gcc/shortest-paths.h

   1 /* Template class for Dijkstra's algorithm on directed graphs.
   2    Copyright (C) 2019-2021 Free Software Foundation, Inc.
   3    Contributed by David Malcolm <dmalcolm@redhat.com>.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it
   8 under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15 General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #ifndef GCC_SHORTEST_PATHS_H
  22 #define GCC_SHORTEST_PATHS_H
  23
  24 #include "timevar.h"
  25
  26 enum shortest_path_sense
  27 {
  28   /* Find the shortest path from the given origin node to each
  29      node in the graph.  */
  30   SPS_FROM_GIVEN_ORIGIN,
  31
  32   /* Find the shortest path from each node in the graph to the
  33      given target node.  */
  34   SPS_TO_GIVEN_TARGET
  35 };
  36
  37 /* A record of the shortest path for each node relative to a special
  38    "given node", either:
  39    SPS_FROM_GIVEN_ORIGIN:
  40      from the given origin node to each node in a graph, or
  41    SPS_TO_GIVEN_TARGET:
  42      from each node in a graph to the given target node.
  43
  44    The constructor runs Dijkstra's algorithm, and the results are
  45    stored in this class.  */
  46
  47 template <typename GraphTraits, typename Path_t>
  48 class shortest_paths
  49 {
  50 public:
  51   typedef typename GraphTraits::graph_t graph_t;
  52   typedef typename GraphTraits::node_t node_t;
  53   typedef typename GraphTraits::edge_t edge_t;
  54   typedef Path_t path_t;
  55
  56   shortest_paths (const graph_t &graph, const node_t *given_node,
  57                   enum shortest_path_sense sense);
  58
  59   path_t get_shortest_path (const node_t *other_node) const;
  60   int get_shortest_distance (const node_t *other_node) const;
  61
  62 private:
  63   const graph_t &m_graph;
  64
  65   enum shortest_path_sense m_sense;
  66
  67   /* For each node (by index), the minimal distance between that node
  68      and the given node (with direction depending on m_sense).  */
  69   auto_vec<int> m_dist;
  70
  71   /* For each node (by index):
  72      SPS_FROM_GIVEN_ORIGIN:
  73        the previous edge in the shortest path from the origin,
  74      SPS_TO_GIVEN_TARGET:
  75        the next edge in the shortest path to the target.  */
  76   auto_vec<const edge_t *> m_best_edge;
  77 };
  78
  79 /* shortest_paths's constructor.
  80
  81    Use Dijkstra's algorithm relative to GIVEN_NODE to populate m_dist and
  82    m_best_edge with enough information to be able to generate Path_t instances
  83    to give the shortest path...
  84    SPS_FROM_GIVEN_ORIGIN: to each node in a graph from the origin node, or
  85    SPS_TO_GIVEN_TARGET: from each node in a graph to the target node.  */
  86
  87 template <typename GraphTraits, typename Path_t>
  88 inline
  89 shortest_paths<GraphTraits, Path_t>::
  90 shortest_paths (const graph_t &graph,
  91                 const node_t *given_node,
  92                 enum shortest_path_sense sense)
  93 : m_graph (graph),
  94   m_sense (sense),
  95   m_dist (graph.m_nodes.length ()),
  96   m_best_edge (graph.m_nodes.length ())
  97 {
  98   auto_timevar tv (TV_ANALYZER_SHORTEST_PATHS);
  99
 100   auto_vec<int> queue (graph.m_nodes.length ());
 101
 102   for (unsigned i = 0; i < graph.m_nodes.length (); i++)
 103     {
 104       m_dist.quick_push (INT_MAX);
 105       m_best_edge.quick_push (NULL);
 106       queue.quick_push (i);
 107     }
 108   m_dist[given_node->m_index] = 0;
 109
 110   while (queue.length () > 0)
 111     {
 112       /* Get minimal distance in queue.
 113          FIXME: this is O(N^2); replace with a priority queue.  */
 114       int idx_with_min_dist = -1;
 115       int idx_in_queue_with_min_dist = -1;
 116       int min_dist = INT_MAX;
 117       for (unsigned i = 0; i < queue.length (); i++)
 118         {
 119           int idx = queue[i];
 120           if (m_dist[queue[i]] < min_dist)
 121             {
 122               min_dist = m_dist[idx];
 123               idx_with_min_dist = idx;
 124               idx_in_queue_with_min_dist = i;
 125             }
 126         }
 127       if (idx_with_min_dist == -1)
 128         break;
 129       gcc_assert (idx_in_queue_with_min_dist != -1);
 130
 131       // FIXME: this is confusing: there are two indices here
 132
 133       queue.unordered_remove (idx_in_queue_with_min_dist);
 134
 135       node_t *n
 136         = static_cast <node_t *> (m_graph.m_nodes[idx_with_min_dist]);
 137
 138       if (m_sense == SPS_FROM_GIVEN_ORIGIN)
 139         {
 140           int i;
 141           edge_t *succ;
 142           FOR_EACH_VEC_ELT (n->m_succs, i, succ)
 143             {
 144               // TODO: only for dest still in queue
 145               node_t *dest = succ->m_dest;
 146               int alt = m_dist[n->m_index] + 1;
 147               if (alt < m_dist[dest->m_index])
 148                 {
 149                   m_dist[dest->m_index] = alt;
 150                   m_best_edge[dest->m_index] = succ;
 151                 }
 152             }
 153         }
 154       else
 155         {
 156           int i;
 157           edge_t *pred;
 158           FOR_EACH_VEC_ELT (n->m_preds, i, pred)
 159             {
 160               // TODO: only for dest still in queue
 161               node_t *src = pred->m_src;
 162               int alt = m_dist[n->m_index] + 1;
 163               if (alt < m_dist[src->m_index])
 164                 {
 165                   m_dist[src->m_index] = alt;
 166                   m_best_edge[src->m_index] = pred;
 167                 }
 168             }
 169         }
 170    }
 171 }
 172
 173 /* Generate an Path_t instance giving the shortest path between OTHER_NODE
 174    and the given node.
 175
 176    SPS_FROM_GIVEN_ORIGIN: shortest path from given origin node to OTHER_NODE
 177    SPS_TO_GIVEN_TARGET: shortest path from OTHER_NODE to given target node.
 178
 179    If no such path exists, return an empty path.  */
 180
 181 template <typename GraphTraits, typename Path_t>
 182 inline Path_t
 183 shortest_paths<GraphTraits, Path_t>::
 184 get_shortest_path (const node_t *other_node) const
 185 {
 186   Path_t result;
 187
 188   while (m_best_edge[other_node->m_index])
 189     {
 190       result.m_edges.safe_push (m_best_edge[other_node->m_index]);
 191       if (m_sense == SPS_FROM_GIVEN_ORIGIN)
 192         other_node = m_best_edge[other_node->m_index]->m_src;
 193       else
 194         other_node = m_best_edge[other_node->m_index]->m_dest;
 195     }
 196
 197   if (m_sense == SPS_FROM_GIVEN_ORIGIN)
 198     result.m_edges.reverse ();
 199
 200   return result;
 201 }
 202
 203 /* Get the shortest distance...
 204    SPS_FROM_GIVEN_ORIGIN: ...from given origin node to OTHER_NODE
 205    SPS_TO_GIVEN_TARGET: ...from OTHER_NODE to given target node.  */
 206
 207 template <typename GraphTraits, typename Path_t>
 208 inline int
 209 shortest_paths<GraphTraits, Path_t>::
 210 get_shortest_distance (const node_t *other_node) const
 211 {
 212   return m_dist[other_node->m_index];
 213 }
 214
 215 #endif /* GCC_SHORTEST_PATHS_H */