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decoding.cpp
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decoding.cpp
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// Copyright (c) 2012-2015 Andre Martins
// All Rights Reserved.
//
// This file is part of TurboParser 2.3.
//
// TurboParser 2.3 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 3 of the License, or
// (at your option) any later version.
//
// TurboParser 2.3 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 TurboParser 2.3. If not, see <http://www.gnu.org/licenses/>.
#include <limits>
#include <iostream>
#include <vector>
#include "decoding.h"
using namespace std;
// Decoder for the basic model; it finds a maximum weighted arborescence
// using Edmonds' algorithm (which runs in O(n^2)).
void c_chu_liu_edmonds(
vector<bool> *disabled,
vector<vector<int> > *candidate_heads,
vector<vector<double> > *candidate_scores,
vector<int> *heads,
double *value) {
// Original number of nodes (including the root).
size_t length = disabled->size();
// Pick the best incoming arc for each node.
heads->resize(length);
vector<double> best_scores(length);
for (int m = 1; m < length; ++m) {
if ((*disabled)[m]) continue;
int best = -1;
for (int k = 0; k < (*candidate_heads)[m].size(); ++k) {
if (best < 0 ||
(*candidate_scores)[m][k] >(*candidate_scores)[m][best]) {
best = k;
}
}
if (best < 0) {
// No spanning tree exists. Assign the parent of this node
// to the root, and give it a minus infinity score.
(*heads)[m] = 0;
best_scores[m] = -std::numeric_limits<double>::infinity();
} else {
(*heads)[m] = (*candidate_heads)[m][best]; //best;
best_scores[m] = (*candidate_scores)[m][best]; //best;
}
}
// Look for cycles. Return after the first cycle is found.
vector<int> cycle;
vector<int> visited(length, 0);
for (int m = 1; m < length; ++m) {
if ((*disabled)[m]) continue;
// Examine all the ancestors of m until the root or a cycle is found.
int h = m;
while (h != 0) {
// If already visited, break and check if it is part of a cycle.
// If visited[h] < m, the node was visited earlier and seen not
// to be part of a cycle.
if (visited[h]) break;
visited[h] = m;
h = (*heads)[h];
}
// Found a cycle to which h belongs.
// Obtain the full cycle.
if (visited[h] == m) {
m = h;
do {
cycle.push_back(m);
m = (*heads)[m];
} while (m != h);
break;
}
}
// If there are no cycles, then this is a well formed tree.
if (cycle.empty()) {
*value = 0.0;
for (int m = 1; m < length; ++m) {
*value += best_scores[m];
}
return;
}
// Build a cycle membership vector for constant-time querying and compute the
// score of the cycle.
// Nominate a representative node for the cycle and disable all the others.
double cycle_score = 0.0;
vector<bool> in_cycle(length, false);
int representative = cycle[0];
for (int k = 0; k < cycle.size(); ++k) {
int m = cycle[k];
in_cycle[m] = true;
cycle_score += best_scores[m];
if (m != representative) (*disabled)[m] = true;
}
// Contract the cycle.
// 1) Update the score of each child to the maximum score achieved by a parent
// node in the cycle.
vector<int> best_heads_cycle(length);
for (int m = 1; m < length; ++m) {
if ((*disabled)[m] || m == representative) continue;
double best_score;
// If the list of candidate parents of m is shorter than the length of
// the cycle, use that. Otherwise, loop through the cycle.
int best = -1;
for (int k = 0; k < (*candidate_heads)[m].size(); ++k) {
if (!in_cycle[(*candidate_heads)[m][k]]) continue;
if (best < 0 || (*candidate_scores)[m][k] > best_score) {
best = k;
best_score = (*candidate_scores)[m][best];
}
}
if (best < 0) continue;
best_heads_cycle[m] = (*candidate_heads)[m][best];
// Reconstruct the list of candidate heads for this m.
int l = 0;
for (int k = 0; k < (*candidate_heads)[m].size(); ++k) {
int h = (*candidate_heads)[m][k];
double score = (*candidate_scores)[m][k];
if (!in_cycle[h]) {
(*candidate_heads)[m][l] = h;
(*candidate_scores)[m][l] = score;
++l;
}
}
// If h is in the cycle and is not the representative node,
// it will be dropped from the list of candidate heads.
(*candidate_heads)[m][l] = representative;
(*candidate_scores)[m][l] = best_score;
(*candidate_heads)[m].resize(l + 1);
(*candidate_scores)[m].resize(l + 1);
}
// 2) Update the score of each candidate parent of the cycle supernode.
vector<int> best_modifiers_cycle(length, -1);
vector<int> candidate_heads_representative;
vector<double> candidate_scores_representative;
vector<double> best_scores_cycle(length);
// Loop through the cycle.
for (int k = 0; k < cycle.size(); ++k) {
int m = cycle[k];
for (int l = 0; l < (*candidate_heads)[m].size(); ++l) {
// Get heads out of the cycle.
int h = (*candidate_heads)[m][l];
if (in_cycle[h]) continue;
double score = (*candidate_scores)[m][l] - best_scores[m];
if (best_modifiers_cycle[h] < 0 || score > best_scores_cycle[h]) {
best_modifiers_cycle[h] = m;
best_scores_cycle[h] = score;
}
}
}
for (int h = 0; h < length; ++h) {
if (best_modifiers_cycle[h] < 0) continue;
double best_score = best_scores_cycle[h] + cycle_score;
candidate_heads_representative.push_back(h);
candidate_scores_representative.push_back(best_score);
}
// Reconstruct the list of candidate heads for the representative node.
(*candidate_heads)[representative] = candidate_heads_representative;
(*candidate_scores)[representative] = candidate_scores_representative;
// Save the current head of the representative node (it will be overwritten).
int head_representative = (*heads)[representative];
// Call itself recursively.
c_chu_liu_edmonds(disabled,
candidate_heads,
candidate_scores,
heads,
value);
// Uncontract the cycle.
int h = (*heads)[representative];
(*heads)[representative] = head_representative;
(*heads)[best_modifiers_cycle[h]] = h;
for (int m = 1; m < length; ++m) {
if ((*disabled)[m]) continue;
if ((*heads)[m] == representative) {
// Get the right parent from within the cycle.
(*heads)[m] = best_heads_cycle[m];
}
}
for (int k = 0; k < cycle.size(); ++k) {
int m = cycle[k];
(*disabled)[m] = false;
}
}