new recursive backtracking builder

script.js

@@ -1214,7 +1214,8 @@ class Game {
   }
 
   placeOrbs(template) {
-    let placer = new RadialOrbPlacer(this, template);
+    // let placer = new RadialOrbPlacer(this, template);
+    let placer = new RecursiveOrbPlacer(this, template);
     return placer.place();
   }
 
@@ -1960,6 +1961,78 @@ class RadialOrbPlacer extends BaseOrbPlacer {
   }
 }
 
+class RecursiveOrbPlacer extends BaseOrbPlacer {
+  doPlace(toPlace) {
+    this.toPlace = toPlace;
+    this.recDepth = 0;
+
+    this.placeOne(0);
+
+    return {};
+  }
+
+  placeOne() {
+    this.recDepth++;
+    if (this.recDepth > 2000) {
+      throw new Error("Too many backtracks");
+    }
+
+    let symbol = this.toPlace.pop();
+
+    let candidates = [[],[],[],[],[],[],[]];
+    for (let n of this.template) {
+      if (!this.board.grid[n]) {
+        // is free
+        const cell = this.getCellInfo(n);
+        if (cell.freeSequence >= 3) {
+          candidates[cell.neighbours].push(n);
+        }
+      }
+    }
+
+    for (let neighs = 6; neighs >= 0; neighs--) {
+      this.rng.arrayShuffle(candidates[neighs]);
+    }
+
+    for (let i = 6; i >= 0; i--) {
+      if (candidates[i].length) {
+        for (let n of candidates[i]) {
+          this.placeOrb(n, symbol);
+
+          // avoid deadlocking
+          if (this.solution.length % 2 === 0) {
+            // this is the second orb in a pair (solution starts with gold)
+            let prevn = this.solution[this.solution.length-1][1];
+            if (!this.isAvailable(prevn)) {
+              this.trace("Avoid deadlock (this is the second of a pair)");
+              this.removeOrb(n);
+              continue;
+            }
+          }
+
+          this.solution.push([symbol, n]);
+
+          if (this.toPlace.length) {
+            if (this.placeOne()) {
+              return true;
+            } else {
+              this.trace("Undo and continue");
+              this.removeOrb(n);
+              this.solution.pop();
+              // and continue the iteration
+            }
+          } else {
+            return true;
+          }
+        }
+      }
+    }
+
+    // give it back
+    this.toPlace.push(symbol);
+    return false;
+  }
+}
 
 /* Start */