Add magic bitboard generation

src/movegen/wizardry/generation.rs

@@ -0,0 +1,74 @@
+use crate::board::{Bitboard, Square};
+use crate::movegen::bishop::bishop_moves;
+use crate::movegen::rook::rook_moves;
+use crate::movegen::Magic;
+
+use super::mask::{generate_bishop_mask, generate_rook_mask};
+
+/// A trait to represent RNG for u64 values.
+#[allow(unused)] // FIXME: remove when used
+pub(crate) trait RandGen {
+    fn gen(&mut self) -> u64;
+}
+
+type MagicGenerationType = (Vec<Magic>, Vec<Bitboard>);
+
+#[allow(unused)] // FIXME: remove when used
+pub fn generate_bishop_magics(rng: &mut dyn RandGen) -> MagicGenerationType {
+    generate_magics(rng, generate_bishop_mask, bishop_moves)
+}
+
+#[allow(unused)] // FIXME: remove when used
+pub fn generate_rook_magics(rng: &mut dyn RandGen) -> MagicGenerationType {
+    generate_magics(rng, generate_rook_mask, rook_moves)
+}
+
+fn generate_magics(
+    rng: &mut dyn RandGen,
+    mask_fn: impl Fn(Square) -> Bitboard,
+    moves_fn: impl Fn(Square, Bitboard) -> Bitboard,
+) -> MagicGenerationType {
+    let mut magics = Vec::new();
+    let mut boards = Vec::new();
+
+    for square in Square::iter() {
+        let mask = mask_fn(square);
+
+        let occupancy_to_moves: Vec<_> = mask
+            .iter_power_set()
+            .map(|occupancy| (occupancy, moves_fn(square, occupancy)))
+            .collect();
+
+        'candidate_search: loop {
+            let mut candidate = Magic {
+                magic: magic_candidate(rng),
+                offset: 0,
+                mask,
+                shift: (64 - mask.count()) as u8,
+            };
+            let mut candidate_moves = vec![Bitboard::EMPTY; occupancy_to_moves.len()];
+
+            for (occupancy, moves) in occupancy_to_moves.iter().cloned() {
+                let index = candidate.get_index(occupancy);
+                // Non-constructive collision, try with another candidate
+                if !candidate_moves[index].is_empty() && candidate_moves[index] != moves {
+                    continue 'candidate_search;
+                }
+                candidate_moves[index] = moves;
+            }
+
+            // We have filled all candidate boards, record the correct offset and add the moves
+            candidate.offset = boards.len();
+            magics.push(candidate);
+            boards.append(&mut candidate_moves);
+            break;
+        }
+    }
+
+    (magics, boards)
+}
+
+fn magic_candidate(rng: &mut dyn RandGen) -> u64 {
+    // Few bits makes for better candidates
+    rng.gen() & rng.gen() & rng.gen()
+}