bisect_kit/math_util.py

# Copyright 2017 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""Math utilities."""

from __future__ import print_function

import copy
import math
import sys


def _log(x):
  """Wrapper of log(x) handling zero and negative value."""
  # Due to arithmetic error, x may become tiny negative.
  if abs(x) < 1e-15:
    return 0.0
  return math.log(x)


def _xlogx(x):
  return x * _log(x)


def least(values):
  """Finds minimum non-zero value.

  Args:
    values: Non-negative values.

  Returns:
    The minimum non-zero value.

  Raises:
    ValueError: if all values are zero or negative.
  """
  return min(x for x in values if x > 0)


class EntropyStats(object):
  r"""A data structure to maintain cross entropy of a set of values.

  This is a math helper for NoisyBinarySearch. Given a set of probability,
  this class can incrementally calculate their cross entropy.

  Algorithm:
    Given unnormalized p_i, their cross entropy could be expressed as
      Let S = \sum_i p_i
      Normalized(p): p_i' = p_i/S
      CrossEntropy(p) = \sum_i { -(p_i/S) * log (p_i/S) }
                      = \sum_i { -(p_i/S) (log p_i - log S) }
                      = \sum_i { -p_i/S * log p_i } +  \sum_i { p_i/S * log S }
                      = -1/S * \sum_i { p_i*log p_i } + log S
                      = log S - 1/S * \sum_i { p_i*log p_i }

    So we can maintain sum=|S| and sum_log=|\sum_i { p_i*log p_i }| for
    incremental update.
  """

  def __init__(self, p):
    """Initializes EntropyStats.

    Args:
        p: A list of probability value. Could be unnormalized.
    """
    self.sum = sum(p)
    self.sum_log = sum(map(_xlogx, p))

  def entropy(self):
    """Returns cross entropy."""
    if self.sum == 0:
      return 0
    return _log(self.sum) - self.sum_log / self.sum

  def replace(self, old, new):
    """Replaces one random variable in the collection with another.

    Args:
      old: original value to be replaced.
      new: new value
    """
    self.sum += new - old
    self.sum_log += _xlogx(new) - _xlogx(old)

  def multiply(self, value):
    """Multiplies all values in the collection by |value|.

    Returns:
      A new instance of EntropyStats with calculated value.
    """
    other = copy.copy(self)
    # \sum_i { (p_i*value) * log (p_i*value) }
    # = \sum_i { (p_i*value) * (log p_i + log value) }
    # = value * \sum_i { p_i log p_i } + \sum_i { p_i * value * log value }
    # = value * \sum_i { p_i log p_i } + \sum_i { p_i } * value * log value
    other.sum_log = value * self.sum_log + self.sum * _xlogx(value)

    other.sum *= value
    return other


class ExtendedFloat(object):
  """Custom floating point number with larger range of exponent.

  Problem:
    In the formula of noisy binary search algorithm, we need to calculate p**n
    (where n is test count) and normalize probabilities. When n is hundreds or
    more, p**n goes too small and becomes zero (underflow), the normalization
    step will fail.

    Since n > 100 is not unreasonable large for noisy bisection, we need a
    numeric type less likely or never underflow.

  Solution:
    Supports a floating number is represented as
        x = mantissa * 2.**exponent
    We use python's float to represent `mantissa` and python's int to represent
    `exponent`.

    For mantissa, we can accept calculated probability is not super accurate
    because the worst case is just running test one or two more times. So we
    keep mantissa in python's built-in float.

    For exponent, we represent it using python's int, which is virtually
    unlimited digits.

  Alternative solutions:
    1. Maybe we can rewrite the formula to avoid underflow. But I want to keep
       the arithmetic code simple.

    2. Use python stdlib's decimal or fractions. But they are very slow --
       decimal is 6-50x slower, fractions is 9-760x slower while ExtendedFloat
       is about 1.2-3.3x slower [1].

       [1] test setup: easy N=1000, oracle=(0.01, 0.2)
                       hard N=1000, oracle=(0.45, 0.55)

  Attributes:
    mantissa: The mantissa part of floating number. Its range is 0.5 <=
        abs(mantissa) < 1.0.
    exponent: The exponent part of floating number.
  """

  def __init__(self, value, exp=0):
    self.mantissa, self.exponent = math.frexp(value)
    self.exponent = self.exponent + exp if value else 0

  def __float__(self):
    return math.ldexp(self.mantissa, self.exponent)

  def __neg__(self):
    return ExtendedFloat(-self.mantissa, self.exponent)

  def __abs__(self):
    return ExtendedFloat(abs(self.mantissa), self.exponent)

  def __add__(self, other):
    if not isinstance(other, ExtendedFloat):
      other = ExtendedFloat(other)
    if self.mantissa == 0:
      return other
    if self.exponent < other.exponent:
      return other.__add__(self)
    value = math.ldexp(other.mantissa, other.exponent - self.exponent)
    return ExtendedFloat(value + self.mantissa, self.exponent)

  __radd__ = __add__

  def __pow__(self, p):
    # Because 0.5 <= abs(mantissa) < 1.0, using float is enough without
    # underflow for small p.
    if p < -sys.float_info.min_exp:
      return ExtendedFloat(self.mantissa**p, self.exponent * p)

    half_pow = self.__pow__(p >> 1)
    result = half_pow.__mul__(half_pow)
    if p & 1:
      result = result.__mul__(self)
    return result

  def __sub__(self, other):
    return self.__add__(-other)

  def __rsub__(self, other):
    return ExtendedFloat(other).__add__(-self)

  def __mul__(self, other):
    if not isinstance(other, ExtendedFloat):
      other = ExtendedFloat(other)
    return ExtendedFloat(self.mantissa * other.mantissa,
                         self.exponent + other.exponent)

  __rmul__ = __mul__

  def __div__(self, other):
    if not isinstance(other, ExtendedFloat):
      other = ExtendedFloat(other)
    return ExtendedFloat(self.mantissa / other.mantissa,
                         self.exponent - other.exponent)

  def __rdiv__(self, other):
    return ExtendedFloat(other).__div__(self)

  def __repr__(self):
    return "ExtendedFloat(%f, %d)" % (self.mantissa, self.exponent)

  def __cmp__(self, other):
    if not isinstance(other, ExtendedFloat):
      other = ExtendedFloat(other)
    diff = self - other
    return cmp(diff.mantissa, 0)