X-Git-Url: https://www.fleuret.org/cgi-bin/gitweb/gitweb.cgi?a=blobdiff_plain;f=mtp_graph.cc;h=5a7da77d86892df1bf28ad38a94ccfdfd99a9c48;hb=c752952df0dd1d781ec80f25c4b9418bddac1b9e;hp=aa294b43bcd6856bd38de8ce945b0588f42b492b;hpb=14e3b33cbe1e0d7deb0b4aa697f6c8b5d43e2963;p=mtp.git

diff --git a/mtp_graph.cc b/mtp_graph.cc
index aa294b4..5a7da77 100644
--- a/mtp_graph.cc
+++ b/mtp_graph.cc
@@ -52,6 +52,8 @@ public:
 
   inline void add_leaving_edge(Edge *e);
   inline void del_leaving_edge(Edge *e);
+  inline void decrease_distance_in_heap(Vertex **heap);
+  inline void increase_distance_in_heap(Vertex **heap, Vertex **heap_bottom);
 };
 
 //////////////////////////////////////////////////////////////////////
@@ -61,9 +63,7 @@ void Edge::invert() {
   positivized_length = - positivized_length;
   origin_vertex->del_leaving_edge(this);
   terminal_vertex->add_leaving_edge(this);
-  Vertex *t = terminal_vertex;
-  terminal_vertex = origin_vertex;
-  origin_vertex = t;
+  swap(terminal_vertex, origin_vertex);
 }
 
 //////////////////////////////////////////////////////////////////////
@@ -91,6 +91,42 @@ void Vertex::del_leaving_edge(Edge *e) {
   }
 }
 
+void Vertex::decrease_distance_in_heap(Vertex **heap) {
+  Vertex **p, **h;
+  // There is some beauty in that
+  h = heap_slot;
+  while(h > heap &&
+        (p = heap + (h - heap + 1) / 2 - 1,
+         (*p)->distance_from_source > (*h)->distance_from_source)) {
+    swap(*p, *h);
+    swap((*p)->heap_slot, (*h)->heap_slot);
+    h = p;
+  }
+}
+
+void Vertex::increase_distance_in_heap(Vertex **heap, Vertex **heap_bottom) {
+  Vertex **c1, **c2, **h;
+  // omg, that's beautiful
+  h = heap_slot;
+  while(c1 = heap + 2 * (h - heap) + 1,
+        c1 < heap_bottom &&
+        (c2 = c1 + 1,
+         (*c1)->distance_from_source < (*h)->distance_from_source
+         ||
+         (c2 < heap_bottom && (*c2)->distance_from_source < (*h)->distance_from_source)
+         )) {
+    if(c2 < heap_bottom && (*c2)->distance_from_source <= (*c1)->distance_from_source) {
+      swap(*c2, *h);
+      swap((*c2)->heap_slot, (*h)->heap_slot);
+      h = c2;
+    } else {
+      swap(*c1, *h);
+      swap((*c1)->heap_slot, (*h)->heap_slot);
+      h = c1;
+    }
+  }
+}
+
 //////////////////////////////////////////////////////////////////////
 
 static int compare_vertex(const void *v1, const void *v2) {
@@ -131,7 +167,7 @@ MTPGraph::MTPGraph(int nb_vertices, int nb_edges,
   paths = 0;
   nb_paths = 0;
 
-  if(compute_dp_distances()) {
+  if(compute_dp_ranks()) {
     // Here the distance_from_source field of every vertex is the
     // number of DP iterations needed to update it. Hence we only have
     // to process the vertex in that order.
@@ -152,6 +188,60 @@ MTPGraph::~MTPGraph() {
   delete[] paths;
 }
 
+int MTPGraph::compute_dp_ranks() {
+  Vertex *v;
+  Edge *e;
+
+  // 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
+  // successively nodes without predecessor: At the first iteration it
+  // removes the source, then the nodes with incoming edge only from
+  // the source, etc. If it can remove all the nodes that way, the
+  // graph is a DAG. If at some point it can not remove node anymore
+  // and there are some remaining nodes, the graph is not a DAG. The
+  // rank of a node is the iteration at which is it removed, and we
+  // set the distance_from_source fields to this value.
+
+  Vertex **unreached = new Vertex *[_nb_vertices];
+
+  // All the nodes are unreached at first
+  for(int k = 0; k < _nb_vertices; k++) {
+    _vertices[k].distance_from_source = 0;
+    unreached[k] = &_vertices[k];
+  }
+
+  scalar_t rank = 1;
+  int nb_unreached = _nb_vertices, pred_nb_unreached;
+
+  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_unreached; f++) {
+      v = unreached[f];
+      for(e = v->leaving_edges; e; e = e->next_leaving_edge) {
+        e->terminal_vertex->distance_from_source = rank;
+      }
+    }
+
+    pred_nb_unreached = nb_unreached;
+    nb_unreached = 0;
+
+    // We keep all the vertices with incoming nodes
+    for(int f = 0; f < pred_nb_unreached; f++) {
+      v = unreached[f];
+      if(v->distance_from_source == rank) {
+        unreached[nb_unreached++] = v;
+      }
+    }
+
+    rank++;
+  } while(nb_unreached < pred_nb_unreached);
+
+  delete[] unreached;
+
+  return nb_unreached == 0;
+}
+
 //////////////////////////////////////////////////////////////////////
 
 void MTPGraph::print(ostream *os) {
@@ -223,86 +313,7 @@ void MTPGraph::force_positivized_lengths() {
 #endif
 }
 
-int MTPGraph::compute_dp_distances() {
-  Vertex *v;
-  Edge *e;
-
-  Vertex **active = new Vertex *[_nb_vertices];
-
-  // We put everybody in the active
-  for(int k = 0; k < _nb_vertices; k++) {
-    _vertices[k].distance_from_source = 0;
-    active[k] = &_vertices[k];
-  }
-
-  int iteration = 1;
-  int nb_active = _nb_vertices, pred_nb_active;
-
-  do {
-    // We set the distance_from_source field of all the vertices with incoming
-    // edges to the current iteration value
-    for(int f = 0; f < nb_active; f++) {
-      v = active[f];
-      for(e = v->leaving_edges; e; e = e->next_leaving_edge) {
-        e->terminal_vertex->distance_from_source = iteration;
-      }
-    }
-
-    pred_nb_active = nb_active;
-    nb_active = 0;
-
-    // We keep all the vertices with incoming nodes
-    for(int f = 0; f < pred_nb_active; f++) {
-      v = active[f];
-      if(v->distance_from_source == iteration) {
-        active[nb_active++] = v;
-      }
-    }
-
-    iteration++;
-  } while(nb_active < pred_nb_active);
-
-  delete[] active;
-
-  return nb_active == 0;
-}
-
-void MTPGraph::decrease_distance_in_heap(Vertex *v) {
-  Vertex **p, **h;
-  // There is some beauty in that
-  h = v->heap_slot;
-  while(h > _heap &&
-        (p = _heap + (h - _heap + 1) / 2 - 1,
-         (*p)->distance_from_source > (*h)->distance_from_source)) {
-    swap(*p, *h);
-    swap((*p)->heap_slot, (*h)->heap_slot);
-    h = p;
-  }
-}
-
-void MTPGraph::increase_distance_in_heap(Vertex *v) {
-  Vertex **c1, **c2, **h;
-  // There is some beauty in that
-  h = v->heap_slot;
-  while(c1 = _heap + 2 * (h - _heap + 1) - 1, c2 = c1 + 1,
-        (c1 < _heap + _heap_size && (*c1)->distance_from_source < (*h)->distance_from_source)
-        ||
-        (c2 < _heap + _heap_size && (*c2)->distance_from_source < (*h)->distance_from_source)
-        ) {
-    if(c1 < _heap + _heap_size &&
-     !(c2 < _heap + _heap_size && (*c2)->distance_from_source < (*c1)->distance_from_source)){
-      swap(*c1, *h);
-      swap((*c1)->heap_slot, (*h)->heap_slot);
-      h = c1;
-    } else {
-      swap(*c2, *h);
-      swap((*c2)->heap_slot, (*h)->heap_slot);
-      h = c2;
-    }
-  }
-}
-
-void MTPGraph::dp_distance_propagation() {
+void MTPGraph::dp_compute_distances() {
   Vertex *v, *tv;
   Edge *e;
   scalar_t d;
@@ -322,7 +333,7 @@ void MTPGraph::dp_distance_propagation() {
       if(d < tv->distance_from_source) {
         tv->distance_from_source = d;
         tv->pred_edge_toward_source = e;
-        decrease_distance_in_heap(tv);
+        tv->decrease_distance_in_heap(_heap);
       }
     }
   }
@@ -344,7 +355,7 @@ void MTPGraph::find_shortest_path() {
 
   _heap_size = _nb_vertices;
   _source->distance_from_source = 0;
-  decrease_distance_in_heap(_source);
+  _source->decrease_distance_in_heap(_heap);
 
   do {
     // Get the closest to the source
@@ -356,7 +367,7 @@ void MTPGraph::find_shortest_path() {
     a = _heap;
     b = _heap + _heap_size;
     swap(*a, *b); swap((*a)->heap_slot, (*b)->heap_slot);
-    increase_distance_in_heap(_heap[0]);
+    _heap[0]->increase_distance_in_heap(_heap, _heap + _heap_size);
 
     // Now update the neighbors of the currently closest to the source
     for(e = v->leaving_edges; e; e = e->next_leaving_edge) {
@@ -366,14 +377,14 @@ void MTPGraph::find_shortest_path() {
         ASSERT(tv->heap_slot - _heap < _heap_size);
         tv->distance_from_source = d;
         tv->pred_edge_toward_source = e;
-        decrease_distance_in_heap(tv);
+        tv->decrease_distance_in_heap(_heap);
       }
     }
   } while(_heap_size > 0);
 }
 
 void MTPGraph::find_best_paths(scalar_t *lengths) {
-  scalar_t total_length;
+  scalar_t shortest_path_length;
   Vertex *v;
   Edge *e;
 
@@ -383,16 +394,21 @@ void MTPGraph::find_best_paths(scalar_t *lengths) {
     _edges[e].positivized_length = _edges[e].length;
   }
 
-  // Update the distance to the source in "good order"
-
-  dp_distance_propagation();
+  // Compute the distance of every node from the source by just
+  // visiting them in the proper DAG ordering we computed when
+  // building the graph
+  dp_compute_distances();
 
   do {
+    // Use the current distance from the source to make all edge
+    // lengths positive
     update_positivized_lengths();
+    // Fix numerical errors
     force_positivized_lengths();
+
     find_shortest_path();
 
-    total_length = 0.0;
+    shortest_path_length = 0.0;
 
     // Do we reach the sink?
     if(_sink->pred_edge_toward_source) {
@@ -400,13 +416,13 @@ void MTPGraph::find_best_paths(scalar_t *lengths) {
       // original edge lengths
       v = _sink;
       while(v->pred_edge_toward_source) {
-        total_length += v->pred_edge_toward_source->length;
+        shortest_path_length += v->pred_edge_toward_source->length;
         v = v->pred_edge_toward_source->origin_vertex;
       }
       // If that length is negative
-      if(total_length < 0.0) {
+      if(shortest_path_length < 0.0) {
 #ifdef VERBOSE
-        cerr << __FILE__ << ": Found a path of length " << total_length << endl;
+        cerr << __FILE__ << ": Found a path of length " << shortest_path_length << endl;
 #endif
         // Invert all the edges along the best path
         v = _sink;
@@ -421,7 +437,7 @@ void MTPGraph::find_best_paths(scalar_t *lengths) {
       }
     }
 
-  } while(total_length < 0.0);
+  } while(shortest_path_length < 0.0);
 
   // Put back the graph in its original state (i.e. invert edges which
   // have been inverted in the process)