perf: alpha-beta pruning

src/search.rs

@@ -20,9 +20,34 @@ use std::cmp::max;
 
 // min can't be represented as positive
 const EVAL_WORST: EvalInt = -(EvalInt::MAX);
+const EVAL_BEST: EvalInt = EvalInt::MAX;
+
+#[cfg(test)]
+mod test_eval_int {
+    use super::*;
+
+    #[test]
+    fn test_eval_worst_best_symm() {
+        // int limits will bite you if you don't test this
+        assert_eq!(EVAL_WORST, -EVAL_BEST);
+        assert_eq!(-EVAL_WORST, EVAL_BEST);
+    }
+}
 
 /// Search the game tree to find the absolute (positive good) eval for the current player.
-fn minmax(board: &mut Board, depth: usize) -> EvalInt {
+///
+/// # Arguments
+///
+/// * board: board position to analyze.
+/// * depth: how deep to analyze the game tree.
+/// * alpha: best score (absolute, from current player perspective) guaranteed for current player.
+/// * beta: best score (absolute, from current player perspective) guaranteed for other player.
+fn minmax(board: &mut Board, depth: usize, alpha: Option<EvalInt>, beta: Option<EvalInt>) -> EvalInt {
+    // default to worst, then gradually improve
+    let mut alpha = alpha.unwrap_or(EVAL_WORST);
+    // our best is their worst
+    let beta = beta.unwrap_or(EVAL_BEST);
+
     if depth == 0 {
         let eval = board.eval();
         match board.turn {
@@ -46,8 +71,19 @@ fn minmax(board: &mut Board, depth: usize) -> EvalInt {
 
     for mv in mvs {
         let anti_mv = mv.make(board);
-        abs_best = max(abs_best, -minmax(board, depth - 1));
+        let abs_score = -minmax(board, depth - 1, Some(-beta), Some(-alpha));
+        abs_best = max(abs_best, abs_score);
+        alpha = max(alpha, abs_best);
         anti_mv.unmake(board);
+        if alpha >= beta  {
+            // alpha-beta prune.
+            //
+            // Beta represents the best eval that the other player can get in sibling branches
+            // (different moves in the parent node). Alpha >= beta means the eval here is _worse_
+            // for the other player, so they will never make the move that leads into this branch.
+            // Therefore, we stop evaluating this branch at all.
+            break;
+        }
     }
 
     abs_best
@@ -64,7 +100,7 @@ fn search(board: &mut Board) -> Option<Move> {
 
     for mv in mvs {
         let anti_mv = mv.make(board);
-        let abs_eval = -minmax(board, DEPTH);
+        let abs_eval = -minmax(board, DEPTH, None, None);
         if abs_eval >= best_eval {
             best_eval = abs_eval;
             best_mv = Some(mv);