private Block findNextBlock(
int sourceStartIndex,
int sourceEndIndex,
int targetStartIndex,
int targetEndIndex,
Alignment a,
SentencePair sp) {
if (targetStartIndex >= sp.getTarget().getWords().length
|| sourceStartIndex >= sp.getSource().getWords().length
|| !a.isTargetAligned(targetStartIndex)
|| !a.isSourceAligned(sourceStartIndex)) {
return new Block(sourceStartIndex, sourceEndIndex, targetStartIndex, targetEndIndex);
}
boolean cont = true;
while (cont) {
cont = false;
for (int sourceIndex = sourceStartIndex; sourceIndex <= sourceEndIndex; sourceIndex++) {
if (a.getMaxTargetIndex(sourceIndex) > targetEndIndex) {
targetEndIndex = a.getMaxTargetIndex(sourceIndex);
cont = true;
}
}
for (int targetIndex = targetStartIndex; targetIndex <= targetEndIndex; targetIndex++) {
if (a.getMaxSourceIndex(targetIndex) > sourceEndIndex) {
sourceEndIndex = a.getMaxSourceIndex(targetIndex);
cont = true;
}
}
}
return new Block(sourceStartIndex, sourceEndIndex, targetStartIndex, targetEndIndex);
}
/**
* This method extends an extracted phrase pair with unaligned boundary words on the target side
*
* @param sourceStartIndex
* @param sourceEndIndex
* @param targetStartIndex
* @param targetEndIndex
* @param a
* @param sp
* @return
*/
private List<Rule> extendUnalignedBoundaryWord(
int sourceStartIndex,
int sourceEndIndex,
int targetStartIndex,
int targetEndIndex,
Alignment a,
SentencePair sp) {
List<Rule> res = new ArrayList<Rule>();
int prev = 0; // number of previous target unaligned words
int targetExtendIndex = targetStartIndex - 1;
// i>0 because i=0 is a position reserved for NULL. In this version, we
// don't make
// use of the alignment to NULL because we use symmetrized alignments
// but that could
// be changed
while (targetExtendIndex >= 0 && !a.isTargetAligned(targetExtendIndex)) {
prev++;
Rule r = new Rule(sourceStartIndex, sourceEndIndex, targetExtendIndex, targetEndIndex, sp, a);
res.add(r);
targetExtendIndex--;
}
int foll = 0; // number of following target unaligned words
targetExtendIndex = targetEndIndex + 1;
while (targetExtendIndex < sp.getTarget().getWords().length
&& !a.isTargetAligned(targetExtendIndex)) {
foll++;
Rule r =
new Rule(sourceStartIndex, sourceEndIndex, targetStartIndex, targetExtendIndex, sp, a);
res.add(r);
targetExtendIndex++;
}
if (prev > 0 && foll > 0) { // if there are unaligned words in both
// sides:
for (targetExtendIndex = 1; targetExtendIndex <= prev; targetExtendIndex++) { // for
// each
// start
// (including
// at
// least
// one
// previous)
int start = targetStartIndex - targetExtendIndex;
for (int k = 1; k <= foll; k++) { // for each end (including at
// least one following)
int end = targetEndIndex + k;
Rule r = new Rule(sourceStartIndex, sourceEndIndex, start, end, sp, a);
res.add(r);
}
}
}
return res;
}
private List<Block> getRegularBlocks(Alignment a, SentencePair sp) {
List<Block> res = new ArrayList<Block>();
int sourceStartIndex = 0;
int sourceEndIndex = 0;
int targetStartIndex = 0;
int targetEndIndex = 0;
while (sourceStartIndex <= sp.getSource().getWords().length
&& targetStartIndex <= sp.getTarget().getWords().length) {
Block next =
findNextBlock(sourceStartIndex, sourceEndIndex, targetStartIndex, targetEndIndex, a, sp);
sourceStartIndex = next.sourceStartIndex;
sourceEndIndex = next.sourceEndIndex;
targetStartIndex = next.targetStartIndex;
targetEndIndex = next.targetEndIndex;
if (targetStartIndex >= sp.getTarget().getWords().length
&& sourceStartIndex >= sp.getSource().getWords().length) {
// do nothing
} else if (targetStartIndex < sp.getTarget().getWords().length
&& !a.isTargetAligned(targetStartIndex)) {
sourceEndIndex--;
} else if (sourceStartIndex < sp.getSource().getWords().length
&& !a.isSourceAligned(sourceStartIndex)) {
targetEndIndex--;
}
if (targetStartIndex >= sp.getTarget().getWords().length
|| sourceStartIndex >= sp.getSource().getWords().length) {
break;
}
if (sourceStartIndex <= sourceEndIndex && targetStartIndex <= targetEndIndex) {
res.add(new Block(sourceStartIndex, sourceEndIndex, targetStartIndex, targetEndIndex));
}
sourceStartIndex = sourceEndIndex + 1;
targetStartIndex = targetEndIndex + 1;
sourceEndIndex = sourceStartIndex;
targetEndIndex = targetStartIndex;
}
return res;
}
/**
* This method extract phrase pairs from a Viterbi alignment and a sentence pair. Protected for
* testing.
*
* @param a
* @param sp
* @return
*/
private List<Rule> extractPhrasePairs(Alignment a, SentencePair sp) {
List<Rule> res = new ArrayList<Rule>();
// loop over source index (beginning of phrase)
for (int sourceStartIndex = 0;
sourceStartIndex < sp.getSource().getWords().length;
sourceStartIndex++) {
// source phrase built on the fly
List<Integer> sourcePhrase = new ArrayList<Integer>();
// maintain the minimum and maximum target index aligned to the
// source phrase
int minTargetIndex = a.getMinTargetIndex(sourceStartIndex);
int maxTargetIndex = a.getMaxTargetIndex(sourceStartIndex);
// loop over source index (end of phrase)
for (int sourceEndIndex = sourceStartIndex;
sourceEndIndex
< Math.min(sourceStartIndex + MAX_SOURCE_PHRASE, sp.getSource().getWords().length);
sourceEndIndex++) {
// update the sourcePhrase
sourcePhrase.add(sp.getSource().getWords()[sourceEndIndex]);
// update minimum and maximum target index aligned to the source
// phrase
int minTargetIndexCandidate = a.getMinTargetIndex(sourceEndIndex);
int maxTargetIndexCandidate = a.getMaxTargetIndex(sourceEndIndex);
if (minTargetIndexCandidate < minTargetIndex) minTargetIndex = minTargetIndexCandidate;
if (maxTargetIndexCandidate > maxTargetIndex) maxTargetIndex = maxTargetIndexCandidate;
if (minTargetIndex > maxTargetIndex) // occurs when haven't
// found any aligned
// word
// yet
continue;
// check if the target phrase between positions minTargetIndex
// and maxTargetIndex and the source phrase are consistent with
// the alignment
boolean consistent = true;
List<Integer> targetPhrase = new ArrayList<Integer>();
for (int targetIndex = minTargetIndex; targetIndex <= maxTargetIndex; targetIndex++) {
targetPhrase.add(sp.getTarget().getWords()[targetIndex]);
if (a.isTargetAligned(targetIndex)
&& (a.getMinSourceIndex(targetIndex) < sourceStartIndex
|| a.getMaxSourceIndex(targetIndex) > sourceEndIndex)) {
consistent = false;
break;
}
}
// we found a phrase pair
if (consistent) {
// TODO the rule may be constructed on the fly
// to avoid duplicated logic as well as duplicated loops.
Rule r =
new Rule(sourceStartIndex, sourceEndIndex, minTargetIndex, maxTargetIndex, sp, a);
res.add(r);
List<Rule> extendedUnaligned =
extendUnalignedBoundaryWord(
sourceStartIndex, sourceEndIndex, minTargetIndex, maxTargetIndex, a, sp);
res.addAll(extendedUnaligned);
}
}
}
return res;
}