feat: compute zobrist hash of the position

1 files changed, 46 insertions, 3 deletions

diff --git a/src/board/mod.rs b/src/board/mod.rs
index 2085894..18e5e7f 100644
--- a/src/board/mod.rs
+++ b/src/board/mod.rs
@@ -1,3 +1,4 @@
+use rand::Rng;
 use crate::{bitboard::{Bitboard, serialize_bitboard, bitscan, pop_count}, moves::{Move, MoveKind}, attacks::Attacks, square::Square};
 mod engine;
 
@@ -22,6 +23,10 @@ pub struct Board {
     /// ```
     pub castling_rights: [[bool; 2]; 2],
 
+    /// Zobrist hash of the current position
+    pub hash: u64,
+
+    zobrist_seed: [u64; 781],
     attacks: Attacks,
 }
 
@@ -105,15 +110,21 @@ impl Board {
             }
         }
 
+        let mut rng = rand::thread_rng();
+        let zobrist_seed = [(); 781].map(|_| rng.gen());
+
         let mut board = Self {
             pieces,
             occupancy: 0,
             ply: 0,
             attacks: Attacks::new(),
             castling_rights: [[true; 2]; 2], // TODO: actualy parse from FEN
-            ep_target: None,
+            ep_target: None, // TODO: parse from FEN
+            hash: 0,
+            zobrist_seed,
         };
         board.update_occupancy();
+        board.update_zobrist_hash();
         board
     }
 
@@ -154,6 +165,40 @@ impl Board {
         occupancy
     }
 
+    /// Compute and store zobrist hash of the current position
+    /// https://www.chessprogramming.org/Zobrist_Hashing
+    fn update_zobrist_hash(&mut self) {
+        self.hash = 0;
+
+        if self.color() == Color::Black {
+            self.hash ^= match self.zobrist_seed.last() {
+                Some(x) => x,
+                None => panic!("Something is wrong with zobrist seed list"),
+            };
+        }
+
+        for (piece_id, bitboard) in self.pieces.iter().enumerate() {
+            for square in serialize_bitboard(*bitboard) {
+                self.hash ^= self.zobrist_seed[piece_id * 64 + square as usize];
+            }
+        }
+
+        for color in 0..2 {
+            for castle_side in 0..2 {
+                if self.castling_rights[color][castle_side] {
+                    self.hash ^= self.zobrist_seed[(12 * 64) + color * 2 + castle_side];
+                }
+            }
+        }
+
+        match self.ep_target {
+            Some(square) => {
+                self.hash ^= self.zobrist_seed[(12 * 64 + 4) + square.file() as usize];
+            },
+            None => {},
+        }
+    }
+
     pub fn print(&self) {
         println!();
         for rank in (0..8).rev() {
@@ -678,8 +723,6 @@ impl Color {
 
 #[cfg(test)]
 mod tests {
-    use std::f32::INFINITY;
-
     use super::*;
     use crate::{bitboard::{pop_count, bitscan, print}, square::Square};