From 34022d5e987cf13ff6db31a6d7263e9266a8b1c0 Mon Sep 17 00:00:00 2001
From: Francois Fleuret <francois@fleuret.org>
Date: Sun, 26 Aug 2012 08:26:23 +0200
Subject: [PATCH] Added is_dag + an ASSERT using it.

---
 mtp_graph.cc | 79 +++++++++++++++++++++++++++++++++++++++++-----------
 mtp_graph.h  |  2 ++
 2 files changed, 64 insertions(+), 17 deletions(-)

diff --git a/mtp_graph.cc b/mtp_graph.cc
index 92e3875..9a811bb 100644
--- a/mtp_graph.cc
+++ b/mtp_graph.cc
@@ -109,8 +109,8 @@ MTPGraph::MTPGraph(int nb_vertices, int nb_edges,
   _source = &_vertices[source];
   _sink = &_vertices[sink];
 
-  for(int v = 0; v < _nb_vertices; v++) {
-    _vertices[v].id = v;
+  for(int k = 0; k < _nb_vertices; k++) {
+    _vertices[k].id = k;
   }
 
   for(int e = 0; e < nb_edges; e++) {
@@ -157,8 +157,6 @@ void MTPGraph::print_dot(ostream *os) {
   (*os) << "        edge [color=gray,arrowhead=open]" << endl;
   (*os) << "        " << _source->id << " [peripheries=2];" << endl;
   (*os) << "        " << _sink->id << " [peripheries=2];" << endl;
-  // (*os) << "        " << _source->id << " [style=bold,color=red];" << endl;
-  // (*os) << "        " << _sink->id << " [style=bold,color=green];" << endl;
   for(int k = 0; k < _nb_edges; k++) {
     Edge *e = _edges + k;
     // (*os) << "  " << e->origin_vertex->id << " -> " << e->terminal_vertex->id
@@ -205,6 +203,50 @@ void MTPGraph::force_positivized_lengths() {
 #endif
 }
 
+int MTPGraph::is_dag() {
+  Vertex *v, *tv;
+  Edge *e;
+
+  // We put everybody in the front
+  for(int k = 0; k < _nb_vertices; k++) {
+    _vertices[k].iteration = 0;
+    _front[k] = &_vertices[k];
+  }
+
+  int front_size = _nb_vertices, nb_with_incoming;
+  int iteration = 0;
+  int new_front_size, pred_front_size;
+
+  do {
+    iteration++;
+    nb_with_incoming = 0;
+
+    // We set the iteration field of all vertex with incoming edges to
+    // the current iteration value
+    for(int f = 0; f < front_size; f++) {
+      v = _front[f];
+      for(e = v->leaving_edges; e; e = e->next_leaving_edge) {
+        tv = e->terminal_vertex;
+        tv->iteration = iteration;
+      }
+    }
+
+    new_front_size = 0;
+    // We remove all vertex without incoming edge
+    for(int f = 0; f < front_size; f++) {
+      v = _front[f];
+      if(v->iteration == iteration) {
+        _front[new_front_size++] = v;
+      }
+    }
+
+    pred_front_size = front_size;
+    front_size = new_front_size;
+  } while(front_size < pred_front_size);
+
+  return front_size == 0;
+}
+
 // This method does not change the edge occupation. It update
 // distance_from_source and pred_edge_toward_source.
 void MTPGraph::find_shortest_path() {
@@ -214,23 +256,23 @@ void MTPGraph::find_shortest_path() {
   Edge *e;
   scalar_t d;
 
-  for(int v = 0; v < _nb_vertices; v++) {
-    _vertices[v].distance_from_source = FLT_MAX;
-    _vertices[v].pred_edge_toward_source = 0;
-    _vertices[v].iteration = 0;
+  for(int k = 0; k < _nb_vertices; k++) {
+    _vertices[k].distance_from_source = FLT_MAX;
+    _vertices[k].pred_edge_toward_source = 0;
+    _vertices[k].iteration = 0;
   }
 
   int iteration = 0;
 
-  int _front_size = 0, _new_front_size;
-  _front[_front_size++] = _source;
+  int front_size = 0, new_front_size;
+  _front[front_size++] = _source;
   _source->distance_from_source = 0;
 
   do {
-    _new_front_size = 0;
+    new_front_size = 0;
     iteration++;
 
-    for(int f = 0; f < _front_size; f++) {
+    for(int f = 0; f < front_size; f++) {
       v = _front[f];
       for(e = v->leaving_edges; e; e = e->next_leaving_edge) {
         d = v->distance_from_source + e->positivized_length;
@@ -239,7 +281,7 @@ void MTPGraph::find_shortest_path() {
           tv->distance_from_source = d;
           tv->pred_edge_toward_source = e;
           if(tv->iteration < iteration) {
-            _new_front[_new_front_size++] = tv;
+            _new_front[new_front_size++] = tv;
             tv->iteration = iteration;
           }
         }
@@ -250,10 +292,10 @@ void MTPGraph::find_shortest_path() {
     _new_front = _front;
     _front = tmp_front;
 
-    tmp_front_size = _new_front_size;
-    _new_front_size = _front_size;
-    _front_size = tmp_front_size;
-  } while(_front_size > 0);
+    tmp_front_size = new_front_size;
+    new_front_size = front_size;
+    front_size = tmp_front_size;
+  } while(front_size > 0);
 }
 
 void MTPGraph::find_best_paths(scalar_t *lengths) {
@@ -267,6 +309,9 @@ void MTPGraph::find_best_paths(scalar_t *lengths) {
     _edges[e].positivized_length = _edges[e].length;
   }
 
+  // Let's be a bit paranoid
+  ASSERT(is_dag());
+
   // We use one iteration of find_shortest_path simply to propagate
   // the distance to make all the edge lengths positive.
   find_shortest_path();
diff --git a/mtp_graph.h b/mtp_graph.h
index 916b604..945f387 100644
--- a/mtp_graph.h
+++ b/mtp_graph.h
@@ -45,6 +45,8 @@ class MTPGraph {
   // Follows the path starting on edge e and returns its length. If
   // nodes is non-null, stores in it the nodes met along the path.
   int retrieve_one_path(Edge *e, Path *path);
+  // Returns if the graph is a DAG
+  int is_dag();
 
   Vertex **_front, **_new_front;
 
-- 
2.39.5