X-Git-Url: https://www.fleuret.org/cgi-bin/gitweb/gitweb.cgi?a=blobdiff_plain;f=mtp_graph.cc;h=ea8f91b93cd974e2c73a4e1dc08ac4bcca5f0bbb;hb=8a3940ee003a04bac70679bdd7b3e2e8bc45ae37;hp=4a73a439acb8cf35c0c509547a0a71163c11acc8;hpb=7f847d18f82e92c8af842f50e0ea16fa66bf9584;p=mtp.git

diff --git a/mtp_graph.cc b/mtp_graph.cc
index 4a73a43..ea8f91b 100644
--- a/mtp_graph.cc
+++ b/mtp_graph.cc
@@ -46,16 +46,14 @@ public:
   Edge *leaving_edges;
   scalar_t distance_from_source;
   Edge *pred_edge_toward_source;
-
-  int last_change; // Used to mark which edges have already been
-                   // processed in some methods
-
-  Vertex **heap_position;
+  Vertex **heap_slot;
 
   Vertex();
 
   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);
 };
 
 //////////////////////////////////////////////////////////////////////
@@ -65,9 +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);
 }
 
 //////////////////////////////////////////////////////////////////////
@@ -95,10 +91,51 @@ 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) {
-  return (*((Vertex **) v1))->last_change - (*((Vertex **) v2))->last_change;
+  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;
 }
 
 MTPGraph::MTPGraph(int nb_vertices, int nb_edges,
@@ -124,16 +161,16 @@ MTPGraph::MTPGraph(int nb_vertices, int nb_edges,
 
   for(int v = 0; v < _nb_vertices; v++) {
     _heap[v] = &_vertices[v];
-    _vertices[v].heap_position = &_heap[v];
+    _vertices[v].heap_slot = &_heap[v];
   }
 
   paths = 0;
   nb_paths = 0;
 
-  if(check_DAG_and_set_last_change()) {
-    // Here the last_change field of every vertex tells us how many
-    // iterations of DP we need to reach it. Hence we only have to
-    // process the vertex in that order.
+  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.
     for(int v = 0; v < _nb_vertices; v++) { _dp_order[v] = &_vertices[v]; }
     qsort(_dp_order, _nb_vertices, sizeof(Vertex *), compare_vertex);
   } else {
@@ -151,6 +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 **active = new Vertex *[_nb_vertices];
+
+  // All the nodes are active at first
+  for(int k = 0; k < _nb_vertices; k++) {
+    _vertices[k].distance_from_source = 0;
+    active[k] = &_vertices[k];
+  }
+
+  scalar_t rank = 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 rank 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 = rank;
+      }
+    }
+
+    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 == rank) {
+        active[nb_active++] = v;
+      }
+    }
+
+    rank++;
+  } while(nb_active < pred_nb_active);
+
+  delete[] active;
+
+  return nb_active == 0;
+}
+
 //////////////////////////////////////////////////////////////////////
 
 void MTPGraph::print(ostream *os) {
@@ -211,109 +302,18 @@ void MTPGraph::force_positivized_lengths() {
     if(e->positivized_length < 0) {
 
 #ifdef VERBOSE
-      if((e->origin_vertex->last_change < 0 && e->terminal_vertex->last_change >= 0) ||
-         (e->origin_vertex->last_change >= 0 && e->terminal_vertex->last_change < 0)) {
-        cout << "Inconsistent non-connexity (this should never happen)." << endl;
-        abort();
-      }
-      if(e->origin_vertex->last_change >= 0 &&
-         e->terminal_vertex->last_change >= 0 &&
-         e->positivized_length < 0) {
-        residual_error -= e->positivized_length;
-        max_error = max(max_error, - e->positivized_length);
-      }
+      residual_error -= e->positivized_length;
+      max_error = max(max_error, - e->positivized_length);
 #endif
       e->positivized_length = 0.0;
     }
   }
 #ifdef VERBOSE
-  cerr << "residual_error " << residual_error << " max_error " << residual_error << endl;
+  cerr << __FILE__ << ": residual_error " << residual_error << " max_error " << residual_error << endl;
 #endif
 }
 
-int MTPGraph::check_DAG_and_set_last_change() {
-  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].last_change = 0;
-    active[k] = &_vertices[k];
-  }
-
-  int iteration = 1;
-  int nb_active = _nb_vertices, pred_nb_active;
-
-  do {
-    // We set the last_change 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->last_change = 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->last_change == 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_position;
-  while(h > _heap &&
-        (p = _heap + (h - _heap + 1) / 2 - 1,
-         (*p)->distance_from_source > (*h)->distance_from_source)) {
-    swap(*p, *h);
-    swap((*p)->heap_position, (*h)->heap_position);
-    h = p;
-  }
-}
-
-void MTPGraph::increase_distance_in_heap(Vertex *v) {
-  Vertex **c1, **c2, **h;
-  // There is some beauty in that
-  h = v->heap_position;
-  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_position, (*h)->heap_position);
-      h = c1;
-    } else {
-      swap(*c2, *h);
-      swap((*c2)->heap_position, (*h)->heap_position);
-      h = c2;
-    }
-  }
-}
-
-void MTPGraph::dp_distance_propagation() {
+void MTPGraph::dp_compute_distances() {
   Vertex *v, *tv;
   Edge *e;
   scalar_t d;
@@ -333,7 +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);
       }
     }
   }
@@ -355,7 +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
@@ -366,18 +366,18 @@ void MTPGraph::find_shortest_path() {
     _heap_size--;
     a = _heap;
     b = _heap + _heap_size;
-    swap(*a, *b); swap((*a)->heap_position, (*b)->heap_position);
-    increase_distance_in_heap(_heap[0]);
+    swap(*a, *b); swap((*a)->heap_slot, (*b)->heap_slot);
+    _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) {
       d = v->distance_from_source + e->positivized_length;
       tv = e->terminal_vertex;
       if(d < tv->distance_from_source) {
-        ASSERT(tv->heap_position - _heap < _heap_size);
+        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);
@@ -394,13 +394,18 @@ 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;
@@ -417,7 +422,7 @@ void MTPGraph::find_best_paths(scalar_t *lengths) {
       // If that length is negative
       if(total_length < 0.0) {
 #ifdef VERBOSE
-        cerr << "Found a path of length " << total_length << endl;
+        cerr << __FILE__ << ": Found a path of length " << total_length << endl;
 #endif
         // Invert all the edges along the best path
         v = _sink;
@@ -464,12 +469,12 @@ int MTPGraph::retrieve_one_path(Edge *e, Path *path) {
 
 #ifdef DEBUG
     if(nb_occupied_next == 0) {
-      cerr << "retrieve_one_path: Non-sink end point." << endl;
+      cerr << __FILE__ << ": retrieve_one_path: Non-sink end point." << endl;
       abort();
     }
 
     else if(nb_occupied_next > 1) {
-      cerr << "retrieve_one_path: Non node-disjoint paths." << endl;
+      cerr << __FILE__ << ": retrieve_one_path: Non node-disjoint paths." << endl;
       abort();
     }
 #endif