From af2bc841119a6751c240dec95dd5511d4ee31d36 Mon Sep 17 00:00:00 2001
From: Kai Stevenson <kai@kaistevenson.com>
Date: Wed, 7 May 2025 23:08:15 -0700
Subject: init ; most of tic tac toe done

---
 tic_tac_toe/src/ai.rs | 111 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 tic_tac_toe/src/ai.rs

(limited to 'tic_tac_toe/src/ai.rs')

diff --git a/tic_tac_toe/src/ai.rs b/tic_tac_toe/src/ai.rs
new file mode 100644
index 0000000..e2d8918
--- /dev/null
+++ b/tic_tac_toe/src/ai.rs
@@ -0,0 +1,111 @@
+use crate::board::{Board, Coord, Tile};
+use crate::{GameState, check_state};
+
+#[derive(Debug)]
+pub struct BestMove {
+    pub coord: Coord,
+    eval: i8,
+    depth: u32,
+}
+
+fn eval_for_player(board: &Board, player: Tile) -> i8 {
+    match player {
+        Tile::PlayerOne => {
+            return match check_state(board) {
+                GameState::PlayerTwoWin => -100,
+                GameState::Draw => -10,
+                GameState::InProgress => 0,
+                GameState::PlayerOneWin => 100,
+            };
+        }
+        Tile::PlayerTwo => {
+            return match check_state(board) {
+                GameState::PlayerOneWin => -100,
+                GameState::Draw => -10,
+                GameState::InProgress => 0,
+                GameState::PlayerTwoWin => 100,
+            };
+        }
+        Tile::Unowned => {
+            return match check_state(board) {
+                GameState::PlayerOneWin => -100,
+                GameState::PlayerTwoWin => -100,
+                GameState::InProgress => 0,
+                GameState::Draw => 100,
+            };
+        }
+    }
+}
+
+pub fn get_best_move(board: &Board, player: Tile, is_tl: bool, depth: u32) -> BestMove {
+    //base case
+    //game is over, return eval
+    let eval = eval_for_player(board, player);
+
+    //eval 0 means the game is in progress
+    if eval != 0 {
+        return BestMove {
+            coord: (99, 99),
+            eval,
+            depth,
+        };
+    }
+
+    //get all possible moves
+    let possible_moves: Vec<Coord> = (0..9)
+        .map(|i| {
+            return (i % 3, i / 3);
+        })
+        .filter(|coord| {
+            return board.get_at_coord(*coord) == Tile::Unowned;
+        })
+        .collect();
+
+    let mut cur_best = BestMove {
+        eval: i8::min_value(),
+        coord: possible_moves[0],
+        depth,
+    };
+
+    for p_move in possible_moves {
+        let mut new_board = board.clone();
+        new_board.set_at_coord(p_move, player);
+
+        let other_player = match player {
+            Tile::PlayerOne => Tile::PlayerTwo,
+            Tile::PlayerTwo => Tile::PlayerOne,
+            //this allows the AI to set as player 'unowned', which will cause it to try to draw
+            //todo: fix this
+            Tile::Unowned => Tile::PlayerOne,
+        };
+
+        let response_move = get_best_move(&new_board, other_player, false, depth + 1);
+        if response_move.coord.0 != 99 {
+            new_board.set_at_coord(response_move.coord, other_player);
+        }
+
+        //this is the inverse of the response eval
+        let response_eval = -response_move.eval;
+
+        if is_tl {
+            println!("AI > If I make move {p_move:?}, the best response is {response_move:?}");
+        }
+
+        //slightly bias for the centre to beat weak players
+        let centre_bias = if p_move == (1, 1) { 1 } else { 0 };
+
+        if response_eval > cur_best.eval {
+            cur_best = BestMove {
+                coord: p_move,
+                eval: response_eval + centre_bias,
+                depth: response_move.depth,
+            };
+        }
+    }
+
+    if is_tl {
+        println!("AI > I'll play {cur_best:?}")
+    }
+
+    return cur_best;
+}
-- 
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