Solving bad cache value, Added weak solver

1 files changed, 36 insertions, 25 deletions

diff --git a/src/solve.rs b/src/solve.rs
index b950ab6..9561ff1 100644
--- a/src/solve.rs
+++ b/src/solve.rs
@@ -1,18 +1,22 @@
 use std::collections::HashMap;
 
-use crate::position::{Position, WIDTH, HEIGHT};
+use crate::position::{Position, WIDTH, HEIGHT, MIN_SCORE};
 
 const COLUMNS_ORDER: [u64; 7] = [3, 2, 4, 1, 5, 0, 6];
 
 pub fn solve(p: Position) -> i32 {
     let mut cache = HashMap::with_capacity(30000);
-    solve_rec(p, -100000, 100000, &mut cache)
+    solve_rec(p, -1000, 1000, &mut cache)
+}
+
+// the weak solver only tells if the position is a win/lose/draw
+// it's faster but less precise
+pub fn solve_weak(p: Position) -> i32 {
+    let mut cache = HashMap::with_capacity(30000);
+    solve_rec(p, -1, 1, &mut cache)
 }
 
 fn solve_rec(p: Position, a: i32, b: i32, cache: &mut HashMap<u64, i32>) -> i32 {
-    if let Some(score) = cache.get(&p.key()) {
-        return *score;
-    }
     if p.is_draw() {
         return 0;
     }
@@ -25,32 +29,39 @@ fn solve_rec(p: Position, a: i32, b: i32, cache: &mut HashMap<u64, i32>) -> i32
     let mut alpha = a;
     let mut beta = b;
 
-    let max = (((WIDTH * HEIGHT + 1) as i32) - (p.play_count as i32)) / 2;
-
-    if beta > max {
-        beta = max;
-        if alpha >= beta {
-            return beta;
+    if let Some(max_score) = cache.get(&p.key()) {
+        // can't return max_score directly
+        // because the alpha-beta context in the cache may be
+        // different than the current alpha-beta
+        if beta > *max_score {
+            beta = *max_score;
+            if alpha >= beta {
+                return beta;
+            }
         }
     }
 
-    for x_unordered in 0..(WIDTH as usize) {
-        let x = COLUMNS_ORDER[x_unordered];
-        if !p.is_valid_play(x) {
-            continue
-        }
-
+    let mut best = MIN_SCORE;
+    for x in (0..(WIDTH as usize))
+                .map(|x| COLUMNS_ORDER[x])
+                .filter(|x| p.is_valid_play(*x))
+    {
+        // using negamax, variante of minimax where:
+        // max(player1, player2) == -min(-player1, -player2)
         let score = -solve_rec(p.play(x), -beta, -alpha, cache);
-
-        if score >= beta {
-            return score;
+        if score > best {
+            best = score;
         }
-
-        if score > alpha {
-            alpha = score;
+        // reduce alpha-beta range if found better score
+        if best > alpha {
+            alpha = best;
+        }
+        // impossible alpha-beta range reached (alpha is supposed to be < to beta)
+        if alpha >= beta {
+            return score;
         }
     }
-    cache.insert(p.key(), alpha);
-    return alpha;
+    cache.insert(p.key(), best);
+    return best;
 }