make the missing - a type error. because it's funny

minmax-rs/src/connect4/board.rs

@@ -4,7 +4,7 @@ use std::{
 };
 
 use crate::{
-    state::{position_as_int, Position},
+    state::{position_as_int, IgnorePlayer, Position},
     Game, Player, Score, State,
 };
 
@@ -99,7 +99,7 @@ impl Connect4 {
         }
     }
 
-    fn rate(&self, player: Player) -> Score {
+    fn rate(&self, player: Player) -> Score<IgnorePlayer> {
         #[rustfmt::skip]
         const WIN_COUNT_TABLE: [i16; BOARD_POSITIONS] = [
             3, 4, 6, 7, 6, 4, 3,
@@ -117,7 +117,9 @@ impl Connect4 {
                 .sum::<i16>()
         };
 
-        Score::new(i32::from(score_player(player) - score_player(player.opponent())))
+        Score::new(i32::from(
+            score_player(player) - score_player(player.opponent()),
+        ))
     }
 
     pub fn drop_player(&self, position: usize) -> usize {
@@ -185,7 +187,7 @@ impl Game for Connect4 {
         }
     }
 
-    fn rate(&self, player: Player) -> Score {
+    fn rate(&self, player: Player) -> Score<IgnorePlayer> {
         Connect4::rate(&self, player)
     }
 }
@@ -215,7 +217,7 @@ impl Display for Connect4 {
 
 #[cfg(test)]
 mod tests {
-    use crate::{Player, Score, State};
+    use crate::{Player, Score, State, state::IgnorePlayer};
 
     use super::Connect4;
 
@@ -244,7 +246,7 @@ mod tests {
         assert_eq!(board.result(), state);
     }
 
-    fn test_rate(board: &str, player: Player, score: Score) {
+    fn test_rate(board: &str, player: Player, score: Score<IgnorePlayer>) {
         let board = parse_board(board);
         assert_eq!(board.rate(player), score);
     }
@@ -298,7 +300,7 @@ mod tests {
         ______X
         ",
             Player::X,
-            Score(3),
+            Score::new(3),
         )
     }
 
@@ -312,7 +314,7 @@ mod tests {
         ___X___
         ",
             Player::X,
-            Score(7),
+            Score::new(7),
         )
     }
 
@@ -326,7 +328,7 @@ mod tests {
         O__X___
         ",
             Player::X,
-            Score(4),
+            Score::new(4),
         )
     }
 
@@ -340,7 +342,7 @@ mod tests {
         O_____X
         ",
             Player::X,
-            Score(0),
+            Score::new(0),
         )
     }
 }

minmax-rs/src/lib.rs

@@ -3,7 +3,6 @@
     try_trait_v2,
     return_position_impl_trait_in_trait,
     let_chains,
-    adt_const_params
 )]
 #![allow(incomplete_features)]
 
@@ -15,6 +14,8 @@ pub mod tic_tac_toe;
 
 use std::fmt::Display;
 
+use state::IgnorePlayer;
+
 pub use self::minmax::PerfectPlayer;
 pub use self::state::{Player, Score, State};
 
@@ -47,7 +48,7 @@ pub trait Game: Display {
     fn result(&self) -> State;
 
     /// Only called if [`GameBoard::REASONABLE_SEARCH_DEPTH`] is `Some`.
-    fn rate(&self, player: Player) -> Score;
+    fn rate(&self, player: Player) -> Score<IgnorePlayer>;
 
     fn make_move(&mut self, position: Self::Move, player: Player);
 

minmax-rs/src/minmax.rs

@@ -3,7 +3,10 @@
 
 use std::time::Instant;
 
-use crate::{Game, GamePlayer, Player, Score, State};
+use crate::{
+    state::{GoodPlayer, MinmaxPlayer},
+    Game, GamePlayer, Player, Score, State,
+};
 
 #[derive(Clone)]
 pub struct PerfectPlayer<G: Game> {
@@ -36,30 +39,30 @@ impl<G: Game> PerfectPlayer<G> {
         self.best_move.expect("no move made yet")
     }
 
-    fn minmax(
+    fn minmax<P: MinmaxPlayer>(
         &mut self,
         board: &mut G,
         maximizing_player: Player,
-        grandparents_favourite_child_alpha: Score,
+        grandparents_favourite_child_alpha: Score<P>,
-        parents_favourite_child_beta: Score,
+        parents_favourite_child_beta: Score<P>,
         depth: usize,
-    ) -> Score {
+    ) -> Score<P> {
         // FIXME: Make depth decrease not increase.
         if let Some(max_depth) = self.max_depth && depth >= max_depth {
-            return board.rate(maximizing_player);
+            return board.rate(maximizing_player).for_player::<P>();
         }
 
         match board.result() {
             State::Winner(winner) => {
                 if winner == maximizing_player {
                     // Our maximizing player wins the game, so this node is a win for it.
-                    Score::WON
+                    Score::WON.for_player::<P>()
                 } else {
                     // The maximizing player lost the board here, so the node is a loss.
-                    Score::LOST
+                    Score::LOST.for_player::<P>()
                 }
             }
-            State::Draw => Score::TIE,
+            State::Draw => Score::TIE.for_player::<P>(),
             State::InProgress => {
                 // The board isn't done yet, go deeper!
                 // The alpha is the favourite (highest reward) child of our grandparent (who's on our side!).
@@ -77,7 +80,7 @@ impl<G: Game> PerfectPlayer<G> {
                     // O A(10) B(  ) <- we are here in the loop and about to call D
                     //          /  \
                     // X    C(11) D(  )
-                    let value = -self.minmax(
+                    let value = -self.minmax::<P::Enemy>(
                         board,
                         // The player that will maximize this round is now the opponent. This layer it was our original
                         // opponent, O, but in the nested round it's X's turn again so they will try to maximize their score.
@@ -130,7 +133,13 @@ impl<G: Game> GamePlayer<G> for PerfectPlayer<G> {
         self.best_move = None;
 
         // Get the rating for the one move we will make.
-        self.minmax(board, this_player, Score::LOST, Score::WON, 0);
+        self.minmax::<GoodPlayer>(
+            board,
+            this_player,
+            Score::LOST.for_player::<GoodPlayer>(),
+            Score::WON.for_player::<GoodPlayer>(),
+            0,
+        );
 
         board.make_move(
             self.best_move

minmax-rs/src/state.rs

@@ -1,6 +1,7 @@
 use std::{
-    fmt::{Display, Debug},
+    fmt::{Debug, Display},
-    ops::{ControlFlow, Try, Neg},
+    marker::PhantomData,
+    ops::{ControlFlow, Neg, Try},
 };
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -96,17 +97,44 @@ impl Try for State {
     }
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+// This fun generic setup ensures that we never compare two scores from different layers.
-pub struct Score(pub i32);
+pub struct Score<P>(pub i32, PhantomData<P>);
 
-impl Score {
+pub trait MinmaxPlayer {
+    type Enemy: MinmaxPlayer<Enemy = Self>;
+}
+pub struct GoodPlayer;
+pub struct EvilPlayer;
+pub struct IgnorePlayer;
+
+impl MinmaxPlayer for GoodPlayer {
+    type Enemy = EvilPlayer;
+}
+impl MinmaxPlayer for EvilPlayer {
+    type Enemy = GoodPlayer;
+}
+impl MinmaxPlayer for IgnorePlayer {
+    type Enemy = Self;
+}
+
+impl Score<IgnorePlayer> {
     // Due to the nature of two's completement, we can't actually negate this properly, so add 1.
-    pub const LOST: Self = Self(i32::MIN + 1);
+    pub const LOST: Self = Self(i32::MIN + 1, PhantomData);
-    pub const TIE: Self = Self(0);
+    pub const TIE: Self = Self(0, PhantomData);
-    pub const WON: Self = Self(i32::MAX);
+    pub const WON: Self = Self(i32::MAX, PhantomData);
 
+    pub fn for_player<P>(self) -> Score<P> {
+        Score(self.0, PhantomData)
+    }
+}
+
+impl<P> Score<P> {
     pub fn new(int: i32) -> Self {
-        Self(int)
+        Self(int, PhantomData)
+    }
+
+    pub fn ignore_side(self) -> Score<IgnorePlayer> {
+        Score(self.0, PhantomData)
     }
 
     #[allow(unused)]
@@ -117,20 +145,51 @@ impl Score {
     }
 }
 
-impl Neg for Score {
+impl<P: MinmaxPlayer> Neg for Score<P> {
-    type Output = Self;
+    type Output = Score<P::Enemy>;
 
     fn neg(self) -> Self::Output {
-        Self(-self.0)
+        Score(-self.0, PhantomData)
     }
 }
 
-impl Debug for Score {
+impl<P> PartialEq for Score<P> {
+    fn eq(&self, other: &Self) -> bool {
+        self.0 == other.0
+    }
+}
+
+impl<P> Eq for Score<P> {}
+
+impl<P> PartialOrd for Score<P> {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.0.partial_cmp(&other.0)
+    }
+}
+
+impl<P> Ord for Score<P> {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.0.cmp(&other.0)
+    }
+}
+
+impl<P> Clone for Score<P> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<P> Copy for Score<P> {}
+
+impl<P> Debug for Score<P> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match *self {
+        // this should be a match but StructuralEq does not like me
-            Self::WON => f.write_str("WON"),
+        if self.ignore_side() == Score::WON {
-            Self::LOST => f.write_str("LOST"),
+            f.write_str("WON")
-            Self(other) => Debug::fmt(&other, f),
+        } else if Score::LOST == self.ignore_side() {
+            f.write_str("LOST")
+        } else {
+            Debug::fmt(&self.0, f)
         }
     }
 }

minmax-rs/src/tic_tac_toe/board.rs

@@ -1,6 +1,6 @@
 use std::fmt::{Display, Write};
 
-use crate::{Game, Player, Score, State};
+use crate::{Game, Player, Score, State, state::IgnorePlayer};
 
 #[derive(Clone)]
 pub struct TicTacToe(u32);
@@ -138,7 +138,7 @@ impl Game for TicTacToe {
         TicTacToe::result(self)
     }
 
-    fn rate(&self, player: Player) -> Score {
+    fn rate(&self, player: Player) -> Score<IgnorePlayer> {
         match self.result() {
             State::Winner(winner) if player == winner => Score::WON,
             State::Winner(_) => Score::LOST,