feat(core): AI Alpha-Beta 搜索 — Negamax + 剪枝 + 启发式排序

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/src/ai/search.rs

@@ -1,2 +1,189 @@
-//! AI 搜索模块 — 占位符, 待实现 Alpha-Beta 搜索
-//! TODO: 在 Task 6 中实现完整搜索逻辑
+use crate::ai::evaluate::evaluate_board;
+use crate::ai::AiEngine;
+use crate::board::Board;
+use crate::types::{Color, Position};
+
+/// Alpha-Beta AI 引擎
+pub struct AlphaBetaAi {
+    depth: usize,
+}
+
+impl AlphaBetaAi {
+    pub fn new(depth: usize) -> Self {
+        Self { depth }
+    }
+}
+
+impl AiEngine for AlphaBetaAi {
+    fn best_move(&self, board: &Board, color: Color) -> Option<Position> {
+        let candidates = board.get_candidate_moves();
+        if candidates.is_empty() {
+            return None;
+        }
+
+        let mut best_pos = None;
+        let mut best_score = f64::NEG_INFINITY;
+
+        for &pos in &candidates {
+            if let Ok(new_board) = board.place(pos, color) {
+                if new_board.check_win(pos) {
+                    return Some(pos);
+                }
+                let score = -self.negamax(
+                    &new_board,
+                    self.depth - 1,
+                    f64::NEG_INFINITY,
+                    f64::INFINITY,
+                    color.opponent(),
+                );
+                if score > best_score {
+                    best_score = score;
+                    best_pos = Some(pos);
+                }
+            }
+        }
+
+        best_pos
+    }
+}
+
+impl AlphaBetaAi {
+    fn negamax(
+        &self,
+        board: &Board,
+        depth: usize,
+        mut alpha: f64,
+        beta: f64,
+        color: Color,
+    ) -> f64 {
+        if depth == 0 {
+            return evaluate_board(board, color);
+        }
+
+        let candidates = board.get_candidate_moves();
+        if candidates.is_empty() {
+            return evaluate_board(board, color);
+        }
+
+        // 启发式排序：先评估每步棋，优先搜索高分走法
+        let mut scored: Vec<(Position, f64)> = candidates
+            .into_iter()
+            .filter_map(|pos| {
+                board.place(pos, color).ok().map(|b| {
+                    if b.check_win(pos) {
+                        (pos, f64::INFINITY)
+                    } else {
+                        let s = evaluate_board(&b, color);
+                        (pos, s)
+                    }
+                })
+            })
+            .collect();
+        scored.sort_by(|a, b| b.1.partial_cmp(&a.1).unwrap_or(std::cmp::Ordering::Equal));
+
+        let mut max_val = f64::NEG_INFINITY;
+        for (pos, _) in scored {
+            if let Ok(new_board) = board.place(pos, color) {
+                if new_board.check_win(pos) {
+                    return f64::INFINITY;
+                }
+                let val = -self.negamax(
+                    &new_board,
+                    depth - 1,
+                    -beta,
+                    -alpha,
+                    color.opponent(),
+                );
+                if val > max_val {
+                    max_val = val;
+                }
+                if val > alpha {
+                    alpha = val;
+                }
+                if alpha >= beta {
+                    break;
+                }
+            }
+        }
+        max_val
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::ai::AiEngine;
+    use crate::board::Board;
+    use crate::types::{Color, Position};
+
+    #[test]
+    fn test_ai_returns_center_on_empty_board() {
+        let board = Board::new(15);
+        let ai = AlphaBetaAi::new(1);
+        let mv = ai.best_move(&board, Color::Black);
+        assert!(mv.is_some());
+        let pos = mv.unwrap();
+        assert!(pos.x >= 6 && pos.x <= 8);
+        assert!(pos.y >= 6 && pos.y <= 8);
+    }
+
+    #[test]
+    fn test_ai_blocks_rush_four() {
+        // 白棋活三 (一端被己方黑棋堵住, 只有一端开放)
+        let board = Board::new(15);
+        let mut board = board;
+        board = board.place(Position::new(7, 1), Color::Black).unwrap();
+        board = board.place(Position::new(7, 2), Color::White).unwrap();
+        board = board.place(Position::new(7, 3), Color::White).unwrap();
+        board = board.place(Position::new(7, 4), Color::White).unwrap();
+        board = board.place(Position::new(7, 5), Color::White).unwrap();
+        let ai = AlphaBetaAi::new(3);
+        let mv = ai.best_move(&board, Color::Black).unwrap();
+        assert_eq!(
+            mv,
+            Position::new(7, 6),
+            "AI should block rush four at (7,6), got ({},{})",
+            mv.x,
+            mv.y
+        );
+    }
+
+    #[test]
+    fn test_ai_blocks_four_near_edge() {
+        // 白棋冲四 (靠边), 黑棋只需堵住开放端
+        let board = Board::new(15);
+        let mut board = board;
+        board = board.place(Position::new(7, 0), Color::White).unwrap();
+        board = board.place(Position::new(7, 1), Color::White).unwrap();
+        board = board.place(Position::new(7, 2), Color::White).unwrap();
+        board = board.place(Position::new(7, 3), Color::White).unwrap();
+        let ai = AlphaBetaAi::new(3);
+        let mv = ai.best_move(&board, Color::Black).unwrap();
+        assert_eq!(
+            mv,
+            Position::new(7, 4),
+            "AI should block four at (7,4), got ({},{})",
+            mv.x,
+            mv.y
+        );
+    }
+
+    #[test]
+    fn test_ai_takes_win() {
+        // 黑棋连四, (7,2) 和 (7,7) 都是胜着
+        let board = Board::new(15);
+        let mut board = board;
+        board = board.place(Position::new(7, 3), Color::Black).unwrap();
+        board = board.place(Position::new(7, 4), Color::Black).unwrap();
+        board = board.place(Position::new(7, 5), Color::Black).unwrap();
+        board = board.place(Position::new(7, 6), Color::Black).unwrap();
+        let ai = AlphaBetaAi::new(3);
+        let mv = ai.best_move(&board, Color::Black).unwrap();
+        assert!(
+            (mv.x == 7 && mv.y == 2) || (mv.x == 7 && mv.y == 7),
+            "AI should take winning move, got ({},{})",
+            mv.x,
+            mv.y
+        );
+    }
+}