dependency_decoding/decoding.cpp at master · andersjo/dependency_decoding · GitHub

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// 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;

}

dependency_decoding/decoding.cpp at master · andersjo/dependency_decoding · GitHub

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