1 files changed, 76 insertions, 1 deletions
diff --git a/rst_parser/rst.cc b/rst_parser/rst.cc
index f6b295b3..bc91330b 100644
--- a/rst_parser/rst.cc
+++ b/rst_parser/rst.cc
@@ -2,6 +2,81 @@
 
 using namespace std;
 
-StochasticForest::StochasticForest(const ArcFactoredForest& af) {
+// David B. Wilson. Generating Random Spanning Trees More Quickly than the Cover Time.
+// this is an awesome algorithm
+TreeSampler::TreeSampler(const ArcFactoredForest& af) : forest(af), usucc(af.size() + 1) {
+  // edges are directed from modifiers to heads, and finally to the root
+  vector<double> p;
+  for (int m = 1; m <= forest.size(); ++m) {
+#if USE_ALIAS_SAMPLER
+    p.clear();
+#else
+    SampleSet<double>& ss = usucc[m];
+#endif
+    double z = 0;
+    for (int h = 0; h <= forest.size(); ++h) {
+      double u = forest(h-1,m-1).edge_prob.as_float();
+      z += u;
+#if USE_ALIAS_SAMPLER
+      p.push_back(u);
+#else
+      ss.add(u);
+#endif
+    }
+#if USE_ALIAS_SAMPLER
+    for (int i = 0; i < p.size(); ++i) { p[i] /= z; }
+    usucc[m].Init(p);
+#endif
+  }
 }
 
+void TreeSampler::SampleRandomSpanningTree(EdgeSubset* tree, MT19937* prng) {
+  MT19937& rng = *prng;
+  const int r = 0;
+  bool success = false;
+  while (!success) {
+    int roots = 0;
+    tree->h_m_pairs.clear();
+    tree->roots.clear();
+    vector<int> next(forest.size() + 1, -1);
+    vector<char> in_tree(forest.size() + 1, 0);
+    in_tree[r] = 1;
+    //cerr << "Forest size: " << forest.size() << endl;
+    for (int i = 0; i <= forest.size(); ++i) {
+      //cerr << "Sampling starting at u=" << i << endl;
+      int u = i;
+      if (in_tree[u]) continue;
+      while(!in_tree[u]) {
+#if USE_ALIAS_SAMPLER
+        next[u] = usucc[u].Draw(rng);
+#else
+        next[u] = rng.SelectSample(usucc[u]);
+#endif
+        u = next[u];
+      }
+      u = i;
+      //cerr << (u-1);
+      int prev = u-1;
+      while(!in_tree[u]) {
+        in_tree[u] = true;
+        u = next[u];
+        //cerr << " > " << (u-1);
+        if (u == r) {
+          ++roots;
+          tree->roots.push_back(prev);
+        } else {
+          tree->h_m_pairs.push_back(make_pair<short,short>(u-1,prev));
+        }
+        prev = u-1;
+      }
+      //cerr << endl;
+    }
+    assert(roots > 0);
+    if (roots > 1) {
+      //cerr << "FAILURE\n";
+    } else {
+      success = true;
+    }
+  }
+};
+