-# Copyright (c) 2019 Cisco and/or its affiliates.
+# Copyright (c) 2021 Cisco and/or its affiliates.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at:
TODO: Figure out a more performant way of handling -inf.
-The functions handle the common task of adding or substracting
+The functions handle the common task of adding or subtracting
two numbers where both operands and the result is given in logarithm form.
There are conditionals to make sure overflow does not happen (if possible)
during the computation."""
def log_plus(first, second):
- """Return logarithm of the sum of two exponentials.
+ """Return logarithm of the sum of two exponents.
Basically math.log(math.exp(first) + math.exp(second))
which avoids overflow and uses None as math.log(0.0).
:returns: Logarithm of the sum (or None if zero).
:rtype: float
"""
-
if first is None:
return second
if second is None:
return first
if second > first:
- return second + math.log(1.0 + math.exp(first - second))
+ retval = second + math.log(1.0 + math.exp(first - second))
else:
- return first + math.log(1.0 + math.exp(second - first))
+ retval = first + math.log(1.0 + math.exp(second - first))
+ return retval
def log_minus(first, second):
- """Return logarithm of the difference of two exponentials.
+ """Return logarithm of the difference of two exponents.
Basically math.log(math.exp(first) - math.exp(second))
which avoids overflow and uses None as math.log(0.0).
:rtype: float
:raises RuntimeError: If the difference would be non-positive.
"""
-
if first is None:
- raise RuntimeError("log_minus: does not suport None first")
+ raise RuntimeError(u"log_minus: does not support None first")
if second is None:
return first
if second >= first:
- raise RuntimeError("log_minus: first has to be bigger than second")
+ raise RuntimeError(u"log_minus: first has to be bigger than second")
factor = -math.expm1(second - first)
if factor <= 0.0:
- raise RuntimeError("log_minus: non-positive number to log")
+ msg = u"log_minus: non-positive number to log"
else:
return first + math.log(factor)
+ raise RuntimeError(msg)
def safe_exp(log_value):