Pre-generate the magic bitboard seeds

My naive RNG implementation takes about ~40 seconds to generate the
magic bitboards for both bishops and rooks (or ~1 second in release
mode).

If we pre-generate the seeds, we can instead make it ~instantaneous.

The self-modifying code is inspired by matklad [1].

[1]: https://matklad.github.io/2022/03/26/self-modifying-code.html

src/movegen/moves.rs

@@ -4,25 +4,35 @@ use crate::{
     board::{Bitboard, Color, File, Square},
     movegen::{
         naive,
-        wizardry::{generate_bishop_magics, generate_rook_magics, MagicMoves, RandGen},
+        wizardry::{
+            generate_bishop_magics, generate_rook_magics, MagicMoves, RandGen, BISHOP_SEED,
+            ROOK_SEED,
+        },
     },
 };
 
-// A simple XOR-shift RNG implementation.
-struct SimpleRng(u64);
+// A pre-rolled RNG for magic bitboard generation, using pre-determined values.
+struct PreRolledRng {
+    numbers: [u64; 64],
+    current_index: usize,
+}
 
-impl SimpleRng {
-    pub fn new() -> Self {
-        Self(4) // https://xkcd.com/221/
+impl PreRolledRng {
+    pub fn new(numbers: [u64; 64]) -> Self {
+        Self {
+            numbers,
+            current_index: 0,
+        }
     }
 }
 
-impl RandGen for SimpleRng {
+impl RandGen for PreRolledRng {
     fn gen(&mut self) -> u64 {
-        self.0 ^= self.0 >> 12;
-        self.0 ^= self.0 << 25;
-        self.0 ^= self.0 >> 27;
-        self.0
+        // We roll 3 numbers per square to bitwise-and them together.
+        // Just return the same one 3 times as a work-around.
+        let res = self.numbers[self.current_index / 3];
+        self.current_index += 1;
+        res
     }
 }
 
@@ -86,7 +96,7 @@ pub fn bishop_moves(square: Square, blockers: Bitboard) -> Bitboard {
     static BISHOP_MAGICS: OnceLock<MagicMoves> = OnceLock::new();
     BISHOP_MAGICS
         .get_or_init(|| {
-            let (magics, moves) = generate_bishop_magics(&mut SimpleRng::new());
+            let (magics, moves) = generate_bishop_magics(&mut PreRolledRng::new(BISHOP_SEED));
             // SAFETY: we used the generator function to compute these values
             unsafe { MagicMoves::new(magics, moves) }
         })
@@ -98,7 +108,7 @@ pub fn rook_moves(square: Square, blockers: Bitboard) -> Bitboard {
     static ROOK_MAGICS: OnceLock<MagicMoves> = OnceLock::new();
     ROOK_MAGICS
         .get_or_init(|| {
-            let (magics, moves) = generate_rook_magics(&mut SimpleRng::new());
+            let (magics, moves) = generate_rook_magics(&mut PreRolledRng::new(ROOK_SEED));
             // SAFETY: we used the generator function to compute these values
             unsafe { MagicMoves::new(magics, moves) }
         })

src/movegen/wizardry/mod.rs

@@ -56,3 +56,228 @@ impl MagicMoves {
         }
     }
 }
+
+// region:sourcegen
+/// A set of magic numbers for bishop move generation.
+pub(crate) const BISHOP_SEED: [u64; 64] = [
+    4908958787341189172,
+    1157496606860279808,
+    289395876198088778,
+    649648646467355137,
+    19162426089930848,
+    564067194896448,
+    18586170375029026,
+    9185354800693760,
+    72172012436987968,
+    317226351607872,
+    2597178509285688384,
+    1162205282238464,
+    144154788211329152,
+    172197832046936160,
+    4625762105940000802,
+    1477217245166903296,
+    2251937789583872,
+    289373902621379585,
+    4616200855845409024,
+    2251909637357568,
+    3532510975437640064,
+    563517968228352,
+    562953309660434,
+    1196005458310201856,
+    2350914225914520576,
+    2287018679861376,
+    13836188353273790593,
+    11267795163676832,
+    297519119119499264,
+    18588344158519552,
+    10453428171813953792,
+    72128237668534272,
+    1298164929055953920,
+    865575144395900952,
+    9293076573325312,
+    108104018148197376,
+    578503662094123152,
+    4665870505495102224,
+    6066493872259301520,
+    285877477613857,
+    2328941618281318466,
+    721165292771739652,
+    4899973577790523400,
+    75050392749184,
+    2305878200632215680,
+    11530099074925593616,
+    290561512873919880,
+    18652187227888000,
+    3379933716168704,
+    9223409493537718272,
+    22273835729926,
+    1152921524003672064,
+    4647812741240848385,
+    1244225087719112712,
+    7367907171013001728,
+    9263922034316951570,
+    300758214358598160,
+    4611686331973636096,
+    2377900605806479360,
+    6958097192913601024,
+    864691130877743617,
+    703824948904066,
+    612700674899317536,
+    180742128018784384,
+];
+
+/// A set of magic numbers for rook move generation.
+pub(crate) const ROOK_SEED: [u64; 64] = [
+    2341871943948451840,
+    18015635528220736,
+    72066665545773824,
+    1188959097794342912,
+    12141713393631625314,
+    720649693658353672,
+    36029896538981888,
+    36033359356363520,
+    140746619355268,
+    1158339898446446661,
+    36591886560003650,
+    578853633228023808,
+    2392554490300416,
+    140814806160384,
+    180706952366596608,
+    10696087878779396,
+    1153260703948210820,
+    310748649170673678,
+    36311372044308544,
+    9223444604757615104,
+    1267187285230592,
+    282574622818306,
+    18722484274726152,
+    2271591090110593,
+    1153063519847989248,
+    10168327557107712,
+    4507998211276833,
+    1153203035420233728,
+    4631961017139660032,
+    2454499182462107776,
+    289367288355753288,
+    18015815850820609,
+    9268726066908758912,
+    11547264697673728000,
+    2314929519368081536,
+    140943655192577,
+    20266215511427202,
+    180706969441535248,
+    1302683805944911874,
+    11534000122299940994,
+    22676602724843520,
+    4639271120198041668,
+    1302104069046927376,
+    9184220895313928,
+    4612249105954373649,
+    562984581726212,
+    2312678200579457040,
+    4647736876550193157,
+    3170604524138139776,
+    4684447574787096704,
+    20283792725901696,
+    1152992019380963840,
+    117383863558471808,
+    1153488854922068096,
+    17596884583424,
+    90074759127192064,
+    4900502436426416706,
+    4573968656793901,
+    1161084564408385,
+    1657887889314811910,
+    4614501455660058690,
+    4612530729109422081,
+    642458506527236,
+    1116704154754,
+];
+// endregion:sourcegen
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    // A simple XOR-shift RNG implementation.
+    struct SimpleRng(u64);
+
+    impl SimpleRng {
+        pub fn new() -> Self {
+            Self(4) // https://xkcd.com/221/
+        }
+    }
+
+    impl RandGen for SimpleRng {
+        fn gen(&mut self) -> u64 {
+            self.0 ^= self.0 >> 12;
+            self.0 ^= self.0 << 25;
+            self.0 ^= self.0 >> 27;
+            self.0
+        }
+    }
+
+    #[test]
+    fn rng() {
+        let mut rng = SimpleRng::new();
+
+        assert_eq!(rng.gen(), 134217733);
+        assert_eq!(rng.gen(), 4504699139039237);
+        assert_eq!(rng.gen(), 13512173405898766);
+        assert_eq!(rng.gen(), 9225626310854853124);
+        assert_eq!(rng.gen(), 29836777971867270);
+    }
+
+    fn split_twice<'a>(
+        text: &'a str,
+        start_marker: &str,
+        end_marker: &str,
+    ) -> Option<(&'a str, &'a str, &'a str)> {
+        let (prefix, rest) = text.split_once(start_marker)?;
+        let (mid, suffix) = rest.split_once(end_marker)?;
+        Some((prefix, mid, suffix))
+    }
+
+    fn array_string(piece_type: &str, values: &[Magic]) -> String {
+        let mut res = format!(
+            "/// A set of magic numbers for {} move generation.\n",
+            piece_type
+        );
+        res.push_str(&format!(
+            "pub(crate) const {}_SEED: [u64; 64] = [\n",
+            piece_type.to_uppercase()
+        ));
+        for magic in values {
+            res.push_str(&format!("    {},\n", magic.magic));
+        }
+        res.push_str("];\n");
+        res
+    }
+
+    #[test]
+    #[ignore = "slow"]
+    // Regenerates the magic bitboard numbers.
+    fn regen_magic_seeds() {
+        // We only care about the magics, the moves can be recomputed at runtime ~cheaply.
+        let (bishop_magics, _) = generate_bishop_magics(&mut SimpleRng::new());
+        let (rook_magics, _) = generate_rook_magics(&mut SimpleRng::new());
+
+        let original_text = std::fs::read_to_string(file!()).unwrap();
+
+        let bishop_array = array_string("bishop", &bishop_magics[..]);
+        let rook_array = array_string("rook", &rook_magics[..]);
+
+        let new_text = {
+            let start_marker = "// region:sourcegen\n";
+            let end_marker = "// endregion:sourcegen\n";
+            let (prefix, _, suffix) =
+                split_twice(&original_text, start_marker, end_marker).unwrap();
+            format!("{prefix}{start_marker}{bishop_array}\n{rook_array}{end_marker}{suffix}")
+        };
+
+        if new_text != original_text {
+            std::fs::write(file!(), new_text).unwrap();
+            panic!("source was not up-to-date")
+        }
+    }
+}