feat: precompute knight and king attacks

1 files changed, 99 insertions, 0 deletions

diff --git a/src/attacks.rs b/src/attacks.rs
new file mode 100644
index 0000000..ece87b6
--- /dev/null
+++ b/src/attacks.rs
@@ -0,0 +1,99 @@
+use crate::bitboard::Bitboard;
+
+static NOT_A_FILE: Bitboard = 0xFEFEFEFEFEFEFEFE;
+static NOT_B_FILE: Bitboard = 0xFDFDFDFDFDFDFDFD;
+static NOT_G_FILE: Bitboard = 0xBFBFBFBFBFBFBFBF;
+static NOT_H_FILE: Bitboard = 0x7F7F7F7F7F7F7F7F;
+
+type AttackTable = [Bitboard; 64];
+
+#[derive(Debug)]
+pub struct Attacks {
+    pub knight: AttackTable,
+    pub king: AttackTable,
+}
+
+impl Attacks {
+    pub fn new() -> Self {
+        let mut knight = [0; 64];
+        let mut king = [0; 64];
+        Self::precompute_knight_attacks(&mut knight);
+        Self::precompute_king_attacks(&mut king);
+        Self { knight, king }
+    }
+
+    fn precompute_knight_attacks(attacks: &mut AttackTable) {
+        for index in 0..64 {
+            let position = 1u64 << index;
+            attacks[index] =
+                ((position & NOT_A_FILE & NOT_B_FILE) << 6) |
+                ((position & NOT_G_FILE & NOT_H_FILE) << 10) |
+                ((position & NOT_A_FILE) << 15) |
+                ((position & NOT_H_FILE) << 17) |
+                ((position & NOT_G_FILE & NOT_H_FILE) >> 6) |
+                ((position & NOT_A_FILE & NOT_B_FILE) >> 10) |
+                ((position & NOT_H_FILE) >> 15) |
+                ((position & NOT_A_FILE) >> 17);
+        }
+    }
+
+    fn precompute_king_attacks(attacks: &mut AttackTable) {
+        for index in 0..64 {
+            let position = 1u64 << index;
+            attacks[index] =
+                ((position & NOT_A_FILE) >> 1) |
+                ((position & NOT_A_FILE) << 7) |
+                ((position & NOT_A_FILE) >> 9) |
+                ((position & NOT_H_FILE) << 1) |
+                ((position & NOT_H_FILE) >> 7) |
+                ((position & NOT_H_FILE) << 9) |
+                (position << 8) |
+                (position >> 8);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{bitboard::pop_count, board::Square};
+
+    use super::*;
+
+    #[test]
+    fn test_knight_attacks() {
+        let attacks = Attacks::new();
+        let e4_attacks = attacks.knight[Square::E4 as usize];
+
+        assert_ne!(e4_attacks & 1 << Square::G5 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::G3 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::C5 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::C3 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::D2 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::F2 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::D6 as usize, 0);
+        assert_ne!(e4_attacks & 1 << Square::F6 as usize, 0);
+
+        assert_eq!(e4_attacks & 1 << Square::E5 as usize, 0);
+        assert_eq!(e4_attacks & 1 << Square::D4 as usize, 0);
+        assert_eq!(e4_attacks & 1 << Square::A1 as usize, 0);
+
+        assert_eq!(pop_count(attacks.knight[Square::G1 as usize]), 3);
+        assert_eq!(pop_count(attacks.knight[Square::H8 as usize]), 2);
+    }
+
+    #[test]
+    fn test_king_attacks() {
+        let attacks = Attacks::new();
+        assert_eq!(pop_count(attacks.king[Square::E4 as usize]), 8);
+
+        assert_eq!(pop_count(attacks.king[Square::A1 as usize]), 3);
+        assert_eq!(pop_count(attacks.king[Square::A8 as usize]), 3);
+        assert_eq!(pop_count(attacks.king[Square::H1 as usize]), 3);
+        assert_eq!(pop_count(attacks.king[Square::H8 as usize]), 3);
+
+        assert_eq!(pop_count(attacks.king[Square::E1 as usize]), 5);
+        assert_eq!(pop_count(attacks.king[Square::E8 as usize]), 5);
+        assert_eq!(pop_count(attacks.king[Square::A4 as usize]), 5);
+        assert_eq!(pop_count(attacks.king[Square::H4 as usize]), 5);
+    }
+}