytearray = source_bytearray[3:]
        else:
            _encoding, _ = util.find_python_encoding(source_bytearray, pyversion)
            # check that the coding isn't mypy. We skip it since
            # registering may not have happened yet
            if _encoding != 'mypy':
                encoding = _encoding

        remainder = f.read()
        m.update(remainder)
        source_bytearray.extend(remainder)
        try:
            source_text = source_bytearray.decode(encoding)
        except LookupError as lookuperr:
            raise DecodeError(str(lookuperr))
        return source_text, m.hexdigest()


def get_cache_names(id: str, path: str, manager: BuildManager) -> Tuple[str, str]:
    """Return the file names for the cache files.

    Args:
      id: module ID
      path: module path (used to recognize packages)
      cache_dir: cache directory
      pyversion: Python version (major, minor)

    Returns:
      A tuple with the file names to be used for the meta JSON and the
      data JSON, respectively.
    """
    cache_dir = manager.options.cache_dir
    pyversion = manager.options.python_version
    prefix = os.path.join(cache_dir, '%d.%d' % pyversion, *id.split('.'))
    is_package = os.path.basename(path).startswith('__init__.py')
    if is_package:
        prefix = os.path.join(prefix, '__init__')
    return (prefix + '.meta.json', prefix + '.data.json')


def find_cache_meta(id: str, path: str, manager: BuildManager) -> Optional[CacheMeta]:
    """Find cache data for a module.

    Args:
      id: module ID
      path: module path
      manager: the build manager (for pyversion, log/trace, and build options)

    Returns:
      A CacheMeta instance if the cache data was found and appears
      valid; otherwise None.
    """
    saved_cache = manager.saved_cache
    if id in saved_cache:
        m, t, types = saved_cache[id]
        manager.add_stats(reused_metas=1)
        manager.trace("Reusing saved metadata for %s" % id)
        # Note: it could still be skipped if the mtime/size/hash mismatches.
        return m

    # TODO: May need to take more build options into account
    meta_json, data_json = get_cache_names(id, path, manager)
    manager.trace('Looking for {} at {}'.format(id, meta_json))
    try:
        with open(meta_json, 'r') as f:
            meta_str = f.read()
            manager.trace('Meta {} {}'.format(id, meta_str.rstrip()))
            meta = json.loads(meta_str)  # TODO: Errors
    except IOError:
        manager.log('Could not load cache for {}: could not find {}'.format(id, meta_json))
        return None
    if not isinstance(meta, dict):
        manager.log('Could not load cache for {}: meta cache is not a dict: {}'
                    .format(id, repr(meta)))
        return None
    m = cache_meta_from_dict(meta, data_json)
    # Don't check for path match, that is dealt with in validate_meta().
    if (m.id != id or
            m.mtime is None or m.size is None or
            m.dependencies is None or m.data_mtime is None):
        manager.log('Metadata abandoned for {}: attributes are missing'.format(id))
        return None

    # Ignore cache if generated by an older mypy version.
    if ((m.version_id != manager.version_id and not manager.options.skip_version_check)
            or m.options is None
            or len(m.dependencies) != len(m.dep_prios)
            or len(m.dependencies) != len(m.dep_lines)):
        manager.log('Metadata abandoned for {}: new attributes are missing'.format(id))
        return None

    # Ignore cache if (relevant) options aren't the same.
    # Note that it's fine to mutilate cached_options since it's only used here.
    cached_options = m.options
    current_options = manager.options.clone_for_module(id).select_options_affecting_cache()
    if manager.options.quick_and_dirty:
        # In quick_and_dirty mode allow non-quick_and_dirty cache files.
        cached_options['quick_and_dirty'] = True
    if manager.options.skip_version_check:
        # When we're lax about version we're also lax about platform.
        cached_options['platform'] = current_options['platform']
    if 'debug_cache' in cached_options:
        # Older versions included debug_cache, but it's silly to compare it.
        del cached_options['debug_cache']
    if cached_options != current_options:
        manager.log('Metadata abandoned for {}: options differ'.format(id))
        if manager.options.verbosity >= 2:
            for key in sorted(set(cached_options) | set(current_options)):
                if cached_options.get(key) != current_options.get(key):
                    manager.trace('    {}: {} != {}'
                                  .format(key, cached_options.get(key), current_options.get(key)))
        return None

    manager.add_stats(fresh_metas=1)
    return m


def random_string() -> str:
    return binascii.hexlify(os.urandom(8)).decode('ascii')


def atomic_write(filename: str, *lines: str) -> bool:
    tmp_filename = filename + '.' + random_string()
    try:
        with open(tmp_filename, 'w') as f:
            for line in lines:
                f.write(line)
        os.replace(tmp_filename, filename)
    except os.error as err:
        return False
    return True


def validate_meta(meta: Optional[CacheMeta], id: str, path: Optional[str],
                  ignore_all: bool, manager: BuildManager) -> Optional[CacheMeta]:
    '''Checks whether the cached AST of this module can be used.

    Returns:
      None, if the cached AST is unusable.
      Original meta, if mtime/size matched.
      Meta with mtime updated to match source file, if hash/size matched but mtime/path didn't.
    '''
    # This requires two steps. The first one is obvious: we check that the module source file
    # contents is the same as it was when the cache data file was created. The second one is not
    # too obvious: we check that the cache data file mtime has not changed; it is needed because
    # we use cache data file mtime to propagate information about changes in the dependencies.

    if meta is None:
        manager.log('Metadata not found for {}'.format(id))
        return None

    if meta.ignore_all and not ignore_all:
        manager.log('Metadata abandoned for {}: errors were previously ignored'.format(id))
        return None

    if meta.memory_only:
        # Special case for fine-grained incremental mode when the JSON file is missing but
        # we want to cache the module anyway.
        manager.log('Memory-only metadata for {}'.format(id))
        return meta

    assert path is not None, "Internal error: meta was provided without a path"
    # Check data_json; assume if its mtime matches it's good.
    # TODO: stat() errors
    data_mtime = getmtime(meta.data_json)
    if data_mtime != meta.data_mtime:
        manager.log('Metadata abandoned for {}: data cache is modified'.format(id))
        return None

    # TODO: Share stat() outcome with find_module()
    path = os.path.abspath(path)
    st = manager.get_stat(path)  # TODO: Errors
    if not stat.S_ISREG(st.st_mode):
        manager.log('Metadata abandoned for {}: file {} does not exist'.format(id, path))
        return None

    # When we are using a fine-grained cache, we want our initial
    # build() to load all of the cache information and then do a
    # fine-grained incremental update to catch anything that has
    # changed since the cache was generated. We *don't* want to do a
    # coarse-grained incremental rebuild, so we accept the cache
    # metadata even if it doesn't match the source file.
    if manager.options.use_fine_grained_cache:
        manager.log('Using potentially stale metadata for {}'.format(id))
        return meta

    size = st.st_size
    if size != meta.size:
        manager.log('Metadata abandoned for {}: file {} has different size'.format(id, path))
        return None

    mtime = int(st.st_mtime)
    if mtime != meta.mtime or path != meta.path:
        with open(path, 'rb') as f:
            source_hash = hashlib.md5(f.read()).hexdigest()
        if source_hash != meta.hash:
            manager.log('Metadata abandoned for {}: file {} has different hash'.format(id, path))
            return None
        else:
            # Optimization: update mtime and path (otherwise, this mismatch will reappear).
            meta = meta._replace(mtime=mtime, path=path)
            # Construct a dict we can pass to json.dumps() (compare to write_cache()).
            meta_dict = {
                'id': id,
                'path': path,
                'mtime': mtime,
                'size': size,
                'hash': source_hash,
                'data_mtime': data_mtime,
                'dependencies': meta.dependencies,
                'suppressed': meta.suppressed,
                'child_modules': meta.child_modules,
                'options': (manager.options.clone_for_module(id)
                            .select_options_affecting_cache()),
                'dep_prios': meta.dep_prios,
                'dep_lines': meta.dep_lines,
                'interface_hash': meta.interface_hash,
                'version_id': manager.version_id,
                'ignore_all': meta.ignore_all,
            }
            if manager.options.debug_cache:
                meta_str = json.dumps(meta_dict, indent=2, sort_keys=True)
            else:
                meta_str = json.dumps(meta_dict)
            meta_json, _ = get_cache_names(id, path, manager)
            manager.log('Updating mtime for {}: file {}, meta {}, mtime {}'
                        .format(id, path, meta_json, meta.mtime))
            atomic_write(meta_json, meta_str, '\n')  # Ignore errors, it's just an optimization.
            return meta

    # It's a match on (id, path, size, hash, mtime).
    manager.trace('Metadata fresh for {}: file {}'.format(id, path))
    return meta


def compute_hash(text: str) -> str:
    # We use md5 instead of the builtin hash(...) function because the output of hash(...)
    # can differ between runs due to hash randomization (enabled by default in Python 3.3).
    # See the note in https://docs.python.org/3/reference/datamodel.html#object.__hash__.
    return hashlib.md5(text.encode('utf-8')).hexdigest()


def write_cache(id: str, path: str, tree: MypyFile,
                serialized_fine_grained_deps: Dict[str, List[str]],
                dependencies: List[str], suppressed: List[str],
                child_modules: List[str], dep_prios: List[int], dep_lines: List[int],
                old_interface_hash: str, source_hash: str,
                ignore_all: bool, manager: BuildManager) -> Tuple[str, Optional[CacheMeta]]:
    """Write cache files for a module.

    Note that this mypy's behavior is still correct when any given
    write_cache() call is replaced with a no-op, so error handling
    code that bails without writing anything is okay.

    Args:
      id: module ID
      path: module path
      tree: the fully checked module data
      dependencies: module IDs on which this module depends
      suppressed: module IDs which were suppressed as dependencies
      child_modules: module IDs which are this package's direct submodules
      dep_prios: priorities (parallel array to dependencies)
      dep_lines: import line locations (parallel array to dependencies)
      old_interface_hash: the hash from the previous version of the data cache file
      source_hash: the hash of the source code
      ignore_all: the ignore_all flag for this module
      manager: the build manager (for pyversion, log/trace)

    Returns:
      A tuple containing the interface hash and CacheMeta
      corresponding to the metadata that was written (the latter may
      be None if the cache could not be written).
    """
    # Obtain file paths
    path = os.path.abspath(path)
    meta_json, data_json = get_cache_names(id, path, manager)
    manager.log('Writing {} {} {} {}'.format(id, path, meta_json, data_json))

    # Make sure directory for cache files exists
    parent = os.path.dirname(data_json)
    assert os.path.dirname(meta_json) == parent

    # Serialize data and analyze interface
    data = {'tree': tree.serialize(),
            'fine_grained_deps': serialized_fine_grained_deps,
            }
    if manager.options.debug_cache:
        data_str = json.dumps(data, indent=2, sort_keys=True)
    else:
        data_str = json.dumps(data, sort_keys=True)
    interface_hash = compute_hash(data_str)

    # Obtain and set up metadata
    try:
        os.makedirs(parent, exist_ok=True)
        st = manager.get_stat(path)
    except OSError as err:
        manager.log("Cannot get stat for {}: {}".format(path, err))
        # Remove apparently-invalid cache files.
        # (This is purely an optimization.)
        for filename in [data_json, meta_json]:
            try:
                os.remove(filename)
            except OSError:
                pass
        # Still return the interface hash we computed.
        return interface_hash, None

    # Write data cache file, if applicable
    if old_interface_hash == interface_hash:
        # If the interface is unchanged, the cached data is guaranteed
        # to be equivalent, and we only need to update the metadata.
        data_mtime = getmtime(data_json)
        manager.trace("Interface for {} is unchanged".format(id))
    else:
        manager.trace("Interface for {} has changed".format(id))
        if not atomic_write(data_json, data_str, '\n'):
            # Most likely the error is the replace() call
            # (see https://github.com/python/mypy/issues/3215).
            manager.log("Error writing data JSON file {}".format(data_json))
            # Let's continue without writing the meta file.  Analysis:
            # If the replace failed, we've changed nothing except left
            # behind an extraneous temporary file; if the replace
            # worked but the getmtime() call failed, the meta file
            # will be considered invalid on the next run because the
            # data_mtime field won't match the data file's mtime.
            # Both have the effect of slowing down the next run a
            # little bit due to an out-of-date cache file.
            return interface_hash, None
        data_mtime = getmtime(data_json)

    mtime = int(st.st_mtime)
    size = st.st_size
    options = manager.options.clone_for_module(id)
    assert source_hash is not None
    meta = {'id': id,
            'path': path,
            'mtime': mtime,
            'size': size,
            'hash': source_hash,
            'data_mtime': data_mtime,
            'dependencies': dependencies,
            'suppressed': suppressed,
            'child_modules': child_modules,
            'options': options.select_options_affecting_cache(),
            'dep_prios': dep_prios,
            'dep_lines': dep_lines,
            'interface_hash': interface_hash,
            'version_id': manager.version_id,
            'ignore_all': ignore_all,
            }

    # Write meta cache file
    if manager.options.debug_cache:
        meta_str = json.dumps(meta, indent=2, sort_keys=True)
    else:
        meta_str = json.dumps(meta)
    if not atomic_write(meta_json, meta_str, '\n'):
        # Most likely the error is the replace() call
        # (see https://github.com/python/mypy/issues/3215).
        # The next run will simply find the cache entry out of date.
        manager.log("Error writing meta JSON file {}".format(meta_json))

    return interface_hash, cache_meta_from_dict(meta, data_json)


def delete_cache(id: str, path: str, manager: BuildManager) -> None:
    """Delete cache files for a module.

    The cache files for a module are deleted when mypy finds errors there.
    This avoids inconsistent states with cache files from different mypy runs,
    see #4043 for an example.
    """
    path = os.path.abspath(path)
    meta_json, data_json = get_cache_names(id, path, manager)
    manager.log('Deleting {} {} {} {}'.format(id, path, meta_json, data_json))
    if id in manager.saved_cache:
        del manager.saved_cache[id]

    for filename in [data_json, meta_json]:
        try:
            os.remove(filename)
        except OSError as e:
            if e.errno != errno.ENOENT:
                manager.log("Error deleting cache file {}: {}".format(filename, e.strerror))


"""Dependency manager.

Design
======

Ideally
-------

A. Collapse cycles (each SCC -- strongly connected component --
   becomes one "supernode").

B. Topologically sort nodes based on dependencies.

C. Process from leaves towards roots.

Wrinkles
--------

a. Need to parse source modules to determine dependencies.

b. Processing order for modules within an SCC.

c. Must order mtimes of files to decide whether to re-process; depends
   on clock never resetting.

d. from P import M; checks filesystem whether module P.M exists in
   filesystem.

e. Race conditions, where somebody modifies a file while we're
   processing.  I propose not to modify the algorithm to handle this,
   but to detect when this could lead to inconsistencies.  (For
   example, when we decide on the dependencies based on cache
   metadata, and then we decide to re-parse a file because of a stale
   dependency, if the re-parsing leads to a different list of
   dependencies we should warn the user or start over.)

Steps
-----

1. For each explicitly given module find the source file location.

2. For each such module load and check the cache metadata, and decide
   whether it's valid.

3. Now recursively (or iteratively) find dependencies and add those to
   the graph:

   - for cached nodes use the list of dependencies from the cache
     metadata (this will be valid even if we later end up re-parsing
     the same source);

   - for uncached nodes parse the file and process all imports found,
     taking care of (a) above.

Step 3 should also address (d) above.

Once step 3 terminates we have the entire dependency graph, and for
each module we've either loaded the cache metadata or parsed the
source code.  (However, we may still need to parse those modules for
which we have cache metadata but that depend, directly or indirectly,
on at least one module for which the cache metadata is stale.)

Now we can execute steps A-C from the first section.  Finding SCCs for
step A shouldn't be hard; there's a recipe here:
http://code.activestate.com/recipes/578507/.  There's also a plethora
of topsort recipes, e.g. http://code.activestate.com/recipes/577413/.

For single nodes, processing is simple.  If the node was cached, we
deserialize the cache data and fix up cross-references.  Otherwise, we
do semantic analysis followed by type checking.  We also handle (c)
above; if a module has valid cache data *but* any of its
dependencies was processed from source, then the module should be
processed from source.

A relatively simple optimization (outside SCCs) we might do in the
future is as follows: if a node's cache data is valid, but one or more
of its dependencies are out of date so we have to re-parse the node
from source, once we have fully type-checked the node, we can decide
whether its symbol table actually changed compared to the cache data
(by reading the cache data and comparing it to the data we would be
writing).  If there is no change we can declare the node up to date,
and any node that depends (and for which we have cached data, and
whose other dependencies are up to date) on it won't need to be
re-parsed from source.

Import cycles
-------------

Finally we have to decide how to handle (c), import cycles.  Here
we'll need a modified version of the original state machine
(build.py), but we only need to do this per SCC, and we won't have to
deal with changes to the list of nodes while we're processing it.

If all nodes in the SCC have valid cache metadata and all dependencies
outside the SCC are still valid, we can proceed as follows:

  1. Load cache data for all nodes in the SCC.

  2. Fix up cross-references for all nodes in the SCC.

Otherwise, the simplest (but potentially slow) way to proceed is to
invalidate all cache data in the SCC and re-parse all nodes in the SCC
from source.  We can do this as follows:

  1. Parse source for all nodes in the SCC.

  2. Semantic analysis for all nodes in the SCC.

  3. Type check all nodes in the SCC.

(If there are more passes the process is the same -- each pass should
be done for all nodes before starting the next pass for any nodes in
the SCC.)

We could process the nodes in the SCC in any order.  For sentimental
reasons, I've decided to process them in the reverse order in which we
encountered them when originally constructing the graph.  That's how
the old build.py deals with cycles, and at least this reproduces the
previous implementation more accurately.

Can we do better than re-parsing all nodes in the SCC when any of its
dependencies are out of date?  It's doubtful.  The optimization
mentioned at the end of the previous section would require re-parsing
and type-checking a node and then comparing its symbol table to the
cached data; but because the node is part of a cycle we can't
technically type-check it until the semantic analysis of all other
nodes in the cycle has completed.  (This is an important issue because
Dropbox has a very large cycle in production code.  But I'd like to
deal with it later.)

Additional wrinkles
-------------------

During implementation more wrinkles were found.

- When a submodule of a package (e.g. x.y) is encountered, the parent
  package (e.g. x) must also be loaded, but it is not strictly a
  dependency.  See State.add_ancestors() below.
"""


class ModuleNotFound(Exception):
    """Control flow exception to signal that a module was not found."""


class State:
    """The state for a module.

    The source is only used for the -c command line option; in that
    case path is None.  Otherwise source is None and path isn't.
    """

    manager = None  # type: BuildManager
    order_counter = 0  # Class variable
    order = None  # type: int  # Order in which modules were encountered
    id = None  # type: str  # Fully qualified module name
    path = None  # type: Optional[str]  # Path to module source
    xpath = None  # type: str  # Path or '<string>'
    source = None  # type: Optional[str]  # Module source code
    source_hash = None  # type: str  # Hash calculated based on the source code
    meta = None  # type: Optional[CacheMeta]
    data = None  # type: Optional[str]
    tree = None  # type: Optional[MypyFile]
    is_from_saved_cache = False  # True if the tree came from the in-memory cache
    dependencies = None  # type: List[str]  # Modules directly imported by the module
    suppressed = None  # type: List[str]  # Suppressed/missing dependencies
    priorities = None  # type: Dict[str, int]

    # Map each dependency to the line number where it is first imported
    dep_line_map = None  # type: Dict[str, int]

    # Parent package, its parent, etc.
    ancestors = None  # type: Optional[List[str]]

    # A list of all direct submodules of a given module
    child_modules = None  # type: Set[str]

    # List of (path, line number) tuples giving context for import
    import_context = None  # type: List[Tuple[str, int]]

    # The State from which this module was imported, if any
    caller_state = None  # type: Optional[State]

    # If caller_state is set, the line number in the caller where the import occurred
    caller_line = 0

    # If True, indicate that the public interface of this module is unchanged
    externally_same = True

    # Contains a hash of the public interface in incremental mode
    interface_hash = ""  # type: str

    # Options, specialized for this file
    options = None  # type: Options

    # Whether to ignore all errors
    ignore_all = False

    # Whether the module has an error or any of its dependencies have one.
    transitive_error = False

    fine_grained_deps = None  # type: Dict[str, Set[str]]

    # Type checker used for checking this file.  Use type_checker() for
    # access and to construct this on demand.
    _type_checker = None  # type: Optional[TypeChecker]

    def __init__(self,
                 id: Optional[str],
                 path: Optional[str],
                 source: Optional[str],
                 manager: BuildManager,
                 caller_state: 'Optional[State]' = None,
                 caller_line: int = 0,
                 ancestor_for: 'Optional[State]' = None,
                 root_source: bool = False,
                 ) -> None:
        assert id or path or source is not None, "Neither id, path nor source given"
        self.manager = manager
        State.order_counter += 1
        self.order = State.order_counter
        self.caller_state = caller_state
        self.caller_line = caller_line
        if caller_state:
            self.import_context = caller_state.import_context[:]
            self.import_context.append((caller_state.xpath, caller_line))
        else:
            self.import_context = []
        self.id = id or '__main__'
        self.options = manager.options.clone_for_module(self.id)
        self._type_checker = None
        self.fine_grained_deps = {}
        if not path and source is None:
            assert id is not None
            file_id = id
            if id == 'builtins' and self.options.python_version[0] == 2:
                # The __builtin__ module is called internally by mypy
                # 'builtins' in Python 2 mode (similar to Python 3),
                # but the stub file is __builtin__.pyi.  The reason is
                # that a lot of code hard-codes 'builtins.x' and it's
                # easier to work it around like this.  It also means
                # that the implementation can mostly ignore the
                # difference and just assume 'builtins' everywhere,
                # which simplifies code.
                file_id = '__builtin__'
            path = find_module(file_id, manager.lib_path)
            if path:
                # For non-stubs, look at options.follow_imports:
                # - normal (default) -> fully analyze
                # - silent -> analyze but silence errors
                # - skip -> don't analyze, make the type Any
                follow_imports = self.options.follow_imports
                if (follow_imports != 'normal'
                        and not root_source  # Honor top-level modules
                        and path.endswith('.py')  # Stubs are always normal
                        and id != 'builtins'):  # Builtins is always normal
                    if follow_imports == 'silent':
                        # Still import it, but silence non-blocker errors.
                        manager.log("Silencing %s (%s)" % (path, id))
                        self.ignore_all = True
                    else:
                        # In 'error' mode, produce special error messages.
                        manager.log("Skipping %s (%s)" % (path, id))
                        if follow_imports == 'error':
                            if ancestor_for:
                                self.skipping_ancestor(id, path, ancestor_for)
                            else:
                                self.skipping_module(id, path)
                        path = None
                        manager.missing_modules.add(id)
                        raise ModuleNotFound
            else:
                # Could not find a module.  Typically the reason is a
                # misspelled module name, missing stub, module not in
                # search path or the module has not been installed.
                if caller_state:
                    if not self.options.ignore_missing_imports:
                        save_import_context = manager.errors.import_context()
                        manager.errors.set_import_context(caller_state.import_context)
                        manager.module_not_found(caller_state.xpath, caller_state.id,
                                                 caller_line, id)
                        manager.errors.set_import_context(save_import_context)
                    manager.missing_modules.add(id)
                    raise ModuleNotFound
                else:
                    # If we can't find a root source it's always fatal.
                    # TODO: This might hide non-fatal errors from
                    # root sources processed earlier.
                    raise CompileError(["mypy: can't find module '%s'" % id])
        self.path = path
        self.xpath = path or '<string>'
        self.source = source
        if path and source is None and self.options.incremental:
            self.meta = find_cache_meta(self.id, path, manager)
            # TODO: Get mtime if not cached.
            if self.meta is not None:
                self.interface_hash = self.meta.interface_hash
        self.add_ancestors()
        self.meta = validate_meta(self.meta, self.id, self.path, self.ignore_all, manager)
        if self.meta:
            # Make copies, since we may modify these and want to
            # compare them to the originals later.
            self.dependencies = list(self.meta.dependencies)
            self.suppressed = list(self.meta.suppressed)
            assert len(self.meta.dependencies) == len(self.meta.dep_prios)
            self.priorities = {id: pri
                               for id, pri in zip(self.meta.dependencies, self.meta.dep_prios)}
            assert len(self.meta.dependencies) == len(self.meta.dep_lines)
            self.dep_line_map = {id: line
                                 for id, line in zip(self.meta.dependencies, self.meta.dep_lines)}
            self.child_modules = set(self.meta.child_modules)
        else:
            # Parse the file (and then some) to get the dependencies.
            self.parse_file()
            self.suppressed = []
            self.child_modules = set()

    def skipping_ancestor(self, id: str, path: str, ancestor_for: 'State') -> None:
        # TODO: Read the path (the __init__.py file) and return
        # immediately if it's empty or only contains comments.
        # But beware, some package may be the ancestor of many modules,
        # so we'd need to cache the decision.
        manager = self.manager
        manager.errors.set_import_context([])
        manager.errors.set_file(ancestor_for.xpath, ancestor_for.id)
        manager.errors.report(-1, -1, "Ancestor package '%s' ignored" % (id,),
                              severity='note', only_once=True)
        manager.errors.report(-1, -1,
                              "(Using --follow-imports=error, submodule passed on command line)",
                              severity='note', only_once=True)

    def skipping_module(self, id: str, path: str) -> None:
        assert self.caller_state, (id, path)
        manager = self.manager
        save_import_context = manager.errors.import_context()
        manager.errors.set_import_context(self.caller_state.import_context)
        manager.errors.set_file(self.caller_state.xpath, self.caller_state.id)
        line = self.caller_line
        manager.errors.report(line, 0,
                              "Import of '%s' ignored" % (id,),
                              severity='note')
        manager.errors.report(line, 0,
                              "(Using --follow-imports=error, module not passed on command line)",
                              severity='note', only_once=True)
        manager.errors.set_import_context(save_import_context)

    def add_ancestors(self) -> None:
        if self.path is not None:
            _, name = os.path.split(self.path)
            base, _ = os.path.splitext(name)
            if '.' in base:
                # This is just a weird filename, don't add anything
                self.ancestors = []
                return
        # All parent packages are new ancestors.
        ancestors = []
        parent = self.id
        while '.' in parent:
            parent, _ = parent.rsplit('.', 1)
            ancestors.append(parent)
        self.ancestors = ancestors

    def is_fresh(self) -> bool:
        """Return whether the cache data for this file is fresh."""
        # NOTE: self.dependencies may differ from
        # self.meta.dependencies when a dependency is dropped due to
        # suppression by silent mode.  However when a suppressed
        # dependency is added back we find out later in the process.
        return (self.meta is not None
                and self.is_interface_fresh()
                and self.dependencies == self.meta.dependencies
                and self.child_modules == set(self.meta.child_modules))

    def is_interface_fresh(self) -> bool:
        return self.externally_same

    def has_new_submodules(self) -> bool:
        """Return if this module has new submodules after being loaded from a warm cache."""
        return self.meta is not None and self.child_modules != set(self.meta.child_modules)

    def mark_as_rechecked(self) -> None:
        """Marks this module as having been fully re-analyzed by the type-checker."""
        self.manager.rechecked_modules.add(self.id)

    def mark_interface_stale(self, *, on_errors: bool = False) -> None:
        """Marks this module as having a stale public interface, and discards the cache data."""
        self.externally_same = False
        if not on_errors:
            self.manager.stale_modules.add(self.id)

    def check_blockers(self) -> None:
        """Raise CompileError if a blocking error is detected."""
        if self.manager.errors.is_blockers():
            self.manager.log("Bailing due to blocking errors")
            self.manager.errors.raise_error()

    @contextlib.contextmanager
    def wrap_context(self) -> Iterator[None]:
        save_import_context = self.manager.errors.import_context()
        self.manager.errors.set_import_context(self.import_context)
        try:
            yield
        except CompileError:
            raise
        except Exception as err:
            report_internal_error(err, self.path, 0, self.manager.errors, self.options)
        self.manager.errors.set_import_context(save_import_context)
        self.check_blockers()

    # Methods for processing cached modules.

    def load_tree(self) -> None:
        assert self.meta is not None, "Internal error: this method must be called only" \
                                      " for cached modules"
        with open(self.meta.data_json) as f:
            data = json.load(f)
        # TODO: Assert data file wasn't changed.
        self.tree = MypyFile.deserialize(data['tree'])
        self.fine_grained_deps = {k: set(v) for k, v in data['fine_grained_deps'].items()}

        self.manager.modules[self.id] = self.tree
        self.manager.add_stats(fresh_trees=1)

    def fix_cross_refs(self) -> None:
        assert self.tree is not None, "Internal error: method must be called on parsed file only"
        fixup_module_pass_one(self.tree, self.manager.modules,
                              self.manager.options.quick_and_dirty)

    def calculate_mros(self) -> None:
        assert self.tree is not None, "Internal error: method must be called on parsed file only"
        fixup_module_pass_two(self.tree, self.manager.modules,
                              self.manager.options.quick_and_dirty)

    def patch_dependency_parents(self) -> None:
        """
        In Python, if a and a.b are both modules, running `import a.b` will
        modify not only the current module's namespace, but a's namespace as
        well -- see SemanticAnalyzerPass2.add_submodules_to_parent_modules for more
        details.

        However, this patching process can occur after `a` has been parsed and
        serialized during increment mode. Consequently, we need to repeat this
        patch when deserializing a cached file.

        This function should be called only when processing fresh SCCs -- the
        semantic analyzer will perform this patch for us when processing stale
        SCCs.
        """
        for dep in self.dependencies:
            self.manager.semantic_analyzer.add_submodules_to_parent_modules(dep, True)

    def fix_suppressed_dependencies(self, graph: Graph) -> None:
        """Corrects whether dependencies are considered stale in silent mode.

        This method is a hack to correct imports in silent mode + incremental mode.
        In particular, the problem is that when running mypy with a cold cache, the
        `parse_file(...)` function is called *at the start* of the `load_graph(...)` function.
        Note that load_graph will mark some dependencies as suppressed if they weren't specified
        on the command line in silent mode.

        However, if the interface for a module is changed, parse_file will be called within
        `process_stale_scc` -- *after* load_graph is finished, wiping out the changes load_graph
        previously made.

        This method is meant to be run after parse_file finishes in process_stale_scc and will
        recompute what modules should be considered suppressed in silent mode.
        """
        # TODO: See if it's possible to move this check directly into parse_file in some way.
        # TODO: Find a way to write a test case for this fix.
        silent_mode = (self.options.ignore_missing_imports or
                       self.options.follow_imports == 'skip')
        if not silent_mode:
            return

        new_suppressed = []
        new_dependencies = []
        entry_points = self.manager.source_set.source_modules
        for dep in self.dependencies + self.suppressed:
            ignored = dep in self.suppressed and dep not in entry_points
            if ignored or dep not in graph:
                new_suppressed.append(dep)
            else:
                new_dependencies.append(dep)
        self.dependencies = new_dependencies
        self.suppressed = new_suppressed

    # Methods for processing modules from source code.

    def parse_file(self) -> None:
        """Parse file and run first pass of semantic analysis.

        Everything done here is local to the file. Don't depend on imported
        modules in any way. Also record module dependencies based on imports.
        """
        if self.tree is not None:
            # The file was already parsed (in __init__()).
            return

        manager = self.manager
        modules = manager.modules
        manager.log("Parsing %s (%s)" % (self.xpath, self.id))

        with self.wrap_context():
            source = self.source
            self.source = None  # We won't need it again.
            if self.path and source is None:
                try:
                    path = manager.maybe_swap_for_shadow_path(self.path)
                    source, self.source_hash = read_with_python_encoding(
                        path, self.options.python_version)
                except IOError as ioerr:
                    raise CompileError([
                        "mypy: can't read file '{}': {}".format(self.path, ioerr.strerror)])
                except (UnicodeDecodeError, DecodeError) as decodeerr:
                    raise CompileError([
                        "mypy: can't decode file '{}': {}".format(self.path, str(decodeerr))])
            assert source is not None
            self.tree = manager.parse_file(self.id, self.xpath, source,
                                           self.ignore_all or self.options.ignore_errors)

        modules[self.id] = self.tree

        # Do the first pass of semantic analysis: add top-level
        # definitions in the file to the symbol table.  We must do
        # this before processing imports, since this may mark some
        # import statements as unreachable.
        first = SemanticAnalyzerPass1(manager.semantic_analyzer)
        with self.wrap_context():
            first.visit_file(self.tree, self.xpath, self.id, self.options)

        # Initialize module symbol table, which was populated by the
        # semantic analyzer.
        # TODO: Why can't SemanticAnalyzerPass1 .analyze() do this?
        self.tree.names = manager.semantic_analyzer.globals

        # Compute (direct) dependencies.
        # Add all direct imports (this is why we needed the first pass).
        # Also keep track of each dependency's source line.
        dependencies = []
        suppressed = []
        priorities = {}  # type: Dict[str, int]  # id -> priority
        dep_line_map = {}  # type: Dict[str, int]  # id -> line
        for pri, id, line in manager.all_imported_modules_in_file(self.tree):
            priorities[id] = min(pri, priorities.get(id, PRI_ALL))
            if id == self.id:
                continue
            # Omit missing modules, as otherwise we could not type-check
            # programs with missing modules.
            if id in manager.missing_modules:
                if id not in dep_line_map:
                    suppressed.append(id)
                    dep_line_map[id] = line
                continue
            if id == '':
                # Must be from a relative import.
                manager.errors.set_file(self.xpath, self.id)
                manager.errors.report(line, 0,
                                      "No parent module -- cannot perform relative import",
                                      blocker=True)
                continue
            if id not in dep_line_map:
                dependencies.append(id)
                dep_line_map[id] = line
        # Every module implicitly depends on builtins.
        if self.id != 'builtins' and 'builtins' not in dep_line_map:
            dependencies.append('builtins')

        # If self.dependencies is already set, it was read from the
        # cache, but for some reason we're re-parsing the file.
        # NOTE: What to do about race conditions (like editing the
        # file while mypy runs)?  A previous version of this code
        # explicitly checked for this, but ran afoul of other reasons
        # for differences (e.g. silent mode).
        self.dependencies = dependencies
        self.suppressed = suppressed
        self.priorities = priorities
        self.dep_line_map = dep_line_map
        self.check_blockers()

    def semantic_analysis(self) -> None:
        assert self.tree is not None, "Internal error: method must be called on parsed file only"
        patches = []  # type: List[Tuple[int, Callable[[], None]]]
        with self.wrap_context():
            self.manager.semantic_analyzer.visit_file(self.tree, self.xpath, self.options, patches)
        self.patches = patches

    def semantic_analysis_pass_three(self) -> None:
        assert self.tree is not None, "Internal error: method must be called on parsed file only"
        patches = []  # type: List[Tuple[int, Callable[[], None]]]
        with self.wrap_context():
            self.manager.semantic_analyzer_pass3.visit_file(self.tree, self.xpath,
                                                            self.options, patches)
            if self.options.dump_type_stats:
                dump_type_stats(self.tree, self.xpath)
        self.patches = patches + self.patches

    def semantic_analysis_apply_patches(self) -> None:
        patches_by_priority = sorted(self.patches, key=lambda x: x[0])
        for priority, patch_func in patches_by_priority:
            patch_func()

    def type_check_first_pass(self) -> None:
        if self.options.semantic_analysis_only:
            return
        with self.wrap_context():
            self.type_checker().check_first_pass()

    def type_checker(self) -> TypeChecker:
        if not self._type_checker:
            assert self.tree is not None, "Internal error: must be called on parsed file only"
            manager = self.manager
            self._type_checker = TypeChecker(manager.errors, manager.modules, self.options,
                                             self.tree, self.xpath, manager.plugin)
        return self._type_checker

    def type_map(self) -> Dict[Expression, Type]:
        return self.type_checker().type_map

    def type_check_second_pass(self) -> bool:
        if self.options.semantic_analysis_only:
            return False
        with self.wrap_context():
            return self.type_checker().check_second_pass()

    def finish_passes(self) -> None:
        assert self.tree is not None, "Internal error: method must be called on parsed file only"
        manager = self.manager
        if self.options.semantic_analysis_only:
            return
        with self.wrap_context():
            # Some tests want to look at the set of all types.
            if manager.options.use_builtins_fixtures or manager.options.dump_deps:
                manager.all_types.update(self.type_map())

            if self.options.incremental:
                self._patch_indirect_dependencies(self.type_checker().module_refs,
                                                  self.type_map())

            if self.options.dump_inference_stats:
                dump_type_stats(self.tree, self.xpath, inferred=True,
                                typemap=self.type_map())
            manager.report_file(self.tree, self.type_map(), self.options)

    def _patch_indirect_dependencies(self,
                                     module_refs: Set[str],
                                     type_map: Dict[Expression, Type]) -> None:
        types = set(type_map.values())
        assert None not in types
        valid = self.valid_references()

        encountered = self.manager.indirection_detector.find_modules(types) | module_refs
        extra = encountered - valid

        for dep in sorted(extra):
            if dep not in self.manager.modules:
                continue
            if dep not in self.suppressed and dep not in self.manager.missing_modules:
                self.dependencies.append(dep)
                self.priorities[dep] = PRI_INDIRECT
            elif dep not in self.suppressed and dep in self.manager.missing_modules:
                self.suppressed.append(dep)

    def compute_fine_grained_deps(self) -> None:
        assert self.tree is not None
        if '/typeshed/' in self.xpath or self.xpath.startswith('typeshed/'):
            # We don't track changes to typeshed -- the assumption is that they are only changed
            # as part of mypy updates, which will invalidate everything anyway.
            #
            # TODO: Not a reliable test, as we could have a package named typeshed.
            # TODO: Consider relaxing this -- maybe allow some typeshed changes to be tracked.
            return
        self.fine_grained_deps = get_dependencies(target=self.tree,
                                                  type_map=self.type_map(),
                                                  python_version=self.options.python_version)

    def valid_references(self) -> Set[str]:
        assert self.ancestors is not None
        valid_refs = set(self.dependencies + self.suppressed + self.ancestors)
        valid_refs.add(self.id)

        if "os" in valid_refs:
            valid_refs.add("os.path")

        return valid_refs

    def write_cache(self) -> None:
        assert self.tree is not None, "Internal error: method must be called on parsed file only"
        if not self.path or self.options.cache_dir == os.devnull:
            return
        if self.manager.options.quick_and_dirty:
            is_errors = self.manager.errors.is_errors_for_file(self.path)
        else:
            is_errors = self.transitive_error
        if is_errors:
            delete_cache(self.id, self.path, self.manager)
            self.meta = None
            self.mark_interface_stale(on_errors=True)
            return
        dep_prios = self.dependency_priorities()
        dep_lines = self.dependency_lines()
        new_interface_hash, self.meta = write_cache(
            self.id, self.path, self.tree,
            {k: list(v) for k, v in self.fine_grained_deps.items()},
            list(self.dependencies), list(self.suppressed), list(self.child_modules),
            dep_prios, dep_lines, self.interface_hash, self.source_hash, self.ignore_all,
            self.manager)
        if new_interface_hash == self.interface_hash:
            self.manager.log("Cached module {} has same interface".format(self.id))
        else:
            self.manager.log("Cached module {} has changed interface".format(self.id))
            self.mark_interface_stale()
            self.interface_hash = new_interface_hash

    def dependency_priorities(self) -> List[int]:
        return [self.priorities.get(dep, PRI_HIGH) for dep in self.dependencies]

    def dependency_lines(self) -> List[int]:
        return [self.dep_line_map.get(dep, 1) for dep in self.dependencies]

    def generate_unused_ignore_notes(self) -> None:
        if self.options.warn_unused_ignores:
            self.manager.errors.generate_unused_ignore_notes(self.xpath)


def dispatch(sources: List[BuildSource], manager: BuildManager) -> Graph:
    set_orig = set(manager.saved_cache)
    manager.log()
    manager.log("Mypy version %s" % __version__)
    t0 = time.time()
    graph = load_graph(sources, manager)
    t1 = time.time()
    manager.add_stats(graph_size=len(graph),
                      stubs_found=sum(g.path is not None and g.path.endswith('.pyi')
                                      for g in graph.values()),
                      graph_load_time=(t1 - t0),
                      fm_cache_size=len(find_module_cache),
                      fm_dir_cache_size=len(find_module_dir_cache),
                      fm_listdir_cache_size=len(find_module_listdir_cache),
                      fm_is_file_cache_size=len(find_module_is_file_cache),
                      fm_isdir_cache_size=len(find_module_isdir_cache),
                      )
    if not graph:
        print("Nothing to do?!")
        return graph
    manager.log("Loaded graph with %d nodes (%.3f sec)" % (len(graph), t1 - t0))
    if manager.options.dump_graph:
        dump_graph(graph)
        return graph
    process_graph(graph, manager)
    updated = preserve_cache(graph)
    set_updated = set(updated)
    manager.saved_cache.clear()
    manager.saved_cache.update(updated)
    set_final = set(manager.saved_cache)
    # These keys have numbers in them to force a sort order.
    manager.add_stats(saved_cache_1orig=len(set_orig),
                      saved_cache_2updated=len(set_updated & set_orig),
                      saved_cache_3added=len(set_final - set_orig),
                      saved_cache_4removed=len(set_orig - set_final),
                      saved_cache_5final=len(set_final))
    if manager.options.dump_deps:
        # This speeds up startup a little when not using the daemon mode.
        from mypy.server.deps import dump_all_dependencies
        dump_all_dependencies(manager.modules, manager.all_types, manager.options.python_version)
    return graph


def preserve_cache(graph: Graph) -> SavedCache:
    saved_cache = {}
    for id, state in graph.items():
        assert state.id == id
        if state.meta is not None and state.tree is not None:
            saved_cache[id] = (state.meta, state.tree, state.type_map())
    return saved_cache


class NodeInfo:
    """Some info about a node in the graph of SCCs."""

    def __init__(self, index: int, scc: List[str]) -> None:
        self.node_id = "n%d" % index
        self.scc = scc
        self.sizes = {}  # type: Dict[str, int]  # mod -> size in bytes
        self.deps = {}  # type: Dict[str, int]  # node_id -> pri

    def dumps(self) -> str:
        """Convert to JSON string."""
        total_size = sum(self.sizes.values())
        return "[%s, %s, %s,\n     %s,\n     %s]" % (json.dumps(self.node_id),
                                                     json.dumps(total_size),
                                                     json.dumps(self.scc),
                                                     json.dumps(self.sizes),
                                                     json.dumps(self.deps))


def dump_graph(graph: Graph) -> None:
    """Dump the graph as a JSON string to stdout.

    This copies some of the work by process_graph()
    (sorted_components() and order_ascc()).
    """
    nodes = []
    sccs = sorted_components(graph)
    for i, ascc in enumerate(sccs):
        scc = order_ascc(graph, ascc)
        node = NodeInfo(i, scc)
        nodes.append(node)
    inv_nodes = {}  # module -> node_id
    for node in nodes:
        for mod in node.scc:
            inv_nodes[mod] = node.node_id
    for node in nodes:
        for mod in node.scc:
            state = graph[mod]
            size = 0
            if state.path:
                try:
                    size = os.path.getsize(state.path)
                except os.error:
                    pass
            node.sizes[mod] = size
            for dep in state.dependencies:
                if dep in state.priorities:
                    pri = state.priorities[dep]
                    if dep in inv_nodes:
                        dep_id = inv_nodes[dep]
                        if (dep_id != node.node_id and
                                (dep_id not in node.deps or pri < node.deps[dep_id])):
                            node.deps[dep_id] = pri
    print("[" + ",\n ".join(node.dumps() for node in nodes) + "\n]")


def load_graph(sources: List[BuildSource], manager: BuildManager) -> Graph:
    """Given some source files, load the full dependency graph.

    As this may need to parse files, this can raise CompileError in case
    there are syntax errors.
    """
    graph = {}  # type: Graph
    # The deque is used to implement breadth-first traversal.
    # TODO: Consider whether to go depth-first instead.  This may
    # affect the order in which we process files within import cycles.
    new = collections.deque()  # type: Deque[State]
    entry_points = set()  # type: Set[str]
    # Seed the graph with the initial root sources.
    for bs in sources:
        try:
            st = State(id=bs.module, path=bs.path, source=bs.text, manager=manager,
                       root_source=True)
        except ModuleNotFound:
            continue
        if st.id in graph:
            manager.errors.set_file(st.xpath, st.id)
            manager.errors.report(-1, -1, "Duplicate module named '%s'" % st.id)
            manager.errors.raise_error()
        graph[st.id] = st
        new.append(st)
        entry_points.add(bs.module)
    # Collect dependencies.  We go breadth-first.
    while new:
        st = new.popleft()
        assert st.ancestors is not None
        # Strip out indirect dependencies.  These will be dealt with
        # when they show up as direct dependencies, and there's a
        # scenario where they hurt:
        # - Suppose A imports B and B imports C.
        # - Suppose on the next round:
        #   - C is deleted;
        #   - B is updated to remove the dependency on C;
        #   - A is unchanged.
        # - In this case A's cached *direct* dependencies are still valid
        #   (since direct dependencies reflect the imports found in the source)
        #   but A's cached *indirect* dependency on C is wrong.
        dependencies = [dep for dep in st.dependencies if st.priorities.get(dep) != PRI_INDIRECT]
        for dep in st.ancestors + dependencies + st.suppressed:
            # We don't want to recheck imports marked with '# type: ignore'
            # so we ignore any suppressed module not explicitly re-included
            # from the command line.
            ignored = dep in st.suppressed and dep not in entry_points
            if ignored:
                manager.missing_modules.add(dep)
            elif dep not in graph:
                try:
                    if dep in st.ancestors:
                        # TODO: Why not 'if dep not in st.dependencies' ?
                        # Ancestors don't have import context.
                        newst = State(id=dep, path=None, source=None, manager=manager,
                                      ancestor_for=st)
                    else:
                        newst = State(id=dep, path=None, source=None, manager=manager,
                                      caller_state=st, caller_line=st.dep_line_map.get(dep, 1))
                except ModuleNotFound:
                    if dep in st.dependencies:
                        st.dependencies.remove(dep)
                        st.suppressed.append(dep)
                else:
                    assert newst.id not in graph, newst.id
                    graph[newst.id] = newst
                    new.append(newst)
            if dep in st.ancestors and dep in graph:
                graph[dep].child_modules.add(st.id)
            if dep in graph and dep in st.suppressed:
                # Previously suppressed file is now visible
                if dep in st.suppressed:
                    st.suppressed.remove(dep)
                    st.dependencies.append(dep)
    for id, g in graph.items():
        if g.has_new_submodules():
            g.parse_file()
            g.fix_suppressed_dependencies(graph)
            g.mark_interface_stale()
    return graph


class FreshState(State):
    meta = None  # type: CacheMeta


def process_graph(graph: Graph, manager: BuildManager) -> None:
    """Process everything in dependency order."""
    sccs = sorted_components(graph)
    manager.log("Found %d SCCs; largest has %d nodes" %
                (len(sccs), max(len(scc) for scc in sccs)))

    fresh_scc_queue = []  # type: List[List[str]]

    # We're processing SCCs from leaves (those without further
    # dependencies) to roots (those from which everything else can be
    # reached).
    for ascc in sccs:
        # Order the SCC's nodes using a heuristic.
        # Note that ascc is a set, and scc is a list.
        scc = order_ascc(graph, ascc)
        # If builtins is in the list, move it last.  (This is a bit of
        # a hack, but it's necessary because the builtins module is
        # part of a small cycle involving at least {builtins, abc,
        # typing}.  Of these, builtins must be processed last or else
        # some builtin objects will be incompletely processed.)
        if 'builtins' in ascc:
            scc.remove('builtins')
            scc.append('builtins')
        if manager.options.verbosity >= 2:
            for id in scc:
                manager.trace("Priorities for %s:" % id,
                              " ".join("%s:%d" % (x, graph[id].priorities[x])
                                       for x in graph[id].dependencies
                                       if x in ascc and x in graph[id].priorities))
        # Because the SCCs are presented in topological sort order, we
        # don't need to look at dependencies recursively for staleness
        # -- the immediate dependencies are sufficient.
        stale_scc = {id for id in scc if not graph[id].is_fresh()}
        fresh = not stale_scc
        deps = set()
        for id in scc:
            deps.update(graph[id].dependencies)
        deps -= ascc
        stale_deps = {id for id in deps if id in graph and not graph[id].is_interface_fresh()}
        if not manager.options.quick_and_dirty:
            fresh = fresh and not stale_deps
        undeps = set()
        if fresh:
            # Check if any dependencies that were suppressed according
            # to the cache have heen added back in this run.
            # NOTE: Newly suppressed dependencies are handled by is_fresh().
            for id in scc:
                undeps.update(graph[id].suppressed)
            undeps &= graph.keys()
            if undeps:
                fresh = False
        if fresh:
            # All cache files are fresh.  Check that no dependency's
            # cache file is newer than any scc node's cache file.
            fresh_graph = cast(Dict[str, FreshState], graph)
            oldest_in_scc = min(fresh_graph[id].meta.data_mtime for id in scc)
            viable = {id for id in stale_deps if graph[id].meta is not None}
            newest_in_deps = 0 if not viable else max(fresh_graph[dep].meta.data_mtime
                                                      for dep in viable)
            if manager.options.verbosity >= 3:  # Dump all mtimes for extreme debugging.
                all_ids = sorted(ascc | viable, key=lambda id: fresh_graph[id].meta.data_mtime)
                for id in all_ids:
                    if id in scc:
                        if fresh_graph[id].meta.data_mtime < newest_in_deps:
                            key = "*id:"
                        else:
                            key = "id:"
                    else:
                        if fresh_graph[id].meta.data_mtime > oldest_in_scc:
                            key = "+dep:"
                        else:
                            key = "dep:"
                    manager.trace(" %5s %.0f %s" % (key, fresh_graph[id].meta.data_mtime, id))
            # If equal, give the benefit of the doubt, due to 1-sec time granularity
            # (on some platforms).
            if manager.options.quick_and_dirty and stale_deps:
                fresh_msg = "fresh(ish)"
            elif oldest_in_scc < newest_in_deps:
                fresh = False
                fresh_msg = "out of date by %.0f seconds" % (newest_in_deps - oldest_in_scc)
            else:
                fresh_msg = "fresh"
        elif undeps:
            fresh_msg = "stale due to changed suppression (%s)" % " ".join(sorted(undeps))
        elif stale_scc:
            fresh_msg = "inherently stale"
            if stale_scc != ascc:
                fresh_msg += " (%s)" % " ".join(sorted(stale_scc))
            if stale_deps:
                fresh_msg += " with stale deps (%s)" % " ".join(sorted(stale_deps))
        else:
            fresh_msg = "stale due to deps (%s)" % " ".join(sorted(stale_deps))

        # Initialize transitive_error for all SCC members from union
        # of transitive_error of dependencies.
        if any(graph[dep].transitive_error for dep in deps if dep in graph):
            for id in scc:
                graph[id].transitive_error = True

        scc_str = " ".join(scc)
        if fresh:
            if maybe_reuse_in_memory_tree(graph, scc, manager):
                manager.add_stats(sccs_kept=1, nodes_kept=len(scc))
            else:
                manager.trace("Queuing %s SCC (%s)" % (fresh_msg, scc_str))
                fresh_scc_queue.append(scc)
        else:
            if len(fresh_scc_queue) > 0:
                manager.log("Processing {} queued fresh SCCs".format(len(fresh_scc_queue)))
                # Defer processing fresh SCCs until we actually run into a stale SCC
                # and need the earlier modules to be loaded.
                #
                # Note that `process_graph` may end with us not having processed every
                # single fresh SCC. This is intentional -- we don't need those modules
                # loaded if there are no more stale SCCs to be rechecked.
                #
                # Also note we shouldn't have to worry about transitive_error here,
                # since modules with transitive errors aren't written to the cache,
                # and if any dependencies were changed, this SCC would be stale.
                # (Also, in quick_and_dirty mode we don't care about transitive errors.)
                #
                # TODO: see if it's possible to determine if we need to process only a
                # _subset_ of the past SCCs instead of having to process them all.
                for prev_scc in fresh_scc_queue:
                    process_fresh_scc(graph, prev_scc, manager)
                fresh_scc_queue = []
            size = len(scc)
            if size == 1:
                manager.log("Processing SCC singleton (%s) as %s" % (scc_str, fresh_msg))
            else:
                manager.log("Processing SCC of size %d (%s) as %s" % (size, scc_str, fresh_msg))
            process_stale_scc(graph, scc, manager)

    # If we are running in fine-grained incremental mode with caching,
    # we always process fresh SCCs so that we have all of the symbol
    # tables and fine-grained dependencies available.
    if manager.options.use_fine_grained_cache:
        for prev_scc in fresh_scc_queue:
            process_fresh_scc(graph, prev_scc, manager)
        fresh_scc_queue = []

    sccs_left = len(fresh_scc_queue)
    nodes_left = sum(len(scc) for scc in fresh_scc_queue)
    manager.add_stats(sccs_left=sccs_left, nodes_left=nodes_left)
    if sccs_left:
        manager.log("{} fresh SCCs ({} nodes) left in queue (and will remain unprocessed)"
                    .format(sccs_left, nodes_left))
        manager.trace(str(fresh_scc_queue))
    else:
        manager.log("No fresh SCCs left in queue")


def order_ascc(graph: Graph, ascc: AbstractSet[str], pri_max: int = PRI_ALL) -> List[str]:
    """Come up with the ideal processing order within an SCC.

    Using the priorities assigned by all_imported_modules_in_file(),
    try to reduce the cycle to a DAG, by omitting arcs representing
    dependencies of lower priority.

    In the simplest case, if we have A <--> B where A has a top-level
    "import B" (medium priority) but B only has the reverse "import A"
    inside a function (low priority), we turn the cycle into a DAG by
    dropping the B --> A arc, which leaves only A --> B.

    If all arcs have the same priority, we fall back to sorting by
    reverse global order (the order in which modules were first
    encountered).

    The algorithm is recursive, as follows: when as arcs of different
    priorities are present, drop all arcs of the lowest priority,
    identify SCCs in the resulting graph, and apply the algorithm to
    each SCC thus found.  The recursion is bounded because at each
    recursion the spread in priorities is (at least) one less.

    In practice there are only a few priority levels (less than a
    dozen) and in the worst case we just carry out the same algorithm
    for finding SCCs N times.  Thus the complexity is no worse than
    the complexity of the original SCC-finding algorithm -- see
    strongly_connected_components() below for a reference.
    """
    if len(ascc) == 1:
        return [s for s in ascc]
    pri_spread = set()
    for id in ascc:
        state = graph[id]
        for dep in state.dependencies:
            if dep in ascc:
                pri = state.priorities.get(dep, PRI_HIGH)
                if pri < pri_max:
                    pri_spread.add(pri)
    if len(pri_spread) == 1:
        # Filtered dependencies are uniform -- order by global order.
        return sorted(ascc, key=lambda id: -graph[id].order)
    pri_max = max(pri_spread)
    sccs = sorted_components(graph, ascc, pri_max)
    # The recursion is bounded by the len(pri_spread) check above.
    return [s for ss in sccs for s in order_ascc(graph, ss, pri_max)]


def process_fresh_scc(graph: Graph, scc: List[str], manager: BuildManager) -> None:
    """Process the modules in one SCC from their cached data.

    This involves loading the tree from JSON and then doing various cleanups.

    If the tree is loaded from memory ('saved_cache') it's even quicker.
    """
    for id in scc:
        graph[id].load_tree()
    for id in scc:
        graph[id].fix_cross_refs()
    for id in scc:
        graph[id].calculate_mros()
    for id in scc:
        graph[id].patch_dependency_parents()


def maybe_reuse_in_memory_tree(graph: Graph, scc: List[str], manager: BuildManager) -> bool:
    """Set the trees for the given SCC from the in-memory cache, if all valid.

    If any saved tree for this SCC is invalid, set the trees for all
    SCC members to None and mark as not-from-cache.
    """
    if not can_reuse_in_memory_tree(graph, scc, manager):
        for id in scc:
            manager.add_stats(cleared_trees=1)
            manager.trace("Clearing tree %s" % id)
            st = graph[id]
            st.tree = None
            st.is_from_saved_cache = False
            if id in manager.modules:
                del manager.modules[id]
        return False
    trees = {id: manager.saved_cache[id][1] for id in scc}
    for id, tree in trees.items():
        manager.add_stats(reused_trees=1)
        manager.trace("Reusing saved tree %s" % id)
        st = graph[id]
        st.tree = tree
        st.is_from_saved_cache = True
        manager.modules[id] = tree
        # Delete any submodules from the module that aren't
        # dependencies of the module; they will be re-added once
        # imported.  It's possible that the parent module is reused
        # but a submodule isn't; we don't want to accidentally link
        # into the old submodule's tree.  See also
        # patch_dependency_parents() above.  The exception for subname
        # in st.dependencies handles the case where 'import m'
        # guarantees that some submodule of m is also available
        # (e.g. 'os.path'); in those cases the submodule is an
        # explicit dependency of the parent.
        for name in list(tree.names):
            sym = tree.names[name]
            subname = id + '.' + name
            if (sym.kind == MODULE_REF
                    and sym.node is not None
                    and sym.node.fullname() == subname
                    and subname not in st.dependencies):
                manager.trace("Purging %s" % subname)
                del tree.names[name]
    return True


def can_reuse_in_memory_tree(graph: Graph, scc: List[str], manager: BuildManager) -> bool:
    """Check whether the given SCC can safely reuse the trees from saved_cache.

    Assumes the SCC is already considered fresh.
    """
    saved_cache = manager.saved_cache
    # Check that all nodes are available for loading from memory.
    if all(id in saved_cache for id in scc):
        # Check that all dependencies were loaded from memory.
        # If not, some dependency was reparsed but the interface hash
        # wasn't changed -- in that case we can't reuse the tree.
        # TODO: Pass deps in from process_graph(), via maybe_reuse_in_memory_tree()?
        deps = set(dep for id in scc for dep in graph[id].dependencies if dep in graph)
        deps -= set(scc)  # Subtract the SCC itself (else nothing will be safe)
        if all(graph[dep].is_from_saved_cache for dep in deps):
            return True
    return False


def process_stale_scc(graph: Graph, scc: List[str], manager: BuildManager) -> None:
    """Process the modules in one SCC from source code.

    Exception: If quick_and_dirty is set, use the cache for fresh modules.
    """
    if manager.options.quick_and_dirty:
        fresh = [id for id in scc if graph[id].is_fresh()]
        fresh_set = set(fresh)  # To avoid running into O(N**2)
        stale = [id for id in scc if id not in fresh_set]
        if fresh:
            manager.log("  Fresh ids: %s" % (", ".join(fresh)))
        if stale:
            manager.log("  Stale ids: %s" % (", ".join(stale)))
    else:
        fresh = []
        stale = scc
    for id in fresh:
        graph[id].load_tree()
    for id in stale:
        # We may already have parsed the module, or not.
        # If the former, parse_file() is a no-op.
        graph[id].parse_file()
        graph[id].fix_suppressed_dependencies(graph)
    for id in fresh:
        graph[id].fix_cross_refs()
    for id in stale:
        graph[id].semantic_analysis()
    for id in stale:
        graph[id].semantic_analysis_pass_three()
    for id in fresh:
        graph[id].calculate_mros()
    for id in stale:
        graph[id].semantic_analysis_apply_patches()
    for id in stale:
        graph[id].type_check_first_pass()
    more = True
    while more:
        more = False
        for id in stale:
            if graph[id].type_check_second_pass():
                more = True
    if any(manager.errors.is_errors_for_file(graph[id].xpath) for id in stale):
        for id in stale:
            graph[id].transitive_error = True
    for id in stale:
        graph[id].finish_passes()
        if manager.options.cache_fine_grained or manager.options.fine_grained_incremental:
            graph[id].compute_fine_grained_deps()
        graph[id].generate_unused_ignore_notes()
        manager.flush_errors(manager.errors.file_messages(graph[id].xpath), False)
        graph[id].write_cache()
        graph[id].mark_as_rechecked()


def sorted_components(graph: Graph,
                      vertices: Optional[AbstractSet[str]] = None,
                      pri_max: int = PRI_ALL) -> List[AbstractSet[str]]:
    """Return the graph's SCCs, topologically sorted by dependencies.

    The sort order is from leaves (nodes without dependencies) to
    roots (nodes on which no other nodes depend).

    This works for a subset of the full dependency graph too;
    dependencies that aren't present in graph.keys() are ignored.
    """
    # Compute SCCs.
    if vertices is None:
        vertices = set(graph)
    edges = {id: deps_filtered(graph, vertices, id, pri_max) for id in vertices}
    sccs = list(strongly_connected_components(vertices, edges))
    # Topsort.
    sccsmap = {id: frozenset(scc) for scc in sccs for id in scc}
    data = {}  # type: Dict[AbstractSet[str], Set[AbstractSet[str]]]
    for scc in sccs:
        deps = set()  # type: Set[AbstractSet[str]]
        for id in scc:
            deps.update(sccsmap[x] for x in deps_filtered(graph, vertices, id, pri_max))
        data[frozenset(scc)] = deps
    res = []
    for ready in topsort(data):
        # Sort the sets in ready by reversed smallest State.order.  Examples:
        #
        # - If ready is [{x}, {y}], x.order == 1, y.order == 2, we get
        #   [{y}, {x}].
        #
        # - If ready is [{a, b}, {c, d}], a.order == 1, b.order == 3,
        #   c.order == 2, d.order == 4, the sort keys become [1, 2]
        #   and the result is [{c, d}, {a, b}].
        res.extend(sorted(ready,
                          key=lambda scc: -min(graph[id].order for id in scc)))
    return res


def deps_filtered(graph: Graph, vertices: AbstractSet[str], id: str, pri_max: int) -> List[str]:
    """Filter dependencies for id with pri < pri_max."""
    if id not in vertices:
        return []
    state = graph[id]
    return [dep
            for dep in state.dependencies
            if dep in vertices and state.priorities.get(dep, PRI_HIGH) < pri_max]


def strongly_connected_components(vertices: AbstractSet[str],
                                  edges: Dict[str, List[str]]) -> Iterator[Set[str]]:
    """Compute Strongly Connected Components of a directed graph.

    Args:
      vertices: the labels for the vertices
      edges: for each vertex, gives the target vertices of its outgoing edges

    Returns:
      An iterator yielding strongly connected components, each
      represented as a set of vertices.  Each input vertex will occur
      exactly once; vertices not part of a SCC are returned as
      singleton sets.

    From http://code.activestate.com/recipes/578507/.
    """
    identified = set()  # type: Set[str]
    stack = []  # type: List[str]
    index = {}  # type: Dict[str, int]
    boundaries = []  # type: List[int]

    def dfs(v: str) -> Iterator[Set[str]]:
        index[v] = len(stack)
        stack.append(v)
        boundaries.append(index[v])

        for w in edges[v]:
            if w not in index:
                # For Python >= 3.3, replace with "yield from dfs(w)"
                for scc in dfs(w):
                    yield scc
            elif w not in identified:
                while index[w] < boundaries[-1]:
                    boundaries.pop()

        if boundaries[-1] == index[v]:
            boundaries.pop()
            scc = set(stack[index[v]:])
            del stack[index[v]:]
            identified.update(scc)
            yield scc

    for v in vertices:
        if v not in index:
            # For Python >= 3.3, replace with "yield from dfs(v)"
            for scc in dfs(v):
                yield scc


def topsort(data: Dict[AbstractSet[str],
                       Set[AbstractSet[str]]]) -> Iterable[Set[AbstractSet[str]]]:
    """Topological sort.

    Args:
      data: A map from SCCs (represented as frozen sets of strings) to
            sets of SCCs, its dependencies.  NOTE: This data structure
            is modified in place -- for normalization purposes,
            self-dependencies are removed and entries representing
            orphans are added.

    Returns:
      An iterator yielding sets of SCCs that have an equivalent
      ordering.  NOTE: The algorithm doesn't care about the internal
      structure of SCCs.

    Example:
      Suppose the input has the following structure:

        {A: {B, C}, B: {D}, C: {D}}

      This is normalized to:

        {A: {B, C}, B: {D}, C: {D}, D: {}}

      The algorithm will yield the following values:

        {D}
        {B, C}
        {A}

    From http://code.activestate.com/recipes/577413/.
    """
    # TODO: Use a faster algorithm?
    for k, v in data.items():
        v.discard(k)  # Ignore self dependencies.
    for item in set.union(*data.values()) - set(data.keys()):
        data[item] = set()
    while True:
        ready = {item for item, dep in data.items() if not dep}
        if not ready:
            break
        yield ready
        data = {item: (dep - ready)
                for item, dep in data.items()
                if item not in ready}
    assert not data, "A cyclic dependency exists amongst %r" % data
