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import java.util.HashMap;
import java.util.Map;
public class SpellCorrector {
final private CorpusReader cr;
final private ConfusionMatrixReader cmr;
final static char[] ALPHABET = "abcdefghijklmnopqrstuvwxyz'".toCharArray();
public SpellCorrector(CorpusReader cr, ConfusionMatrixReader cmr) {
this.cr = cr;
this.cmr = cmr;
}
public String correctPhrase(String phrase) {
if (phrase == null || phrase.length() == 0) {
throw new IllegalArgumentException("phrase must be non-empty.");
}
String[] words = phrase.split(" ");
String finalSuggestion = "";
/**
* CODE TO BE ADDED *
*/
return finalSuggestion.trim();
}
/**
* Gets all candidate words, resulting from a single insertion, deletion.
* substitution or transposition.
*
* @param word The (wrong) word to find (corrected) candidates for.
* @return Candidate words mapping to the noisy channel model probability.
*/
public Map<String, Double> getCandidateWords(String word) {
Map<String, Double> candidates = new HashMap<>();
// tries to add word2 to the list of words. This word2 was generated
// from "word" by changing "error" to "correct".
TriFunction<String, String, String> push = (word2, error, correct) -> {
if (!cr.inVocabulary(word2)) {
return;
}
// Find the channel model probability (probability of the edit).
// P(x|w) = "corrections count given error" / "errors count"
double correctionCount, errorCount, p_channel;
correctionCount = (double) cmr.getConfusionCount(error, correct);
errorCount = cmr.getErrorsCount(error);
// is this a known correction?
if (errorCount == 0.0) {
// no,
p_channel = 0.0;
} else {
// yes,
p_channel = correctionCount / errorCount;
}
// Sum the probabilities as independent modifications can result in
// the same word ("acess" -> "access" by "a|ac", "e|ce").
double p = candidates.getOrDefault(word2, 0.0);
p += p_channel;
candidates.put(word2, p);
};
makeWordInsertion(word, push);
makeWordSubstitution(word, push);
makeWordDeletion(word, push);
makeWordTransposition(word, push);
return candidates;
}
private void makeWordInsertion(String word,
TriFunction<String, String, String> push) {
// Generate words by insertion of a character
// |p]en -> [ap]en, [p|en -> [pi]en, p[e|n -> p[ei]n, pe[n| -> pe[nm].
for (int i = 0; i <= word.length(); i++) {
String head, head_last, tail;
// the word is split into [0..i] [i..n]
head = word.substring(0, i);
tail = word.substring(i);
for (char c : ALPHABET) {
if (i == 0) {
// insert "c" before current letter (i = 0)
head_last = word.substring(0);
push.call(head + c + tail, head_last, c + head_last);
} else {
// insert "c" after current letter (i > 0)
head_last = word.substring(i - 1, i);
push.call(head + c + tail, head_last, head_last + c);
}
}
}
}
private void makeWordSubstitution(String word,
TriFunction<String, String, String> push) {
// Generate words by substitution of a character.
// |p]en -> [P]en, p|e]n -> p[E]n, pe|n] -> pe[N].
for (int i = 0; i < word.length(); i++) {
String head, middle, tail;
// the word is split into [0..i] [i..i+1] [i+1..n]
head = word.substring(0, i);
middle = word.substring(i, i + 1);
tail = word.substring(i + 1);
for (char c : ALPHABET) {
// substitution of "middle" with "c"
push.call(head + c + tail, middle, "" + c);
}
}
}
private void makeWordDeletion(String word,
TriFunction<String, String, String> push) {
// Generare words by deletion of a character. Requires at least two
// characters in the word (an empty string is invalid).
// |pe]in -> [e]in, p|ei]n -> p[i]n, pe|in] -> pe[n], pe[i|n] -> pe[i].
if (word.length() > 1) {
for (int i = 0; i < word.length(); i++) {
String head, middle, error, tail;
// the word is split into [0..i] [i..i+1] [i+1..n]
head = word.substring(0, i);
middle = word.substring(i, i + 1);
tail = word.substring(i + 1);
if (i + 1 < word.length()) {
// Common case: deletion of the following character.
// p|ei]n -> p[i]n
error = word.substring(i, i + 2);
} else {
// Last case: operate on the previous and next characters.
// pe[i|n] -> pe[i]
error = word.substring(i - 1, i + 1);
}
push.call(head + tail, error, middle);
}
}
}
private void makeWordTransposition(String word,
TriFunction<String, String, String> push) {
// The Damerau-Levenshtein distance also includes transposition of two
// adjacent letters to account for spelling mistakes.
// [p|e]n -> [ep]n, p[e|n] -> p[ne].
for (int i = 1; i < word.length(); i++) {
String head, middle, transposed, tail;
// the word is split into [0..i-1] [i-1..i+1] [i+1..n]
head = word.substring(0, i - 1);
middle = word.substring(i - 1, i + 1);
tail = word.substring(i + 1);
transposed = "" + middle.charAt(1) + middle.charAt(0);
push.call(head + transposed + tail, middle, transposed);
}
}
}
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