From 968e95ae0f9bb6a7c68d1ed80f3e9055787c2311 Mon Sep 17 00:00:00 2001
From: Francois Fleuret <francois@fleuret.org>
Date: Fri, 28 Dec 2012 23:55:59 +0100
Subject: [PATCH] A new version of compute_dp_ranks, orders of magnitude
 faster.

---
 mtp_graph.cc | 71 +++++++++++++++++++++++++++++++---------------------
 1 file changed, 43 insertions(+), 28 deletions(-)

diff --git a/mtp_graph.cc b/mtp_graph.cc
index dc38321..0fe3cf0 100644
--- a/mtp_graph.cc
+++ b/mtp_graph.cc
@@ -192,6 +192,7 @@ MTPGraph::~MTPGraph() {
 void MTPGraph::compute_dp_ranks() {
   Vertex *v;
   Edge *e;
+  int tv;
 
   // This procedure computes for each node the longest link from the
   // source and abort if the graph is not a DAG. It works by removing
@@ -203,47 +204,61 @@ void MTPGraph::compute_dp_ranks() {
   // rank of a node is the iteration at which is it removed, and we
   // set the distance_from_source fields to this value.
 
-  Vertex **with_predecessor = new Vertex *[_nb_vertices];
+  int *nb_predecessors = new int[_nb_vertices];
+  int *without_predecessors = new int[_nb_vertices];
+  int *new_without_predecessors = new int[_nb_vertices];
+  int nb_without_predecessors, new_nb_without_predecessors;
 
-  // All the nodes are with_predecessor at first
   for(int k = 0; k < _nb_vertices; k++) {
-    _vertices[k].distance_from_source = 0;
-    with_predecessor[k] = &_vertices[k];
+    nb_predecessors[k] = 0;
   }
 
-  scalar_t rank = 1;
-  int nb_with_predecessor = _nb_vertices, pred_nb_with_predecessor;
-
-  do {
-    // We set the distance_from_source field of all the vertices with incoming
-    // edges to the current rank value
-    for(int f = 0; f < nb_with_predecessor; f++) {
-      v = with_predecessor[f];
-      for(e = v->leaving_edge_list_root; e; e = e->next_leaving_edge) {
-        e->terminal_vertex->distance_from_source = rank;
-      }
+  for(int k = 0; k < _nb_vertices; k++) {
+    v = _vertices + k;
+    for(e = v->leaving_edge_list_root; e; e = e->next_leaving_edge) {
+      tv = e->terminal_vertex - _vertices;
+      nb_predecessors[tv]++;
     }
+  }
 
-    pred_nb_with_predecessor = nb_with_predecessor;
-    nb_with_predecessor = 0;
+  nb_without_predecessors = 0;
+  for(int k = 0; k < _nb_vertices; k++) {
+    if(nb_predecessors[k] == 0) {
+      without_predecessors[nb_without_predecessors++] = k;
+    }
+  }
 
-    // We keep all the vertices with incoming nodes
-    for(int f = 0; f < pred_nb_with_predecessor; f++) {
-      v = with_predecessor[f];
-      if(v->distance_from_source == rank) {
-        with_predecessor[nb_with_predecessor++] = v;
+  scalar_t rank = 1;
+  while(nb_without_predecessors > 0) {
+    new_nb_without_predecessors = 0;
+    for(int l = 0; l < nb_without_predecessors; l++) {
+      v = _vertices + without_predecessors[l];
+      v->distance_from_source = rank;
+      for(e = v->leaving_edge_list_root; e; e = e->next_leaving_edge) {
+        tv = e->terminal_vertex - _vertices;
+        nb_predecessors[tv]--;
+        ASSERT(nb_predecessors[tv] >= 0);
+        if(nb_predecessors[tv] == 0) {
+          new_without_predecessors[new_nb_without_predecessors++] = tv;
+        }
       }
     }
 
+    swap(without_predecessors, new_without_predecessors);
+    nb_without_predecessors = new_nb_without_predecessors;
     rank++;
-  } while(nb_with_predecessor < pred_nb_with_predecessor);
-
-  delete[] with_predecessor;
+  }
 
-  if(nb_with_predecessor > 0) {
-    cerr << __FILE__ << ": The graph is not a DAG." << endl;
-    abort();
+  for(int k = 0; k < _nb_vertices; k++) {
+    if(nb_predecessors[k] > 0) {
+      cerr << __FILE__ << ": The graph is not a DAG." << endl;
+      abort();
+    }
   }
+
+  delete[] nb_predecessors;
+  delete[] without_predecessors;
+  delete[] new_without_predecessors;
 }
 
 //////////////////////////////////////////////////////////////////////
-- 
2.39.5