//===- Sequence.h - Utility for producing sequences of values ---*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
/// \file
/// Provides some synthesis utilities to produce sequences of values. The names
/// are intentionally kept very short as they tend to occur in common and
/// widely used contexts.
///
/// The `seq(A, B)` function produces a sequence of values from `A` to up to
/// (but not including) `B`, i.e., [`A`, `B`), that can be safely iterated over.
/// `seq` supports both integral (e.g., `int`, `char`, `uint32_t`) and enum
/// types. `seq_inclusive(A, B)` produces a sequence of values from `A` to `B`,
/// including `B`.
///
/// Examples with integral types:
/// ```
/// for (int x : seq(0, 3))
///   outs() << x << " ";
/// ```
///
/// Prints: `0 1 2 `.
///
/// ```
/// for (int x : seq_inclusive(0, 3))
///   outs() << x << " ";
/// ```
///
/// Prints: `0 1 2 3 `.
///
/// Similar to `seq` and `seq_inclusive`, the `enum_seq` and
/// `enum_seq_inclusive` functions produce sequences of enum values that can be
/// iterated over.
/// To enable iteration with enum types, you need to either mark enums as safe
/// to iterate on by specializing `enum_iteration_traits`, or opt into
/// potentially unsafe iteration at every callsite by passing
/// `force_iteration_on_noniterable_enum`.
///
/// Examples with enum types:
/// ```
/// namespace X {
///   enum class MyEnum : unsigned {A = 0, B, C};
/// } // namespace X
///
/// template <> struct enum_iteration_traits<X::MyEnum> {
///   static contexpr bool is_iterable = true;
/// };
///
/// class MyClass {
/// public:
///   enum Safe { D = 3, E, F };
///   enum MaybeUnsafe { G = 1, H = 2, I = 4 };
/// };
///
/// template <> struct enum_iteration_traits<MyClass::Safe> {
///   static contexpr bool is_iterable = true;
/// };
/// ```
///
/// ```
///   for (auto v : enum_seq(MyClass::Safe::D, MyClass::Safe::F))
///     outs() << int(v) << " ";
/// ```
///
/// Prints: `3 4 `.
///
/// ```
///   for (auto v : enum_seq(MyClass::MaybeUnsafe::H, MyClass::MaybeUnsafe::I,
///                          force_iteration_on_noniterable_enum))
///     outs() << int(v) << " ";
/// ```
///
/// Prints: `2 3 `.
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_SEQUENCE_H
#define LLVM_ADT_SEQUENCE_H

#include <cassert>     // assert
#include <iterator>    // std::random_access_iterator_tag
#include <limits>      // std::numeric_limits
#include <type_traits> // std::is_integral, std::is_enum, std::underlying_type,
                       // std::enable_if

#include "llvm/Support/MathExtras.h" // AddOverflow / SubOverflow

namespace llvm {

// Enum traits that marks enums as safe or unsafe to iterate over.
// By default, enum types are *not* considered safe for iteration.
// To allow iteration for your enum type, provide a specialization with
// `is_iterable` set to `true` in the `llvm` namespace.
// Alternatively, you can pass the `force_iteration_on_noniterable_enum` tag
// to `enum_seq` or `enum_seq_inclusive`.
template <typename EnumT> struct enum_iteration_traits {
  static constexpr bool is_iterable = false;
};

struct force_iteration_on_noniterable_enum_t {
  explicit force_iteration_on_noniterable_enum_t() = default;
};

inline constexpr force_iteration_on_noniterable_enum_t
    force_iteration_on_noniterable_enum;

namespace detail {

// Returns whether a value of type U can be represented with type T.
template <typename T, typename U> bool canTypeFitValue(const U Value) {
  const intmax_t BotT = intmax_t(std::numeric_limits<T>::min());
  const intmax_t BotU = intmax_t(std::numeric_limits<U>::min());
  const uintmax_t TopT = uintmax_t(std::numeric_limits<T>::max());
  const uintmax_t TopU = uintmax_t(std::numeric_limits<U>::max());
  return !((BotT > BotU && Value < static_cast<U>(BotT)) ||
           (TopT < TopU && Value > static_cast<U>(TopT)));
}

// An integer type that asserts when:
// - constructed from a value that doesn't fit into intmax_t,
// - casted to a type that cannot hold the current value,
// - its internal representation overflows.
struct CheckedInt {
  // Integral constructor, asserts if Value cannot be represented as intmax_t.
  template <typename Integral,
            std::enable_if_t<std::is_integral<Integral>::value, bool> = 0>
  static CheckedInt from(Integral FromValue) {
    if (!canTypeFitValue<intmax_t>(FromValue))
      assertOutOfBounds();
    CheckedInt Result;
    Result.Value = static_cast<intmax_t>(FromValue);
    return Result;
  }

  // Enum constructor, asserts if Value cannot be represented as intmax_t.
  template <typename Enum,
            std::enable_if_t<std::is_enum<Enum>::value, bool> = 0>
  static CheckedInt from(Enum FromValue) {
    using type = std::underlying_type_t<Enum>;
    return from<type>(static_cast<type>(FromValue));
  }

  // Equality
  bool operator==(const CheckedInt &O) const { return Value == O.Value; }
  bool operator!=(const CheckedInt &O) const { return Value != O.Value; }

  CheckedInt operator+(intmax_t Offset) const {
    CheckedInt Result;
    if (AddOverflow(Value, Offset, Result.Value))
      assertOutOfBounds();
    return Result;
  }

  intmax_t operator-(CheckedInt Other) const {
    intmax_t Result;
    if (SubOverflow(Value, Other.Value, Result))
      assertOutOfBounds();
    return Result;
  }

  // Convert to integral, asserts if Value cannot be represented as Integral.
  template <typename Integral,
            std::enable_if_t<std::is_integral<Integral>::value, bool> = 0>
  Integral to() const {
    if (!canTypeFitValue<Integral>(Value))
      assertOutOfBounds();
    return static_cast<Integral>(Value);
  }

  // Convert to enum, asserts if Value cannot be represented as Enum's
  // underlying type.
  template <typename Enum,
            std::enable_if_t<std::is_enum<Enum>::value, bool> = 0>
  Enum to() const {
    using type = std::underlying_type_t<Enum>;
    return Enum(to<type>());
  }

private:
  static void assertOutOfBounds() { assert(false && "Out of bounds"); }

  intmax_t Value;
};

template <typename T, bool IsReverse> struct SafeIntIterator {
  using iterator_category = std::random_access_iterator_tag;
  using value_type = T;
  using difference_type = intmax_t;
  using pointer = T *;
  using reference = value_type; // The iterator does not reference memory.

  // Construct from T.
  explicit SafeIntIterator(T Value) : SI(CheckedInt::from<T>(Value)) {}
  // Construct from other direction.
  SafeIntIterator(const SafeIntIterator<T, !IsReverse> &O) : SI(O.SI) {}

  // Dereference
  reference operator*() const { return SI.to<T>(); }
  // Indexing
  reference operator[](intmax_t Offset) const { return *(*this + Offset); }

  // Can be compared for equivalence using the equality/inequality operators.
  bool operator==(const SafeIntIterator &O) const { return SI == O.SI; }
  bool operator!=(const SafeIntIterator &O) const { return SI != O.SI; }
  // Comparison
  bool operator<(const SafeIntIterator &O) const { return (*this - O) < 0; }
  bool operator>(const SafeIntIterator &O) const { return (*this - O) > 0; }
  bool operator<=(const SafeIntIterator &O) const { return (*this - O) <= 0; }
  bool operator>=(const SafeIntIterator &O) const { return (*this - O) >= 0; }

  // Pre Increment/Decrement
  void operator++() { offset(1); }
  void operator--() { offset(-1); }

  // Post Increment/Decrement
  SafeIntIterator operator++(int) {
    const auto Copy = *this;
    ++*this;
    return Copy;
  }
  SafeIntIterator operator--(int) {
    const auto Copy = *this;
    --*this;
    return Copy;
  }

  // Compound assignment operators
  void operator+=(intmax_t Offset) { offset(Offset); }
  void operator-=(intmax_t Offset) { offset(-Offset); }

  // Arithmetic
  SafeIntIterator operator+(intmax_t Offset) const { return add(Offset); }
  SafeIntIterator operator-(intmax_t Offset) const { return add(-Offset); }

  // Difference
  intmax_t operator-(const SafeIntIterator &O) const {
    return IsReverse ? O.SI - SI : SI - O.SI;
  }

private:
  SafeIntIterator(const CheckedInt &SI) : SI(SI) {}

  static intmax_t getOffset(intmax_t Offset) {
    return IsReverse ? -Offset : Offset;
  }

  CheckedInt add(intmax_t Offset) const { return SI + getOffset(Offset); }

  void offset(intmax_t Offset) { SI = SI + getOffset(Offset); }

  CheckedInt SI;

  // To allow construction from the other direction.
  template <typename, bool> friend struct SafeIntIterator;
};

} // namespace detail

template <typename T> struct iota_range {
  using value_type = T;
  using reference = T &;
  using const_reference = const T &;
  using iterator = detail::SafeIntIterator<value_type, false>;
  using const_iterator = iterator;
  using reverse_iterator = detail::SafeIntIterator<value_type, true>;
  using const_reverse_iterator = reverse_iterator;
  using difference_type = intmax_t;
  using size_type = std::size_t;

  explicit iota_range(T Begin, T End, bool Inclusive)
      : BeginValue(Begin), PastEndValue(End) {
    assert(Begin <= End && "Begin must be less or equal to End.");
    if (Inclusive)
      ++PastEndValue;
  }

  size_t size() const { return PastEndValue - BeginValue; }
  bool empty() const { return BeginValue == PastEndValue; }

  auto begin() const { return const_iterator(BeginValue); }
  auto end() const { return const_iterator(PastEndValue); }

  auto rbegin() const { return const_reverse_iterator(PastEndValue - 1); }
  auto rend() const { return const_reverse_iterator(BeginValue - 1); }

private:
  static_assert(std::is_integral<T>::value || std::is_enum<T>::value,
                "T must be an integral or enum type");
  static_assert(std::is_same<T, std::remove_cv_t<T>>::value,
                "T must not be const nor volatile");

  iterator BeginValue;
  iterator PastEndValue;
};

/// Iterate over an integral type from Begin up to - but not including - End.
/// Note: Begin and End values have to be within [INTMAX_MIN, INTMAX_MAX] for
/// forward iteration (resp. [INTMAX_MIN + 1, INTMAX_MAX] for reverse
/// iteration).
template <typename T, typename = std::enable_if_t<std::is_integral<T>::value &&
                                                  !std::is_enum<T>::value>>
auto seq(T Begin, T End) {
  return iota_range<T>(Begin, End, false);
}

/// Iterate over an integral type from Begin to End inclusive.
/// Note: Begin and End values have to be within [INTMAX_MIN, INTMAX_MAX - 1]
/// for forward iteration (resp. [INTMAX_MIN + 1, INTMAX_MAX - 1] for reverse
/// iteration).
template <typename T, typename = std::enable_if_t<std::is_integral<T>::value &&
                                                  !std::is_enum<T>::value>>
auto seq_inclusive(T Begin, T End) {
  return iota_range<T>(Begin, End, true);
}

/// Iterate over an enum type from Begin up to - but not including - End.
/// Note: `enum_seq` will generate each consecutive value, even if no
/// enumerator with that value exists.
/// Note: Begin and End values have to be within [INTMAX_MIN, INTMAX_MAX] for
/// forward iteration (resp. [INTMAX_MIN + 1, INTMAX_MAX] for reverse
/// iteration).
template <typename EnumT,
          typename = std::enable_if_t<std::is_enum<EnumT>::value>>
auto enum_seq(EnumT Begin, EnumT End) {
  static_assert(enum_iteration_traits<EnumT>::is_iterable,
                "Enum type is not marked as iterable.");
  return iota_range<EnumT>(Begin, End, false);
}

/// Iterate over an enum type from Begin up to - but not including - End, even
/// when `EnumT` is not marked as safely iterable by `enum_iteration_traits`.
/// Note: `enum_seq` will generate each consecutive value, even if no
/// enumerator with that value exists.
/// Note: Begin and End values have to be within [INTMAX_MIN, INTMAX_MAX] for
/// forward iteration (resp. [INTMAX_MIN + 1, INTMAX_MAX] for reverse
/// iteration).
template <typename EnumT,
          typename = std::enable_if_t<std::is_enum<EnumT>::value>>
auto enum_seq(EnumT Begin, EnumT End, force_iteration_on_noniterable_enum_t) {
  return iota_range<EnumT>(Begin, End, false);
}

/// Iterate over an enum type from Begin to End inclusive.
/// Note: `enum_seq_inclusive` will generate each consecutive value, even if no
/// enumerator with that value exists.
/// Note: Begin and End values have to be within [INTMAX_MIN, INTMAX_MAX - 1]
/// for forward iteration (resp. [INTMAX_MIN + 1, INTMAX_MAX - 1] for reverse
/// iteration).
template <typename EnumT,
          typename = std::enable_if_t<std::is_enum<EnumT>::value>>
auto enum_seq_inclusive(EnumT Begin, EnumT End) {
  static_assert(enum_iteration_traits<EnumT>::is_iterable,
                "Enum type is not marked as iterable.");
  return iota_range<EnumT>(Begin, End, true);
}

/// Iterate over an enum type from Begin to End inclusive, even when `EnumT`
/// is not marked as safely iterable by `enum_iteration_traits`.
/// Note: `enum_seq_inclusive` will generate each consecutive value, even if no
/// enumerator with that value exists.
/// Note: Begin and End values have to be within [INTMAX_MIN, INTMAX_MAX - 1]
/// for forward iteration (resp. [INTMAX_MIN + 1, INTMAX_MAX - 1] for reverse
/// iteration).
template <typename EnumT,
          typename = std::enable_if_t<std::is_enum<EnumT>::value>>
auto enum_seq_inclusive(EnumT Begin, EnumT End,
                        force_iteration_on_noniterable_enum_t) {
  return iota_range<EnumT>(Begin, End, true);
}

} // end namespace llvm

#endif // LLVM_ADT_SEQUENCE_H
