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c2f1443436
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105 lines
2.8 KiB
Rust
105 lines
2.8 KiB
Rust
//! Borland Win16 random-number stream used by the original executable.
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use crate::real48::Real48;
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const MULTIPLIER: u32 = 0x0808_8405;
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#[cfg(test)]
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const TWO_TO_32: f64 = 4_294_967_296.0;
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#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
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pub struct BorlandRandom {
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seed: u32,
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}
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impl BorlandRandom {
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pub const fn new(seed: u32) -> Self {
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Self { seed }
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}
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pub fn next_u32(&mut self) -> u32 {
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self.seed = self.seed.wrapping_mul(MULTIPLIER).wrapping_add(1);
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self.seed
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}
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#[allow(clippy::cast_possible_truncation)]
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pub fn below(&mut self, upper_bound: u16) -> u16 {
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let product = u64::from(self.next_u32()) * u64::from(upper_bound);
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(product >> 32) as u16
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}
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#[allow(clippy::cast_possible_truncation)]
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pub fn real48(&mut self) -> Real48 {
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let mut random = self.next_u32();
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if random == 0 {
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return Real48::ZERO;
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}
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let mut exponent = 0x80_u8;
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while random & 0x8000_0000 == 0 {
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random <<= 1;
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exponent = exponent.wrapping_sub(1);
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}
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random &= 0x7fff_ffff;
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Real48::from_bytes([
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exponent,
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0,
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random as u8,
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(random >> 8) as u8,
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(random >> 16) as u8,
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(random >> 24) as u8,
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])
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}
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/// Exact host representation of the x87 `Random` result in `[0, 1)`.
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#[cfg(test)]
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pub fn unit_interval(&mut self) -> f64 {
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f64::from(self.next_u32()) / TWO_TO_32
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}
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pub const fn seed(self) -> u32 {
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self.seed
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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 stream_matches_the_reconstructed_borland_runtime() {
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let mut random = BorlandRandom::new(0x1234_5678);
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let first = 0x1234_5678_u32.wrapping_mul(MULTIPLIER).wrapping_add(1);
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assert_eq!(random.next_u32(), first);
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assert_eq!(random.below(3_800), 1_810);
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}
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#[test]
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fn zero_bounds_still_advance_the_seed() {
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let mut random = BorlandRandom::new(7);
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assert_eq!(random.below(0), 0);
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assert_eq!(
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random.seed(),
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7_u32.wrapping_mul(MULTIPLIER).wrapping_add(1)
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);
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}
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#[test]
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fn unit_interval_is_the_exact_unsigned_seed_fraction() {
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let mut random = BorlandRandom::new(0xfedc_ba98);
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let expected_seed = 0xfedc_ba98_u32.wrapping_mul(MULTIPLIER).wrapping_add(1);
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assert_eq!(
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random.unit_interval().to_bits(),
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(f64::from(expected_seed) / TWO_TO_32).to_bits()
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);
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}
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#[test]
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fn real48_register_result_matches_the_reconstructed_normalization() {
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let mut random = BorlandRandom::new(7);
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assert_eq!(
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random.real48(),
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Real48::from_bytes([0x7e, 0, 0x90, 0x70, 0xee, 0x60])
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);
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}
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}
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