Implement minmax that is ugly as fuck

rs/src/board.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 
-#[derive(Eq, PartialEq, Clone)]
+#[derive(Eq, PartialEq, Clone, Debug)]
 pub enum Color {
     Black,
     White,
@@ -19,7 +19,7 @@ impl Color {
             Color::White => Rank::_2,
         }
     }
-    fn other(&self) -> Color {
+    pub fn other(&self) -> Color {
         match self {
             Color::Black => Color::White,
             Color::White => Color::Black,
@@ -27,7 +27,7 @@ impl Color {
     }
 }
 
-#[derive(PartialEq, Eq, Hash, Clone)]
+#[derive(PartialEq, Eq, Hash, Clone, Debug)]
 pub struct Position {
     pub file: File,
     pub rank: Rank,
@@ -64,7 +64,7 @@ impl Position {
     }
 }
 
-#[derive(PartialEq, Eq, Hash, Clone, Copy)]
+#[derive(PartialEq, Eq, Hash, Clone, Copy, Debug)]
 pub enum Rank {
     _1,
     _2,
@@ -76,7 +76,7 @@ pub enum Rank {
     _8,
 }
 
-#[derive(PartialEq, Eq, Hash, Clone, Copy)]
+#[derive(PartialEq, Eq, Hash, Clone, Copy, Debug)]
 pub enum File {
     A,
     B,

rs/src/engine.rs

@@ -10,13 +10,14 @@ impl board::PieceType {
     }
 }
 
+#[derive(Debug)]
 pub struct Move_ {
     pub source: board::Position,
     pub target: board::Position,
 }
 
 pub struct Engine {
-    board: board::Board,
+    pub board: board::Board,
 }
 
 impl Engine {
@@ -36,28 +37,27 @@ impl Engine {
     pub fn set_state(&mut self, board: board::Board) {
         self.board = board;
     }
-    pub fn get_legal_moves(
-        &self,
+    pub fn get_legal_moves<'a>(
+        board: &'a board::Board,
         position: &board::Position,
-    ) -> Result<impl Iterator<Item = board::Position> + '_, ()> {
-        Ok(self
-            .board
-            .find_moves(position)?
-            .chain(self.board.find_captures(position)?))
+    ) -> Result<impl Iterator<Item = board::Position> + 'a, ()> {
+        Ok(board.find_moves(position)?.chain(board.find_captures(position)?))
     }
     pub fn make_move(
         &mut self,
         source: &board::Position,
         target: board::Position,
     ) -> Result<(), ()> {
-        if !self.get_legal_moves(source)?.any(|pos| pos == target) {
+        if !Engine::get_legal_moves(&self.board, source)?
+            .any(|pos| pos == target)
+        {
             Err(())
         } else {
             // We checked that there is a piece at source in get_legal_moves
             self.board.relocate(source, target)
         }
     }
-    pub fn evaluate_position(&self, board: &board::Board) -> f32 {
+    pub fn evaluate_position(board: &board::Board) -> f32 {
         board
             .iter()
             .map(|(_, piece)| match piece.color {
@@ -67,24 +67,29 @@ impl Engine {
             .sum()
     }
     pub fn evaluate_move(
-        &self,
-        color: &board::Color,
         move_: &Move_,
+        board: &board::Board,
+        color: &board::Color,
     ) -> Result<f32, ()> {
-        let mut board = self.board.clone();
+        let mut board = board.clone();
         board.relocate(&move_.source, move_.target.clone())?;
         Ok(match color {
             board::Color::White => 1.0,
             board::Color::Black => -1.0,
-        } * self.evaluate_position(&board))
+        } * Engine::evaluate_position(&board))
     }
-    pub fn choose_move(&self, color: &board::Color) -> Option<Move_> {
-        let all_moves = self
-            .board
+    fn minmax(
+        depth: i8,
+        board: &board::Board,
+        color: &board::Color,
+    ) -> Option<(f32, Move_)> {
+        eprintln!("Entering minmax for {} at depth {}", color, depth);
+        let all_moves = board
             .iter()
             .filter_map(|(source, piece)| {
                 if &piece.color == color {
-                    if let Ok(targets) = self.get_legal_moves(source) {
+                    if let Ok(targets) = Engine::get_legal_moves(board, source)
+                    {
                         Some(targets.map(|target| Move_ {
                             source: source.clone(),
                             target,
@@ -97,20 +102,56 @@ impl Engine {
                 }
             })
             .flatten();
+        if depth == 0 {
+            Engine::best_immediate_move(board, color, all_moves)
+        } else if depth > 0 {
+            let best_given_opponent = all_moves.map(|move_| {
+                eprintln!("Finding opponent moves for {:?}", move_);
+                let mut board = board.clone();
+                board.relocate(&move_.source, move_.target.clone()).unwrap();
+                let result = if let Some((opponent_score, _)) =
+                    Engine::minmax(depth - 1, &board, &color.other())
+                {
+                    (-opponent_score, move_)
+                } else {
+                    (0.0, move_)
+                };
+                eprintln!(
+                    "Move {:?} got opponent minmax score {} for {:?}",
+                    result.1, result.0, color
+                );
+                result
+            });
+            best_given_opponent
+                .max_by(|(s1, _), (s2, _)| s1.partial_cmp(s2).unwrap())
+        } else {
+            panic!();
+        }
+    }
+    fn best_immediate_move(
+        board: &board::Board,
+        color: &board::Color,
+        moves: impl Iterator<Item = Move_>,
+    ) -> Option<(f32, Move_)> {
         let mut rng = rand::thread_rng();
         Some(
-            all_moves
+            moves
                 .map(|move_| {
-                    (
-                        self.evaluate_move(color, &move_).unwrap()
-                            + rng.gen::<f32>() * 0.1,
-                        move_,
-                    )
+                    let score = Engine::evaluate_move(&move_, board, color)
+                        .unwrap()
+                        + rng.gen::<f32>() * 0.05;
+                    eprintln!(
+                        "Move {:?} got immediate score {} for {:?}",
+                        move_, score, color
+                    );
+                    (score, move_)
                 })
                 .max_by(|(score1, _), (score2, _)| {
                     score1.partial_cmp(score2).unwrap()
-                })?
-                .1,
+                })?,
         )
     }
+    pub fn choose_move(&self, color: &board::Color) -> Option<Move_> {
+        Some(Engine::minmax(1, &self.board, color)?.1)
+    }
 }

rs/src/ui.rs

@@ -219,7 +219,10 @@ impl Ui {
                 board::Position::parse(position_str).map_err(|_| {
                     format!("Error parsing position {}", position_str)
                 })?;
-            match self.engine.get_legal_moves(&position) {
+            match engine::Engine::get_legal_moves(
+                &self.engine.board,
+                &position,
+            ) {
                 Err(_) => {
                     let error = format!("No moves possible from {}", position);
                     Err(error)