From 74973416aa6a7b0fae9ed2b9911fa18fe87c4e1c Mon Sep 17 00:00:00 2001
From: eug-vs <eugene@eug-vs.xyz>
Date: Sat, 21 Jan 2023 18:24:52 +0300
Subject: feat: precompute first rank attacks

---
 src/attacks.rs  | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 src/bitboard.rs |  3 +++
 2 files changed, 68 insertions(+), 2 deletions(-)

(limited to 'src')

diff --git a/src/attacks.rs b/src/attacks.rs
index ece87b6..141ba44 100644
--- a/src/attacks.rs
+++ b/src/attacks.rs
@@ -1,25 +1,42 @@
-use crate::bitboard::Bitboard;
+use crate::bitboard::{Bitboard, ls1b, pop_count};
 
 static NOT_A_FILE: Bitboard = 0xFEFEFEFEFEFEFEFE;
 static NOT_B_FILE: Bitboard = 0xFDFDFDFDFDFDFDFD;
 static NOT_G_FILE: Bitboard = 0xBFBFBFBFBFBFBFBF;
 static NOT_H_FILE: Bitboard = 0x7F7F7F7F7F7F7F7F;
 
+/// An array where N-th item is an attack bitboard
+/// of a piece on N-th square
 type AttackTable = [Bitboard; 64];
 
+/// One byte stores exactly 256 states which is every
+/// possible state of the occupancy on a rank
+pub type Byte = u8;
+
+/// First index is *occupancy* of the first rank (all possible states)
+/// and the second index is position of the piece on the first rank
+/// ```
+/// rank_attacks = first_rank_attacks[occupancy][piece_file];
+/// ```
+type FirstRankAttacks = [[Byte; 8]; 256];
+
+
 #[derive(Debug)]
 pub struct Attacks {
     pub knight: AttackTable,
     pub king: AttackTable,
+    pub first_rank_attacks: FirstRankAttacks,
 }
 
 impl Attacks {
     pub fn new() -> Self {
         let mut knight = [0; 64];
         let mut king = [0; 64];
+        let mut first_rank_attacks = [[0; 8]; 256];
         Self::precompute_knight_attacks(&mut knight);
         Self::precompute_king_attacks(&mut king);
-        Self { knight, king }
+        Self::precompute_first_rank_attacks(&mut first_rank_attacks);
+        Self { knight, king, first_rank_attacks }
     }
 
     fn precompute_knight_attacks(attacks: &mut AttackTable) {
@@ -51,6 +68,38 @@ impl Attacks {
                 (position >> 8);
         }
     }
+
+    fn precompute_first_rank_attacks(attacks: &mut FirstRankAttacks) {
+        /// Really slow implementation of Most Significant One Bit which is fine to use when pre-computing
+        fn log_base_2(bb: Bitboard) -> u64 {
+            (bb as f64).log2() as u64
+        }
+
+        for occupancy in 0..256 {
+            let right_most_piece = log_base_2(occupancy);
+            let left_most_piece = log_base_2(ls1b(occupancy));
+
+            for file in 0..8 {
+                let left_bound = if left_most_piece == file {
+                    0
+                } else {
+                    left_most_piece
+                };
+
+                let right_bound = if right_most_piece == file {
+                    7
+                } else {
+                    right_most_piece
+                };
+
+                for index in left_bound..right_bound+1 {
+                    if index != file {
+                        attacks[occupancy as usize][file as usize] |= 1 << index;
+                    }
+                }
+            }
+        }
+    }
 }
 
 #[cfg(test)]
@@ -96,4 +145,18 @@ mod tests {
         assert_eq!(pop_count(attacks.king[Square::A4 as usize]), 5);
         assert_eq!(pop_count(attacks.king[Square::H4 as usize]), 5);
     }
+
+    #[test]
+    fn test_first_rank_attacks() {
+        let attacks = Attacks::new();
+        //                                      HGFEDCBA        HGFEDCBA
+        assert_eq!(attacks.first_rank_attacks[0b00010000][4], 0b11101111, "If rook is the only one on the file, it should be able to attack all rank");
+        assert_eq!(attacks.first_rank_attacks[0b00010001][4], 0b11101111, "If only other piece is on A rank, rook should be able to attack all rank");
+        assert_eq!(attacks.first_rank_attacks[0b10010000][4], 0b11101111, "If only other piece is on H rank, rook should be able to attack all rank");
+        assert_eq!(attacks.first_rank_attacks[0b10010001][4], 0b11101111, "If only other pieces are on A and H ranks, rook should be able to attack all rank");
+        assert_eq!(attacks.first_rank_attacks[0b00010100][4], 0b11101100);
+        assert_eq!(attacks.first_rank_attacks[0b01010100][4], 0b01101100);
+        assert_eq!(attacks.first_rank_attacks[0b01010010][4], 0b01101110);
+
+    }
 }
diff --git a/src/bitboard.rs b/src/bitboard.rs
index 6481863..646006c 100644
--- a/src/bitboard.rs
+++ b/src/bitboard.rs
@@ -24,6 +24,9 @@ pub fn pop_count(bb: Bitboard) -> u8 {
 
 /// Return Bitboard with only Least Single Bit
 pub fn ls1b(bb: Bitboard) -> u64 {
+    if bb == 0 {
+        return 0
+    }
     bb & !(bb - 1)
 }
 
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