"""Implementation of HyperLogLog

This implements the HyperLogLog algorithm for cardinality estimation, found
in

    Philippe Flajolet, Éric Fusy, Olivier Gandouet and Frédéric Meunier.
        "HyperLogLog: the analysis of a near-optimal cardinality estimation
        algorithm". 2007 Conference on Analysis of Algorithms. Nice, France
        (2007)

"""
import numpy as np
import pandas as pd
from pandas.util import hash_pandas_object


def compute_first_bit(a):
    "Compute the position of the first nonzero bit for each int in an array."
    # TODO: consider making this less memory-hungry
    bits = np.bitwise_and.outer(a, 1 << np.arange(32))
    bits = bits.cumsum(axis=1).astype(bool)
    return 33 - bits.sum(axis=1)


def compute_hll_array(obj, b):
    # b is the number of bits

    if not 8 <= b <= 16:
        raise ValueError("b should be between 8 and 16")
    num_bits_discarded = 32 - b
    m = 1 << b

    # Get an array of the hashes
    hashes = hash_pandas_object(obj, index=False)
    if isinstance(hashes, pd.Series):
        hashes = hashes._values
    hashes = hashes.astype(np.uint32)

    # Of the first b bits, which is the first nonzero?
    j = hashes >> num_bits_discarded
    first_bit = compute_first_bit(hashes)

    # Pandas can do the max aggregation
    df = pd.DataFrame({"j": j, "first_bit": first_bit})
    series = df.groupby("j").max()["first_bit"]

    # Return a dense array so we can concat them and get a result
    # that is easy to deal with
    return series.reindex(np.arange(m), fill_value=0).values.astype(np.uint8)


def reduce_state(Ms, b):
    m = 1 << b

    # We concatenated all of the states, now we need to get the max
    # value for each j in both
    Ms = Ms.reshape((len(Ms) // m), m)
    return Ms.max(axis=0)


def estimate_count(Ms, b):
    m = 1 << b

    # Combine one last time
    M = reduce_state(Ms, b)

    # Estimate cardinality, no adjustments
    alpha = 0.7213 / (1 + 1.079 / m)
    E = alpha * m / (2.0 ** -(M.astype("f8"))).sum() * m
    #                        ^^^^ starts as unsigned, need a signed type for
    #                             negation operator to do something useful

    # Apply adjustments for small / big cardinalities, if applicable
    if E < 2.5 * m:
        V = (M == 0).sum()
        if V:
            return m * np.log(m / V)
    if E > 2**32 / 30.0:
        return -(2**32) * np.log1p(-E / 2**32)
    return E
