91 lines
2.5 KiB
Rust
91 lines
2.5 KiB
Rust
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use super::hashing::hash;
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const SEED_SIZE_BYTES: usize = 32;
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const RAND_BYTES: usize = 3; // 24 / 8
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const RAND_MAX: u32 = 16_777_215; // 2 ** (rand_bytes * 8) - 1
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/// A pseudo-random number generator which given a seed
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/// uses successive blake2s hashing to generate "entropy".
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pub struct ShuffleRng {
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seed: Vec<u8>,
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idx: usize,
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pub rand_max: u32,
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}
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impl ShuffleRng {
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/// Create a new instance given some "seed" bytes.
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pub fn new(initial_seed: &[u8]) -> Self {
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Self {
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seed: hash(initial_seed),
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idx: 0,
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rand_max: RAND_MAX,
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}
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}
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/// "Regenerates" the seed by hashing it.
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fn rehash_seed(&mut self) {
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self.seed = hash(&self.seed);
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self.idx = 0;
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}
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/// Extracts 3 bytes from the `seed`. Rehashes seed if required.
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fn rand(&mut self) -> u32 {
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self.idx += RAND_BYTES;
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if self.idx >= SEED_SIZE_BYTES {
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self.rehash_seed();
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self.rand()
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} else {
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int_from_byte_slice(&self.seed, self.idx - RAND_BYTES)
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}
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}
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/// Generate a random u32 below the specified maximum `n`.
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///
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/// Provides a filtered result from a higher-level rng, by discarding
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/// results which may bias the output. Because of this, execution time is
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/// not linear and may potentially be infinite.
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pub fn rand_range(&mut self, n: u32) -> u32 {
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assert!(n < RAND_MAX, "RAND_MAX exceed");
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let mut x = self.rand();
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while x >= self.rand_max - (self.rand_max % n) {
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x = self.rand();
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}
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x % n
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}
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}
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/// Reads the next three bytes of `source`, starting from `offset` and
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/// interprets those bytes as a 24 bit big-endian integer.
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/// Returns that integer.
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fn int_from_byte_slice(source: &[u8], offset: usize) -> u32 {
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(u32::from(source[offset + 2]))
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| (u32::from(source[offset + 1]) << 8)
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| (u32::from(source[offset]) << 16)
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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 test_shuffling_int_from_slice() {
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let mut x = int_from_byte_slice(&[0, 0, 1], 0);
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assert_eq!((x as u32), 1);
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x = int_from_byte_slice(&[0, 1, 1], 0);
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assert_eq!(x, 257);
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x = int_from_byte_slice(&[1, 1, 1], 0);
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assert_eq!(x, 65793);
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x = int_from_byte_slice(&[255, 1, 1], 0);
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assert_eq!(x, 16711937);
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x = int_from_byte_slice(&[255, 255, 255], 0);
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assert_eq!(x, 16777215);
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x = int_from_byte_slice(&[0x8f, 0xbb, 0xc7], 0);
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assert_eq!(x, 9419719);
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}
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}
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