Adding log probabilities

log-add.py

from math import log, exp

"""
sources: 
http://windowoffice.tumblr.com/post/33548509/logsum-underflow-trick-re-discovery
https://facwiki.cs.byu.edu/nlp/index.php/Log_Domain_Computations
"""
def log_add(x,y):
	logx = log(x)
	logy = log(y)
	
	if (logy > logx):
		temp = logx
		logx = logy
		logy = temp
	
	negdiff = logy - logx
	
	if negdiff < -20:
		return logx
	
	return logx + log(1.0 + exp(negdiff))

for x in range(1,20):
	print log_add(0.5, 2*(10**(-x) ))