Implement alpha beta pruning.

Will have to test once evaluator is functioning.
joesweeney · Apr 16, 2017 · b7d4203 · b7d4203
1 parent ad0a564
commit b7d4203
Showing 1 changed file with 58 additions and 2 deletions.
diff --git a/src/CheckersAI.java b/src/CheckersAI.java
@@ -7,9 +7,65 @@ public class CheckersAI{
         this.eval = e;
     }
 
-    public Move minimax(CheckersGameState s){
+    public Move minimax(CheckersGameState s, int ply){
+        double alpha = Double.POSITIVE_INFINITY;
+        double beta = Double.NEGATIVE_INFINITY;
+        double v = Double.NEGATIVE_INFINITY;
+        double check;
+        Move max;
+        for(Move a: s.actions()){
+            check = minValue(s.result(a), alpha, beta, ply, depth + 1);
+            if(check > v){
+                v = check;
+                max = a;
+            }
+            if(v > beta){
+                return max;
+            }
+            alpha = Math.max(alpha, v);
+        }
+        returm max;
+    }
+
+   private double maxValue(CheckersGameState s, double alpha, double beta, int ply, int depth){
+
+      if(s.isTerminal() || depth == ply){
+         return eval.evaluate(s); // if terminal, piece ratio should be infinite
+        }
+        double v = Double.NEGATIVE_INFINITY;
+        double check;
+        for(Move a: s.actions()){
+            check = minValue(s.result(a), alpha, beta, ply, depth + 1);
+            if(check > v){
+                v = check;
+            }
+            if (v >= beta){
+               return v;
+            }
+            alpha  = Math.max(alpha, v);
+            }
+        return v;
+    }
+
+   private double minValue(CheckersGameState s, double alpha, double beta, int ply, int depth){
 
-        return null;
+       if(s.isTerminal() || depth == ply){
+           return eval.evaluate(s);
+        }
+       double v = Double.POSITIVE_INFINITY;
+       double check;
+       for(Move a: s.actions()){
+           check = maxValue(s.result(a), alpha, beta, ply, depth + 1);
+           if(check < v){
+               v = check;
+            }
+           if( v <= alpha){
+               return v;
+            }
+           beta = Math.min(beta, v);
+        }
+       return v;
     }
 
+
 }