Bound the frontier. I.e., do beam search.

[slidingtile] / sliding_tile_lib.cc

diff --git a/sliding_tile_lib.cc b/sliding_tile_lib.cc
index 551bc1f018735e373fd03f0211ab7856a2625120..5a4114bd89c6ab621ddec79db2476ef8e0665494 100644 (file)
--- a/sliding_tile_lib.cc
+++ b/sliding_tile_lib.cc
@@ -189,7 +189,7 @@ std::ostream& operator<<(std::ostream& os, const Step& step) {
   return os;
 }
 
-std::shared_ptr<Step> find_path(const std::string& start, const std::string& goal) {
+std::shared_ptr<Step> find_path(const std::string& start, const std::string& goal, unsigned max_frontier) {
   std::istringstream iss_start{start}, iss_goal{goal};
   Board board_start, board_goal;
   iss_start >> board_start;
@@ -200,10 +200,10 @@ std::shared_ptr<Step> find_path(const std::string& start, const std::string& goa
   if (iss_goal.fail() || !iss_goal.eof()) {
     throw std::runtime_error("Could not parse the goal board: " + goal);
   }
-  return find_path(board_start, board_goal);
+  return find_path(board_start, board_goal, max_frontier);
 }
 
-std::shared_ptr<Step> find_path(const Board& start, const Board& goal) {
+std::shared_ptr<Step> find_path(const Board& start, const Board& goal, unsigned max_frontier) {
   InvertedBoard invgoal = goal.invert();
   std::multimap<int, std::shared_ptr<Step>> todo;
   std::set<Board> seen;
@@ -230,6 +230,9 @@ std::shared_ptr<Step> find_path(const Board& start, const Board& goal) {
         }
       }
     }
+    while (todo.size() > max_frontier) {
+      todo.erase(--todo.end());
+    }
   }
   throw std::runtime_error("No path from start to goal");
 }