From 21e5e0dc3de477df4b250673eb89c4e94aa00c2d Mon Sep 17 00:00:00 2001
From: Michael Captain <mac@macaptain.com>
Date: Fri, 2 Apr 2021 18:49:24 +0300
Subject: [PATCH] Add knight attacks with tests

---
 src/attack.zig  | 63 ++++++++++++++++++++++++++++++++++++++++++-------
 src/main.zig    | 17 +++++++++++++
 test/attack.zig | 34 ++++++++++++++++++++++++++
 3 files changed, 105 insertions(+), 9 deletions(-)

diff --git a/src/attack.zig b/src/attack.zig
index 711157c..86afe7c 100644
--- a/src/attack.zig
+++ b/src/attack.zig
@@ -1,21 +1,25 @@
 pub usingnamespace @import("board.zig");
 
 const mask_not_a = 0xfefefefefefefefe;
+const mask_not_ab = 0xfcfcfcfcfcfcfcfc;
 const mask_not_h = 0x7f7f7f7f7f7f7f7f;
+const mask_not_gh = 0x3f3f3f3f3f3f3f3f;
 
 const pawn_attacks = initPawnAttacks();
 
 fn initPawnAttacks() [2][64]u64 {
-    var sq: u6 = 0;
-    var attacks: [2][64]u64 = undefined;
-    while (sq < 63) : (sq += 1) {
-        attacks[0][sq] = maskPawnAttacks(Side.White, @intToEnum(Square, sq));
+    comptime {
+        var sq: u8 = 0;
+        var attacks: [2][64]u64 = undefined;
+        while (sq < 64) : (sq += 1) {
+            attacks[0][sq] = maskPawnAttacks(Side.White, @intToEnum(Square, sq));
+        }
+        sq = 0;
+        while (sq < 64) : (sq += 1) {
+            attacks[1][sq] = maskPawnAttacks(Side.Black, @intToEnum(Square, sq));
+        }
+        return attacks;
     }
-    sq = 0;
-    while (sq < 63) : (sq += 1) {
-        attacks[1][sq] = maskPawnAttacks(Side.Black, @intToEnum(Square, sq));
-    }
-    return attacks;
 }
 
 fn maskPawnAttacks(side: Side, pawn: Square) u64 {
@@ -29,3 +33,44 @@ fn maskPawnAttacks(side: Side, pawn: Square) u64 {
 pub fn pawnAttacks(side: Side, pawn: Square) u64 {
     return pawn_attacks[@enumToInt(side)][@enumToInt(pawn)];
 }
+
+const knight_attacks = initKnightAttacks();
+
+fn initKnightAttacks() [64]u64 {
+    comptime {
+        var sq: u8 = 0;
+        var attacks: [64]u64 = undefined;
+        while (sq < 64) : (sq += 1) {
+            attacks[sq] = maskKnightAttacks(@intToEnum(Square, sq));
+        }
+        return attacks;
+    }
+}
+
+fn maskKnightAttacks(knight: Square) u64 {
+    const bitboard = bitboardFromSquare(knight);
+    // attacks considered clockwise
+    var attacks = bitboard >> 15 & mask_not_a;
+    attacks |= bitboard >> 6 & mask_not_ab;
+    attacks |= bitboard << 10 & mask_not_ab;
+    attacks |= bitboard << 17 & mask_not_a;
+    attacks |= bitboard << 15 & mask_not_h;
+    attacks |= bitboard << 6 & mask_not_gh;
+    attacks |= bitboard >> 10 & mask_not_gh;
+    attacks |= bitboard >> 17 & mask_not_h;
+    return attacks;
+}
+
+pub fn knightAttacks(knight: Square) u64 {
+    return knight_attacks[@enumToInt(knight)];
+}
+
+const king_attacks = initKingAttacks();
+
+fn initKingAttacks() [64]u64 {
+    return 0;
+}
+
+pub fn kingAttacks(king: Square) u64 {
+    return king_attacks[@enumToInt(king)];
+}
diff --git a/src/main.zig b/src/main.zig
index d01e94b..2a99743 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -24,6 +24,23 @@ fn uciLoop() !void {
     }
 }
 
+/// Helper function for getting the value of a specific bitboard.
+fn bitboardBuilder() void {
+    var bitboard: u64 = 0;
+    var rank: u6 = 0;
+    while (rank < 8) : (rank += 1) {
+        var file: u6 = 0;
+        while (file < 8) : (file += 1) {
+            const square = 8 * rank + file;
+            if (file != 6 and file != 7) {
+                bitboard |= @as(u64, 1) << square;
+            }
+        }
+    }
+    board.printBitboard(bitboard);
+}
+
 pub fn main() !void {
     //uciLoop();
+    //bitboardBuilder();
 }
diff --git a/test/attack.zig b/test/attack.zig
index 664ade3..c218e98 100644
--- a/test/attack.zig
+++ b/test/attack.zig
@@ -60,3 +60,37 @@ test "black pawn on 1st rank attacks nothing" {
     const actual = pawnAttacks(side, pawn);
     expectEqual(expected, actual);
 }
+
+test "knight in center attacks all 8 squares" {
+    const knight = Square.e4;
+    const expected_attacked_squares = [8]Square{
+        Square.f6,
+        Square.g5,
+        Square.g3,
+        Square.f2,
+        Square.d2,
+        Square.c3,
+        Square.c5,
+        Square.d6,
+    };
+    const expected = bitboardFromSquares(expected_attacked_squares[0..]);
+    const actual = knightAttacks(knight);
+    expectEqual(expected, actual);
+}
+
+test "knights in corners attack only 2 squares" {
+    const knight_a8 = Square.a8;
+    const knight_h8 = Square.h8;
+    const knight_h1 = Square.h1;
+    const knight_a1 = Square.a1;
+
+    const expected_a8 = [2]Square{ Square.c7, Square.b6 };
+    const expected_h8 = [2]Square{ Square.g6, Square.f7 };
+    const expected_h1 = [2]Square{ Square.f2, Square.g3 };
+    const expected_a1 = [2]Square{ Square.b3, Square.c2 };
+
+    expectEqual(bitboardFromSquares(expected_a8[0..]), knightAttacks(knight_a8));
+    expectEqual(bitboardFromSquares(expected_h8[0..]), knightAttacks(knight_h8));
+    expectEqual(bitboardFromSquares(expected_h1[0..]), knightAttacks(knight_h1));
+    expectEqual(bitboardFromSquares(expected_a1[0..]), knightAttacks(knight_a1));
+}
-- 
2.45.2