#!/usr/local/bin/gawk -f## reverse-lm --#	reverse N-grams in a backoff LM## usage: reverse-lm lm-file > rev-lm-file## $Header: /home/srilm/devel/utils/src/RCS/reverse-lm.gawk,v 1.2 2004/11/02 02:00:35 stolcke Exp $#BEGIN {	start_tag = "<s>";	end_tag = "</s>";	renorm_command = "ngram -debug 1 -order 2 -lm - -renorm -write-lm -";}NF==0 {	print | renorm_command;	next;}/^ngram *[0-9][0-9]*=/ {	order = substr($2,1,index($2,"=")-1);	if (order > 2) {		print "can handle bigram LMs only" >> "/dev/stderr";		exit(2);	}	print | renorm_command;	next;}/^\\[0-9]-grams:/ {	currorder=substr($0,2,1);	print | renorm_command;	next;}/^\\/ {	print | renorm_command;	next;}currorder == 1 {	# unigrams are copied unchanged	# store probs for later use	prob = $1;	word = $2;	if (word == start_tag) {	    ; # get <s> unigram prob from </s>	} else if (word == end_tag) {	    uniprob[start_tag] = uniprob[end_tag] = prob;	} else {	    uniprob[word] = prob;	}	# add dummy backoff weight	$3 = "0";	print | renorm_command;	next;}function map_tags(w) {	if (w == start_tag) {	    return end_tag;	} else if (w == end_tag) {	    return start_tag;	} else {	    return w;	}}currorder == 2 {	# bigrams are reverse and new probabilities are assigned 	prob = $1;	w1 = map_tags($2);	w2 = map_tags($3);	# p_rev(w1|w2) = p(w1) p(w2|w1) / p(w2)	new_prob = uniprob[w1] + prob - uniprob[w2];	if (new_prob > 0) {		print "warning: p(" w1 "|" w2 ") > 0" >> "/dev/stderr";	}	print new_prob "\t" w2 " " w1 | renorm_command;	next;}