From: Francois Fleuret <francois@fleuret.org>
Date: Wed, 2 Jan 2013 02:23:40 +0000 (+0100)
Subject: Simplified and optimized compute_dp_ordering.
X-Git-Url: https://fleuret.org/cgi-bin/gitweb/gitweb.cgi?a=commitdiff_plain;h=b7b7ab505a1cd374370a03cc61ca08f92ab8c075;p=mtp.git

Simplified and optimized compute_dp_ordering.
---

diff --git a/mtp_graph.cc b/mtp_graph.cc
index a03f60e..4cfd1f8 100644
--- a/mtp_graph.cc
+++ b/mtp_graph.cc
@@ -424,23 +424,15 @@ int MTPGraph::retrieve_one_path(Edge *e, Path *path) {
 
 //////////////////////////////////////////////////////////////////////
 
-static int compare_vertices_on_distance(const void *v1, const void *v2) {
-  scalar_t delta =
-    (*((Vertex **) v1))->distance_from_source -
-    (*((Vertex **) v2))->distance_from_source;
-  if(delta < 0) return -1;
-  else if(delta > 0) return 1;
-  else return 0;
-}
-
 void MTPGraph::compute_dp_ordering() {
   Vertex *v;
   Edge *e;
   int ntv;
 
-  // This method computes for each node the length of the longest link
-  // from the source, and orders the node in _dp_order according to
-  // it. It aborts if the graph is not a DAG.
+  // This method orders the nodes by putting first the ones with no
+  // predecessors, then going on adding nodes whose predecessors have
+  // all been already added. Computing the distances from the source
+  // by visiting nodes in that order is equivalent to DP.
 
   int *nb_predecessors = new int[_nb_vertices];
 
@@ -464,12 +456,10 @@ void MTPGraph::compute_dp_ordering() {
     }
   }
 
-  scalar_t rank = 1;
   while(already_processed < front) {
     new_front = front;
     while(already_processed < front) {
       v = *(already_processed++);
-      v->distance_from_source = rank;
       for(e = v->leaving_edge_list_root; e; e = e->next_leaving_edge) {
         ntv = int(e->terminal_vertex - _vertices);
         nb_predecessors[ntv]--;
@@ -480,7 +470,6 @@ void MTPGraph::compute_dp_ordering() {
       }
     }
     front = new_front;
-    rank++;
   }
 
   if(already_processed < _dp_order + _nb_vertices) {
@@ -489,9 +478,6 @@ void MTPGraph::compute_dp_ordering() {
   }
 
   delete[] nb_predecessors;
-
-  for(int v = 0; v < _nb_vertices; v++) { _dp_order[v] = &_vertices[v]; }
-  qsort(_dp_order, _nb_vertices, sizeof(Vertex *), compare_vertices_on_distance);
 }
 
 //////////////////////////////////////////////////////////////////////