From 16d0e2a34df3e32e8992aeda3ad2de7a6e525f14 Mon Sep 17 00:00:00 2001
From: Chris Dyer <cdyer@cs.cmu.edu>
Date: Sat, 28 Jul 2012 18:35:11 -0400
Subject: slight improvement to the way NTs are handled in the FST-CFG
 composition algorithm, new rescoring mode

---
 decoder/hg_remove_eps.cc | 91 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)
 create mode 100644 decoder/hg_remove_eps.cc

(limited to 'decoder/hg_remove_eps.cc')

diff --git a/decoder/hg_remove_eps.cc b/decoder/hg_remove_eps.cc
new file mode 100644
index 00000000..050c4876
--- /dev/null
+++ b/decoder/hg_remove_eps.cc
@@ -0,0 +1,91 @@
+#include "hg_remove_eps.h"
+
+#include <cassert>
+
+#include "trule.h"
+#include "hg.h"
+
+using namespace std;
+
+namespace {
+  TRulePtr kEPSRule;
+  TRulePtr kUnaryRule;
+
+  TRulePtr CreateUnaryRule(int lhs, int rhs) {
+    if (!kUnaryRule) kUnaryRule.reset(new TRule("[X] ||| [X,1] ||| [X,1]"));
+    TRule* r = new TRule(*kUnaryRule);
+    assert(lhs < 0);
+    assert(rhs < 0);
+    r->lhs_ = lhs;
+    r->f_[0] = rhs;
+    return TRulePtr(r);
+  }
+
+  TRulePtr CreateEpsilonRule(int lhs, WordID eps) {
+    if (!kEPSRule) kEPSRule.reset(new TRule("[X] ||| <eps> ||| <eps>"));
+    TRule* r = new TRule(*kEPSRule);
+    r->lhs_ = lhs;
+    assert(lhs < 0);
+    assert(eps > 0);
+    r->e_[0] = eps;
+    r->f_[0] = eps;
+    return TRulePtr(r);
+  }
+}
+
+void RemoveEpsilons(Hypergraph* g, WordID eps) {
+  vector<bool> kill(g->edges_.size(), false);
+  for (unsigned i = 0; i < g->edges_.size(); ++i) {
+    const Hypergraph::Edge& edge = g->edges_[i];
+    if (edge.tail_nodes_.empty() &&
+        edge.rule_->f_.size() == 1 &&
+        edge.rule_->f_[0] == eps) {
+      kill[i] = true;
+      if (!edge.feature_values_.empty()) {
+        Hypergraph::Node& node = g->nodes_[edge.head_node_];
+        if (node.in_edges_.size() != 1) {
+          cerr << "[WARNING] <eps> edge with features going into non-empty node - can't promote\n";
+          // this *probably* means that there are multiple derivations of the
+          // same sequence via different paths through the input forest
+          // this needs to be investigated and fixed
+        } else {
+          for (unsigned j = 0; j < node.out_edges_.size(); ++j)
+            g->edges_[node.out_edges_[j]].feature_values_ += edge.feature_values_;
+          // cerr << "PROMOTED " << edge.feature_values_ << endl;
+	}
+      }
+    }
+  }
+  bool created_eps = false;
+  g->PruneEdges(kill);
+  for (unsigned i = 0; i < g->nodes_.size(); ++i) {
+    const Hypergraph::Node& node = g->nodes_[i];
+    if (node.in_edges_.empty()) {
+      for (unsigned j = 0; j < node.out_edges_.size(); ++j) {
+        Hypergraph::Edge& edge = g->edges_[node.out_edges_[j]];
+        const int lhs = edge.rule_->lhs_;
+        if (edge.rule_->Arity() == 2) {
+          assert(edge.rule_->f_.size() == 2);
+          assert(edge.rule_->e_.size() == 2);
+          unsigned cur = node.id_;
+          int t = -1;
+          assert(edge.tail_nodes_.size() == 2);
+          int rhs = 0;
+          for (unsigned i = 0; i < 2u; ++i) if (edge.tail_nodes_[i] != cur) { t = edge.tail_nodes_[i]; rhs = edge.rule_->f_[i]; }
+          assert(t != -1);
+          edge.tail_nodes_.resize(1);
+          edge.tail_nodes_[0] = t;
+          edge.rule_ = CreateUnaryRule(lhs, rhs);
+        } else {
+          edge.rule_ = CreateEpsilonRule(lhs, eps);
+          edge.tail_nodes_.clear();
+          created_eps = true;
+        }
+      }
+    }
+  }
+  vector<bool> k2(g->edges_.size(), false);
+  g->PruneEdges(k2);
+  if (created_eps) RemoveEpsilons(g, eps);
+}
+
-- 
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