From 6e38d50ba49d7f9be72fcd0294890ba483133942 Mon Sep 17 00:00:00 2001
From: Peter Wu <peter@lekensteyn.nl>
Date: Fri, 3 Apr 2015 20:02:16 +0200
Subject: Consider changes to ngram probabilties in the context

Finally "hme locations" gets corrected to "home locations"!
---
 spellchecker/src/SpellCorrector.java | 38 ++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/spellchecker/src/SpellCorrector.java b/spellchecker/src/SpellCorrector.java
index 53255fd..eee6cf1 100644
--- a/spellchecker/src/SpellCorrector.java
+++ b/spellchecker/src/SpellCorrector.java
@@ -246,7 +246,7 @@ public class SpellCorrector {
             this.word_likelihoods = new double[words.length];
             this.words_readonly = new boolean[words.length];
             for (int i = 0; i < words.length; i++) {
-                word_likelihoods[i] = getWordLikelihood(i, words[i],
+                word_likelihoods[i] = getWordLikelihood(i,
                         LM_PROBABILITY_UNMODIFIED);
             }
             sentence_probability = combineProbabilities(word_likelihoods);
@@ -268,9 +268,9 @@ public class SpellCorrector {
          * Calculates the probability that the word {@code word} is valid at
          * position {@code index}.
          */
-        public double getWordLikelihood(int index, String word,
-                double channel_probability) {
+        public double getWordLikelihood(int index, double channel_probability) {
             double prior, score, p, igram_p;
+            String word = words[index];
             // a suggested word not in the vocabulary is certainly wrong,
             // changed (or consequentive) words should also not be changed.
             if (!cr.inVocabulary(word) || words_readonly[index]) {
@@ -345,11 +345,38 @@ public class SpellCorrector {
          */
         public void tryWord(int index, String word, double channel_probability) {
             double score, p;
+            String old_word = words[index];
             double[] scores;
+            int index_left, index_right;
+
+            // Simulate the change of changing this word.
+            words[index] = word;
 
+            // As changing the word itself can affect the ngram rating of
+            // the context, recalculate the probabilities for those too.
+            index_left = Math.max(0, index - NGRAM_N + 1);
+            index_right = Math.min(words.length, index + NGRAM_N - 1);
             scores = word_likelihoods.clone();
-            score = getWordLikelihood(index, word, channel_probability);
-            scores[index] = score;
+
+            // calculate probabilities for each word that is possible affected
+            // by this change.
+            for (int i = 0; i < words.length; i++) {
+                if (i < index_left || i > index_right) {
+                    // the probability is unchanged, ignore.
+                    continue;
+                }
+                if (i == index) {
+                    // the word that is being modified
+                    score = getWordLikelihood(i, channel_probability);
+                } else {
+                    // the word around the modified word
+                    score = getWordLikelihood(i, LM_PROBABILITY_UNMODIFIED);
+                }
+                scores[i] = score;
+            }
+
+            // restore word
+            words[index] = old_word;
 
             // group the effects of this modifications for tracking.
             WordModification effect = new WordModification(index, word, scores);
@@ -357,7 +384,6 @@ public class SpellCorrector {
             if ((best_modification != null
                     && effect.probability > best_modification.probability)
                     || effect.probability > sentence_probability) {
-                System.err.println("found better word!" + word + " p=" + effect.probability);
                 best_modification = effect;
             }
         }
-- 
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