From 066d4428231629605399607ad2173a12de3f38e5 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Fri, 22 Jul 2022 18:42:05 +0200
Subject: [PATCH] Add magic bitboard generation

---
 src/movegen/magic/mod.rs           |  1 -
 src/movegen/wizardry/generation.rs | 74 ++++++++++++++++++++++++++++++
 src/movegen/wizardry/mask.rs       |  2 -
 src/movegen/wizardry/mod.rs        |  1 +
 4 files changed, 75 insertions(+), 3 deletions(-)
 create mode 100644 src/movegen/wizardry/generation.rs

diff --git a/src/movegen/magic/mod.rs b/src/movegen/magic/mod.rs
index f9d01d1..242a0b4 100644
--- a/src/movegen/magic/mod.rs
+++ b/src/movegen/magic/mod.rs
@@ -14,7 +14,6 @@ pub struct Magic {
 
 impl Magic {
     /// Compute the index into the magics database for this set of `blockers`.
-    #[allow(unused)] // FIXME: remove once used
     pub fn get_index(&self, blockers: Bitboard) -> usize {
         let relevant_occupancy = (blockers & self.mask).0;
         let base_index = ((relevant_occupancy.wrapping_mul(self.magic)) >> self.shift) as usize;
diff --git a/src/movegen/wizardry/generation.rs b/src/movegen/wizardry/generation.rs
new file mode 100644
index 0000000..a5e47e2
--- /dev/null
+++ b/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()
+}
diff --git a/src/movegen/wizardry/mask.rs b/src/movegen/wizardry/mask.rs
index af1b8fa..aca9f4f 100644
--- a/src/movegen/wizardry/mask.rs
+++ b/src/movegen/wizardry/mask.rs
@@ -3,7 +3,6 @@ use crate::movegen::bishop::bishop_moves;
 use crate::movegen::rook::rook_moves;
 
 /// Compute the relevancy mask for a bishop on a given [Square].
-#[allow(unused)] // FIXME: remove once used
 pub fn generate_bishop_mask(square: Square) -> Bitboard {
     let rays = bishop_moves(square, Bitboard::EMPTY);
 
@@ -16,7 +15,6 @@ pub fn generate_bishop_mask(square: Square) -> Bitboard {
 }
 
 /// Compute the relevancy mask for a rook on a given [Square].
-#[allow(unused)] // FIXME: remove once used
 pub fn generate_rook_mask(square: Square) -> Bitboard {
     let rays = rook_moves(square, Bitboard::EMPTY);
 
diff --git a/src/movegen/wizardry/mod.rs b/src/movegen/wizardry/mod.rs
index f6053c6..8b5ba4e 100644
--- a/src/movegen/wizardry/mod.rs
+++ b/src/movegen/wizardry/mod.rs
@@ -1 +1,2 @@
+mod generation;
 mod mask;