mosesdecoder/moses/ChartKBestExtractor.cpp
2014-12-13 12:52:47 +01:00

313 lines
12 KiB
C++

/***********************************************************************
Moses - statistical machine translation system
Copyright (C) 2006-2014 University of Edinburgh
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
***********************************************************************/
#include "ChartKBestExtractor.h"
#include "ChartHypothesis.h"
#include "ScoreComponentCollection.h"
#include "StaticData.h"
#include <boost/scoped_ptr.hpp>
#include <vector>
using namespace std;
namespace Moses
{
// Extract the k-best list from the search graph.
void ChartKBestExtractor::Extract(
const std::vector<const ChartHypothesis*> &topLevelHypos, std::size_t k,
KBestVec &kBestList)
{
kBestList.clear();
if (topLevelHypos.empty()) {
return;
}
// Create a new ChartHypothesis object, supremeHypo, that has the best
// top-level hypothesis as its predecessor and has the same score.
std::vector<const ChartHypothesis*>::const_iterator p = topLevelHypos.begin();
const ChartHypothesis &bestTopLevelHypo = **p;
boost::scoped_ptr<ChartHypothesis> supremeHypo(
new ChartHypothesis(bestTopLevelHypo, *this));
// Do the same for each alternative top-level hypothesis, but add the new
// ChartHypothesis objects as arcs from supremeHypo, as if they had been
// recombined.
for (++p; p != topLevelHypos.end(); ++p) {
// Check that the first item in topLevelHypos really was the best.
UTIL_THROW_IF2((*p)->GetTotalScore() > bestTopLevelHypo.GetTotalScore(),
"top-level hypotheses are not correctly sorted");
// Note: there's no need for a smart pointer here: supremeHypo will take
// ownership of altHypo.
ChartHypothesis *altHypo = new ChartHypothesis(**p, *this);
supremeHypo->AddArc(altHypo);
}
// Create the target vertex then lazily fill its k-best list.
boost::shared_ptr<Vertex> targetVertex = FindOrCreateVertex(*supremeHypo);
LazyKthBest(*targetVertex, k, k);
// Copy the k-best list from the target vertex, but drop the top edge from
// each derivation.
kBestList.reserve(targetVertex->kBestList.size());
for (std::vector<boost::weak_ptr<Derivation> >::const_iterator
q = targetVertex->kBestList.begin();
q != targetVertex->kBestList.end(); ++q) {
const boost::shared_ptr<Derivation> d(*q);
assert(d);
assert(d->subderivations.size() == 1);
kBestList.push_back(d->subderivations[0]);
}
}
// Generate the target-side yield of the derivation d.
Phrase ChartKBestExtractor::GetOutputPhrase(const Derivation &d)
{
FactorType placeholderFactor = StaticData::Instance().GetPlaceholderFactor();
Phrase ret(ARRAY_SIZE_INCR);
const ChartHypothesis &hypo = d.edge.head->hypothesis;
const TargetPhrase &phrase = hypo.GetCurrTargetPhrase();
const AlignmentInfo::NonTermIndexMap &nonTermIndexMap =
phrase.GetAlignNonTerm().GetNonTermIndexMap();
for (std::size_t pos = 0; pos < phrase.GetSize(); ++pos) {
const Word &word = phrase.GetWord(pos);
if (word.IsNonTerminal()) {
std::size_t nonTermInd = nonTermIndexMap[pos];
const Derivation &subderivation = *d.subderivations[nonTermInd];
Phrase subPhrase = GetOutputPhrase(subderivation);
ret.Append(subPhrase);
} else {
ret.AddWord(word);
if (placeholderFactor == NOT_FOUND) {
continue;
}
std::set<std::size_t> sourcePosSet =
phrase.GetAlignTerm().GetAlignmentsForTarget(pos);
if (sourcePosSet.size() == 1) {
const std::vector<const Word*> *ruleSourceFromInputPath =
hypo.GetTranslationOption().GetSourceRuleFromInputPath();
UTIL_THROW_IF2(ruleSourceFromInputPath == NULL,
"Source Words in of the rules hasn't been filled out");
std::size_t sourcePos = *sourcePosSet.begin();
const Word *sourceWord = ruleSourceFromInputPath->at(sourcePos);
UTIL_THROW_IF2(sourceWord == NULL,
"Null source word at position " << sourcePos);
const Factor *factor = sourceWord->GetFactor(placeholderFactor);
if (factor) {
ret.Back()[0] = factor;
}
}
}
}
return ret;
}
// Generate the target tree of the derivation d.
TreePointer ChartKBestExtractor::GetOutputTree(const Derivation &d)
{
const ChartHypothesis &hypo = d.edge.head->hypothesis;
const TargetPhrase &phrase = hypo.GetCurrTargetPhrase();
if (const PhraseProperty *property = phrase.GetProperty("Tree")) {
const std::string *tree = property->GetValueString();
TreePointer mytree (boost::make_shared<InternalTree>(*tree));
//get subtrees (in target order)
std::vector<TreePointer> previous_trees;
for (size_t pos = 0; pos < phrase.GetSize(); ++pos) {
const Word &word = phrase.GetWord(pos);
if (word.IsNonTerminal()) {
size_t nonTermInd = phrase.GetAlignNonTerm().GetNonTermIndexMap()[pos];
const Derivation &subderivation = *d.subderivations[nonTermInd];
const TreePointer prev_tree = GetOutputTree(subderivation);
previous_trees.push_back(prev_tree);
}
}
mytree->Combine(previous_trees);
return mytree;
}
else {
UTIL_THROW2("Error: TreeStructureFeature active, but no internal tree structure found");
}
}
// Create an unweighted hyperarc corresponding to the given ChartHypothesis.
ChartKBestExtractor::UnweightedHyperarc ChartKBestExtractor::CreateEdge(
const ChartHypothesis &h)
{
UnweightedHyperarc edge;
edge.head = FindOrCreateVertex(h);
const std::vector<const ChartHypothesis*> &prevHypos = h.GetPrevHypos();
edge.tail.resize(prevHypos.size());
for (std::size_t i = 0; i < prevHypos.size(); ++i) {
const ChartHypothesis *prevHypo = prevHypos[i];
edge.tail[i] = FindOrCreateVertex(*prevHypo);
}
return edge;
}
// Look for the vertex corresponding to a given ChartHypothesis, creating
// a new one if necessary.
boost::shared_ptr<ChartKBestExtractor::Vertex>
ChartKBestExtractor::FindOrCreateVertex(const ChartHypothesis &h)
{
VertexMap::value_type element(&h, boost::shared_ptr<Vertex>());
std::pair<VertexMap::iterator, bool> p = m_vertexMap.insert(element);
boost::shared_ptr<Vertex> &sp = p.first->second;
if (!p.second) {
return sp; // Vertex was already in m_vertexMap.
}
sp.reset(new Vertex(h));
// Create the 1-best derivation and add it to the vertex's kBestList.
UnweightedHyperarc bestEdge;
bestEdge.head = sp;
const std::vector<const ChartHypothesis*> &prevHypos = h.GetPrevHypos();
bestEdge.tail.resize(prevHypos.size());
for (std::size_t i = 0; i < prevHypos.size(); ++i) {
const ChartHypothesis *prevHypo = prevHypos[i];
bestEdge.tail[i] = FindOrCreateVertex(*prevHypo);
}
boost::shared_ptr<Derivation> bestDerivation(new Derivation(bestEdge));
#ifndef NDEBUG
std::pair<DerivationSet::iterator, bool> q =
#endif
m_derivations.insert(bestDerivation);
assert(q.second);
sp->kBestList.push_back(bestDerivation);
return sp;
}
// Create the 1-best derivation for each edge in BS(v) (except the best one)
// and add it to v's candidate queue.
void ChartKBestExtractor::GetCandidates(Vertex &v, std::size_t k)
{
// Create derivations for all of v's incoming edges except the best. This
// means everything in v.hypothesis.GetArcList() and not the edge defined
// by v.hypothesis itself. The 1-best derivation for that edge will already
// have been created.
const ChartArcList *arcList = v.hypothesis.GetArcList();
if (arcList) {
for (std::size_t i = 0; i < arcList->size(); ++i) {
const ChartHypothesis &recombinedHypo = *(*arcList)[i];
boost::shared_ptr<Vertex> w = FindOrCreateVertex(recombinedHypo);
assert(w->kBestList.size() == 1);
v.candidates.push(w->kBestList[0]);
}
}
}
// Lazily fill v's k-best list.
void ChartKBestExtractor::LazyKthBest(Vertex &v, std::size_t k,
std::size_t globalK)
{
// If this is the first visit to vertex v then initialize the priority queue.
if (v.visited == false) {
// The 1-best derivation should already be in v's k-best list.
assert(v.kBestList.size() == 1);
// Initialize v's priority queue.
GetCandidates(v, globalK);
v.visited = true;
}
// Add derivations to the k-best list until it contains k or there are none
// left to add.
while (v.kBestList.size() < k) {
assert(!v.kBestList.empty());
// Update the priority queue by adding the successors of the last
// derivation (unless they've been seen before).
boost::shared_ptr<Derivation> d(v.kBestList.back());
LazyNext(v, *d, globalK);
// Check if there are any derivations left in the queue.
if (v.candidates.empty()) {
break;
}
// Get the next best derivation and delete it from the queue.
boost::weak_ptr<Derivation> next = v.candidates.top();
v.candidates.pop();
// Add it to the k-best list.
v.kBestList.push_back(next);
}
}
// Create the neighbours of Derivation d and add them to v's candidate queue.
void ChartKBestExtractor::LazyNext(Vertex &v, const Derivation &d,
std::size_t globalK)
{
for (std::size_t i = 0; i < d.edge.tail.size(); ++i) {
Vertex &pred = *d.edge.tail[i];
// Ensure that pred's k-best list contains enough derivations.
std::size_t k = d.backPointers[i] + 2;
LazyKthBest(pred, k, globalK);
if (pred.kBestList.size() < k) {
// pred's derivations have been exhausted.
continue;
}
// Create the neighbour.
boost::shared_ptr<Derivation> next(new Derivation(d, i));
// Check if it has been created before.
std::pair<DerivationSet::iterator, bool> p = m_derivations.insert(next);
if (p.second) {
v.candidates.push(next); // Haven't previously seen it.
}
}
}
// Construct the 1-best Derivation that ends at edge e.
ChartKBestExtractor::Derivation::Derivation(const UnweightedHyperarc &e)
{
edge = e;
std::size_t arity = edge.tail.size();
backPointers.resize(arity, 0);
subderivations.reserve(arity);
for (std::size_t i = 0; i < arity; ++i) {
const Vertex &pred = *edge.tail[i];
assert(pred.kBestList.size() >= 1);
boost::shared_ptr<Derivation> sub(pred.kBestList[0]);
subderivations.push_back(sub);
}
scoreBreakdown = edge.head->hypothesis.GetScoreBreakdown();
score = edge.head->hypothesis.GetTotalScore();
}
// Construct a Derivation that neighbours an existing Derivation.
ChartKBestExtractor::Derivation::Derivation(const Derivation &d, std::size_t i)
{
edge.head = d.edge.head;
edge.tail = d.edge.tail;
backPointers = d.backPointers;
subderivations = d.subderivations;
std::size_t j = ++backPointers[i];
scoreBreakdown = d.scoreBreakdown;
// Deduct the score of the old subderivation.
scoreBreakdown.MinusEquals(subderivations[i]->scoreBreakdown);
// Update the subderivation pointer.
boost::shared_ptr<Derivation> newSub(edge.tail[i]->kBestList[j]);
subderivations[i] = newSub;
// Add the score of the new subderivation.
scoreBreakdown.PlusEquals(subderivations[i]->scoreBreakdown);
score = scoreBreakdown.GetWeightedScore();
}
} // namespace Moses