115 lines
3.7 KiB
Rust
115 lines
3.7 KiB
Rust
//! Deterministic 32-bit xorshift generator, bit-exact with `model/rng.ts`.
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/// Xorshift32, the original inline kernel RNG.
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///
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/// The JavaScript version keeps its state as a raw `number` that the shift operators coerce to
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/// int32 on every draw; after the first draw the state is therefore always an int32, and that is
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/// the signed value checkpoints store verbatim. This port stores it as an `i32` from the start,
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/// which is the same value for every reachable state (the default seed, 22222, is already an
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/// int32).
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#[derive(Debug, Clone, Copy, PartialEq, Eq)]
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pub struct Xorshift32 {
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value: i32,
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}
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/// The seed of the original kernel.
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pub const DEFAULT_SEED: i32 = 22_222;
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impl Default for Xorshift32 {
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fn default() -> Self {
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Self::new(DEFAULT_SEED)
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}
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}
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impl Xorshift32 {
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pub fn new(seed: i32) -> Self {
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Self { value: seed }
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}
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/// Next raw draw as an unsigned 32-bit integer (`value >>> 0` in the original).
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#[inline]
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pub fn next_uint(&mut self) -> u32 {
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// `value ^= value << 13` etc. in JS: the shifts do the int32 coercion, `>>> 17` is the
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// only unsigned one.
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let mut value = self.value;
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value ^= value << 13;
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value ^= ((value as u32) >> 17) as i32;
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value ^= value << 5;
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self.value = value;
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value as u32
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}
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/// Next draw mapped to `[0, 1)` with 2^-32 resolution.
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#[inline]
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pub fn next_f64(&mut self) -> f64 {
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f64::from(self.next_uint()) / 4_294_967_296.0
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}
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/// Signed internal state, as written to and read from checkpoints.
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#[inline]
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pub fn state(&self) -> i32 {
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self.value
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}
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/// Restore a checkpointed state. A JS checkpoint holds the signed int32; anything wider is
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/// reduced the way the next shift operator would reduce it (`ToInt32`).
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#[inline]
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pub fn set_state(&mut self, value: i32) {
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self.value = value;
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}
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/// `ToInt32` of an arbitrary JSON integer, for importing a checkpoint written by a host that
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/// stored the state as an unsigned or oversized number.
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pub fn to_int32(value: f64) -> i32 {
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(value as i64 as u32) as i32
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn reproduces_the_original_kernel_stream() {
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// The reference stream, recomputed here the way tests/model.test.ts recomputes it.
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let mut legacy: i32 = 22_222;
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let mut legacy_uint = move || {
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let mut value = legacy;
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value ^= value << 13;
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value ^= ((value as u32) >> 17) as i32;
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value ^= value << 5;
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legacy = value;
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value as u32
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};
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let mut rng = Xorshift32::default();
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assert_eq!(rng.state(), 22_222);
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for _ in 0..10_000 {
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assert_eq!(rng.next_uint(), legacy_uint());
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}
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let checkpoint = rng.state();
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let draws = [rng.next_f64(), rng.next_f64(), rng.next_f64()];
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rng.set_state(checkpoint);
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assert_eq!([rng.next_f64(), rng.next_f64(), rng.next_f64()], draws);
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assert!(draws.iter().all(|value| *value >= 0.0 && *value < 1.0));
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assert_ne!(
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Xorshift32::new(7).next_uint(),
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Xorshift32::default().next_uint()
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);
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}
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#[test]
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fn first_draws_match_node() {
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// node -e 'let v=22222; const n=()=>{v^=v<<13;v^=v>>>17;v^=v<<5;return v>>>0};
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// console.log(n(),n(),n(),n())'
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let mut rng = Xorshift32::default();
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assert_eq!(
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[
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rng.next_uint(),
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rng.next_uint(),
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rng.next_uint(),
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rng.next_uint()
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],
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[1_374_414_818, 2_413_927_497, 829_730_269, 1_025_812_730]
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);
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}
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}
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