#!/usr/bin/env python

#=========================================================================

# This is OPEN SOURCE SOFTWARE governed by the Gnu General Public

# License (GPL) version 3, as described at www.opensource.org.

# Copyright (C)2016 William H. Majoros (martiandna@gmail.com).

#=========================================================================

import math

NEGATIVE_INFINITY=float("-inf")

def sumLogProbs_ordered(largerValue,smallerValue):

if(smallerValue==NEGATIVE_INFINITY): return largerValue

return largerValue+math.log(1+math.exp(smallerValue-largerValue))

def sumLogProbs2(logP,logQ):

if(logP>logQ): return sumLogProbs_ordered(logP,logQ)

return sumLogProbs_ordered(logQ,logP)

def sumLogProbs(x):

n=len(x)

if(n==1): return x[0]

if(n==0): return NEGATIVE_INFINITY

# Pull out the largest value

largestValue=x[0]

for v in x:

if(v>largestValue): largestValue=v

# Handle the case of all zeros separately

if(largestValue==NEGATIVE_INFINITY): return NEGATIVE_INFINITY

# Apply the Kingsbury-Raynor formula

sum=0.0;

for v in x:

if(v==NEGATIVE_INFINITY): continue

sum+=math.exp(v-largestValue)

return largestValue+math.log(sum)

#v=[math.log(0.0001),math.log(0.0002),math.log(0.0003)]

#print(math.exp(sumLogProbs2(math.log(0.1),math.log(0.2))))

#print(math.exp(sumLogProbs(v)))