Create ZobristHashable trait.

To select which variants can use the `Zobrist` trait, solve issues raised in niklasf#45.

Exposed so third-party variants can implement it.

* `Atomic` cannot use it because `play_unchecked` does not handle explosions.
* `CrazyHouse` is not properly supported because it does not account for the `Pocket` of the player.
* `TreeCheck` is not properly supported because it does not account for the `Check` in the position.

src/lib.rs

@@ -111,4 +111,4 @@ pub use crate::setup::{Castles, Setup};
 pub use crate::movelist::MoveList;
 pub use crate::position::{Chess, Outcome, Position, FromSetup, PlayError, PositionError, PositionErrorKinds};
 pub use crate::perft::perft;
-pub use crate::zobrist::{Zobrist, zobrist_from_pos};
+pub use crate::zobrist::{Zobrist, ZobristHashable, zobrist_from_pos};

src/position.rs

@@ -30,6 +30,7 @@ use crate::types::{CastlingSide, CastlingMode, Move, Piece, Role, RemainingCheck
 use crate::material::Material;
 use crate::setup::{Castles, EpSquare, Setup, SwapTurn};
 use crate::movelist::MoveList;
+use crate::zobrist::ZobristHashable;
 
 /// Outcome of a game.
 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
@@ -671,6 +672,8 @@ impl Position for Chess {
     fn variant_outcome(&self) -> Option<Outcome> { None }
 }
 
+impl ZobristHashable for Chess {}
+
 #[cfg(feature = "variant")]
 pub(crate) mod variant {
     use super::*;
@@ -1022,6 +1025,8 @@ pub(crate) mod variant {
         }
     }
 
+    impl ZobristHashable for Antichess {}
+
     /// A King of the Hill position.
     #[derive(Clone, Debug, Default)]
     pub struct KingOfTheHill {
@@ -1109,6 +1114,8 @@ pub(crate) mod variant {
         }
     }
 
+    impl ZobristHashable for KingOfTheHill {}
+
     /// A Three-Check position.
     #[derive(Clone, Debug, Default)]
     pub struct ThreeCheck {
@@ -1532,6 +1539,8 @@ pub(crate) mod variant {
         }
     }
 
+    impl ZobristHashable for RacingKings {}
+
     /// A Horde position.
     #[derive(Clone, Debug)]
     pub struct Horde {
@@ -1931,6 +1940,8 @@ pub(crate) mod variant {
         }
     }
 
+    impl ZobristHashable for Horde {}
+
     fn add_king_promotions(moves: &mut MoveList) {
         let mut king_promotions = MoveList::new();
 

src/zobrist.rs

@@ -4,20 +4,27 @@ use std::num::NonZeroU32;
 
 include!(concat!(env!("OUT_DIR"), "/zobrist.rs")); // generated by build.rs
 
+/// Used to discriminate which variants support Zobrist hashing.
+pub trait ZobristHashable {}
+
+pub trait PositionHashable: Position + ZobristHashable {} // trait alias
+
 #[derive(Debug)]
-pub struct Zobrist<P> {
-    pos: P,
+pub struct Zobrist<P: PositionHashable> {
+    pos: PositionHashable,
     zobrist: u64
 }
 
-impl <P:Position> Zobrist<P> {
+impl <P:PositionHashable> ZobristHashable for Zobrist<P> {}
+
+impl <P:PositionHashable> Zobrist<P> {
     /// Computes the zobrist hash of the current game state
     pub fn hash(&self) -> u64 {
         self.zobrist
     }
 }
 
-impl <P:Default + Position> Default for Zobrist<P> {
+impl <P:Default + PositionHashable> Default for Zobrist<P> {
     fn default() -> Self {
         let pos = P::default();
         let board = pos.board();
@@ -35,7 +42,7 @@ impl <P:Default + Position> Default for Zobrist<P> {
     }
 }
 
-impl <P:FromSetup + Position> FromSetup for Zobrist<P> {
+impl <P:FromSetup + PositionHashable> FromSetup for Zobrist<P> {
     fn from_setup(setup: &dyn Setup, mode: CastlingMode) -> Result<Self, PositionError<Self>> {
         // create the underlying from the setup
         let pos = match P::from_setup(setup, mode) {
@@ -48,7 +55,7 @@ impl <P:FromSetup + Position> FromSetup for Zobrist<P> {
 }
 
 // Simply call through to the underlying methods
-impl <P: Position> Setup for Zobrist<P> {
+impl <P: PositionHashable> Setup for Zobrist<P> {
     #[inline(always)]
     fn board(&self) -> &Board {
         self.pos.board()
@@ -91,7 +98,7 @@ impl <P: Position> Setup for Zobrist<P> {
 }
 
 // call through to the underlying methods for everything except `play_unchecked`
-impl <P: Position> Position for Zobrist<P> {
+impl <P: PositionHashable> Position for Zobrist<P> {
     #[inline(always)]
     fn legal_moves(&self) -> MoveList {
         self.pos.legal_moves()
@@ -215,7 +222,7 @@ fn zobrist_from_board(board: &Board) -> u64 {
 }
 
 /// Computes the Zobrist hash given a position.
-pub fn zobrist_from_pos<T: Position>(pos: &T) -> u64 {
+pub fn zobrist_from_pos<T: PositionHashable>(pos: &T) -> u64 {
     // compute the zobrist hash from the pieces on the board
     let mut zobrist = zobrist_from_board(&pos.board());
 
@@ -266,7 +273,7 @@ fn castle(color :Color, castle: CastlingSide) -> u64 {
 
 #[cfg(test)]
 mod zobrist_tests {
-    use crate::{Square, Piece, Chess, Position, CastlingMode, Move};
+    use crate::{Square, Piece, Chess, PositionHashable, CastlingMode, Move};
     use crate::fen::{epd, Fen};
     use crate::zobrist::{square, Zobrist};
     use std::collections::{HashSet, HashMap};