Add penalties for turns to avoid staircasing in A* algorithm

src/finders/AStarFinder.js

@@ -16,6 +16,10 @@ var DiagonalMovement = require('../core/DiagonalMovement');
  *     (defaults to manhattan).
  * @param {number} opt.weight Weight to apply to the heuristic to allow for
  *     suboptimal paths, in order to speed up the search.
+ * @param {number} opt.avoidStarcasing Add penalties to discourage turning and
+ * causing a 'staircase' effect (defaults to false).
+ * @param {number} opt.turnPenalty Penalty to add to turning. Higher numbers
+ * discourage turning more (defaults to 1).
  */
 function AStarFinder(opt) {
     opt = opt || {};
@@ -24,6 +28,8 @@ function AStarFinder(opt) {
     this.heuristic = opt.heuristic || Heuristic.manhattan;
     this.weight = opt.weight || 1;
     this.diagonalMovement = opt.diagonalMovement;
+    this.avoidStaircase = opt.avoidStaircase;
+    this.turnPenalty = opt.turnPenalty || 1;
 
     if (!this.diagonalMovement) {
         if (!this.allowDiagonal) {
@@ -59,9 +65,11 @@ AStarFinder.prototype.findPath = function(startX, startY, endX, endY, grid) {
         endNode = grid.getNodeAt(endX, endY),
         heuristic = this.heuristic,
         diagonalMovement = this.diagonalMovement,
+        avoidStaircase = this.avoidStaircase,
+        turnPenalty = this.turnPenalty,
         weight = this.weight,
         abs = Math.abs, SQRT2 = Math.SQRT2,
-        node, neighbors, neighbor, i, l, x, y, ng;
+        lastDirection, node, neighbors, neighbor, i, l, x, y, ng;
 
     // set the `g` and `f` value of the start node to be 0
     startNode.g = 0;
@@ -98,6 +106,14 @@ AStarFinder.prototype.findPath = function(startX, startY, endX, endY, grid) {
             // and calculate the next g score
             ng = node.g + ((x - node.x === 0 || y - node.y === 0) ? 1 : SQRT2);
 
+            // if we're avoiding staircasing, add penalties if the direction 
+            // will change
+            if (avoidStaircase) {
+                lastDirection = node.parent === undefined? undefined : { x : node.x - node.parent.x, y : node.y - node.parent.y };
+                var turned = lastDirection === undefined? 0 : lastDirection.x !== x - node.x || lastDirection.y !== y - node.y;
+                ng += turnPenalty * turned;
+            }
+
             // check if the neighbor has not been inspected yet, or
             // can be reached with smaller cost from the current node
             if (!neighbor.opened || ng < neighbor.g) {

visual/index.html

@@ -72,8 +72,14 @@ <h3>Options</h3>
             <label class="option_label">Bi-directional</label> <br>
             <input type="checkbox" class="dont_cross_corners">
             <label class="option_label">Don't Cross Corners</label> <br>
-            <input class="spinner" name="astar_weight" value="1">
+            <input class="spinner astar_weight" name="astar_weight" value="1">
             <label class="option_label">Weight</label> <br>
+            <br>
+            <input type="checkbox" class="avoid_staircase">
+            <label class="option_label">Avoid staircasing</label> <br>
+            <input class="spinner turn_penalty" name="turn_penalty" value="1">
+            <label class="option_label">Turn Penalty</label> <br>
+
           </div>
         </div>
 

visual/js/panel.js

@@ -41,9 +41,15 @@ var Panel = {
                                      '.bi-directional:checked').val() !=='undefined';
             dontCrossCorners = typeof $('#astar_section ' +
                                      '.dont_cross_corners:checked').val() !=='undefined';
+            avoidStaircase = typeof $('#astar_section ' +
+                                     '.avoid_staircase:checked').val() !=='undefined';
 
             /* parseInt returns NaN (which is falsy) if the string can't be parsed */
-            weight = parseInt($('#astar_section .spinner').val()) || 1;
+            turnPenalty = parseInt($('#astar_section .turn_penalty').val()) || 1;
+            turnPenalty = turnPenalty >= 1 ? turnPenalty : 1; /* if negative or 0, use 1 */
+
+            /* parseInt returns NaN (which is falsy) if the string can't be parsed */
+            weight = parseInt($('#astar_section .astar_weight').val()) || 1;
             weight = weight >= 1 ? weight : 1; /* if negative or 0, use 1 */
 
             heuristic = $('input[name=astar_heuristic]:checked').val();
@@ -59,7 +65,9 @@ var Panel = {
                     allowDiagonal: allowDiagonal,
                     dontCrossCorners: dontCrossCorners,
                     heuristic: PF.Heuristic[heuristic],
-                    weight: weight
+                    weight: weight,
+                    avoidStaircase: avoidStaircase,
+                    turnPenalty: turnPenalty
                 });
             }
             break;