From e2998fc79c9dd549b1c1bad537fdf1052276f82c Mon Sep 17 00:00:00 2001
From: Chris Dyer <cdyer@cs.cmu.edu>
Date: Thu, 8 Mar 2012 01:46:32 -0500
Subject: simple context feature for tagger

---
 decoder/ff_context.cc | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 decoder/ff_context.cc

(limited to 'decoder/ff_context.cc')

diff --git a/decoder/ff_context.cc b/decoder/ff_context.cc
new file mode 100644
index 00000000..19f9a413
--- /dev/null
+++ b/decoder/ff_context.cc
@@ -0,0 +1,99 @@
+#include "ff_context.h"
+
+#include <sstream>
+#include <cassert>
+#include <cmath>
+
+#include "filelib.h"
+#include "stringlib.h"
+#include "sentence_metadata.h"
+#include "lattice.h"
+#include "fdict.h"
+#include "verbose.h"
+
+using namespace std;
+
+namespace {
+  string Escape(const string& x) {
+    string y = x;
+    for (int i = 0; i < y.size(); ++i) {
+      if (y[i] == '=') y[i]='_';
+      if (y[i] == ';') y[i]='_';
+    }
+    return y;
+  }
+}
+
+RuleContextFeatures::RuleContextFeatures(const std::string& param) {
+  kSOS = TD::Convert("<s>");
+  kEOS = TD::Convert("</s>");
+
+  // TODO param lets you pass in a string from the cdec.ini file
+}
+
+void RuleContextFeatures::PrepareForInput(const SentenceMetadata& smeta) {
+  const Lattice& sl = smeta.GetSourceLattice();
+  current_input.resize(sl.size());
+  for (unsigned i = 0; i < sl.size(); ++i) {
+    if (sl[i].size() != 1) {
+      cerr << "Context features not supported with lattice inputs!\nid=" << smeta.GetSentenceId() << endl;
+      abort();
+    }
+    current_input[i] = sl[i][0].label;
+  }
+}
+
+void RuleContextFeatures::TraversalFeaturesImpl(const SentenceMetadata& smeta,
+                                                const Hypergraph::Edge& edge,
+                                                const vector<const void*>& ant_contexts,
+                                                SparseVector<double>* features,
+                                                SparseVector<double>* estimated_features,
+                                                void* context) const {
+  const TRule& rule = *edge.rule_;
+
+  if (rule.Arity() != 0 || // arity = 0, no nonterminals
+      rule.e_.size() != 1) return; // size = 1, predicted label is a single token
+
+
+  // you can see the current label "for free"
+  const WordID cur_label = rule.e_[0];
+  // (if you want to see more labels, you have to be very careful, and muck
+  //  about with contexts and ant_contexts)
+
+  // but... you can look at as much of the source as you want!
+  const int from_src_index = edge.i_;   // start of the span in the input being labeled
+  const int to_src_index = edge.j_;     // end of the span in the input
+  // (note: in the case of tagging the size of the spans being labeled will
+  //  always be 1, but in other formalisms, you can have bigger spans.)
+
+  // this is the current token being labeled:
+  const WordID cur_input = current_input[from_src_index];
+
+  // let's get the previous token in the input (may be to the left of the start
+  // of the sentence!)
+  WordID prev_input = kSOS;
+  if (from_src_index > 0) { prev_input = current_input[from_src_index - 1]; }
+  // let's get the next token (may be to the left of the start of the sentence!)
+  WordID next_input = kEOS;
+  if (to_src_index < current_input.size()) { next_input = current_input[to_src_index]; }
+
+  // now, build a feature string
+  ostringstream os;
+  // TD::Convert converts from the internal integer representation of a token
+  // to the actual token
+  os << "C1:" << TD::Convert(prev_input) << '_' 
+     << TD::Convert(cur_input) << '|' << TD::Convert(cur_label);
+  // C1 is just to prevent a name clash
+
+  // pick a value
+  double fval = 1.0; // can be any real value
+
+  // add it to the feature vector FD::Convert converts the feature string to a
+  // feature int, Escape makes sure the feature string doesn't have any bad
+  // symbols that could confuse a parser somewhere
+  features->add_value(FD::Convert(Escape(os.str())), fval);
+  // that's it!
+
+  // create more features if you like...
+}
+
-- 
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