lighthouse-pulse/lighthouse/state/transition/shuffling/rng.rs

use super::blake2_rfc::blake2s::{ Blake2s, Blake2sResult };

const SEED_SIZE_BYTES: usize = 32;
const RAND_BYTES: usize = 3;      // 24 / 8
const RAND_MAX: u32 = 16777216;   // 2**24

/// A pseudo-random number generator which given a seed
/// uses successive blake2s hashing to generate "entropy".
pub struct ShuffleRng {
    seed: Blake2sResult,
    idx: usize,
    pub rand_max: u32,
}

impl ShuffleRng {
    /// Create a new instance given some "seed" bytes.
    pub fn new(initial_seed: &[u8]) -> Self {
        Self {
            seed: hash(initial_seed),
            idx: 0,
            rand_max: RAND_MAX,
        }
    }

    /// "Regenerates" the seed by hashing it.
    fn rehash_seed(&mut self) {
        self.seed  = hash(self.seed.as_bytes());
        self.idx = 0;
    }

    /// Extracts 3 bytes from the `seed`. Rehashes seed if required.
    fn rand(&mut self) -> u32 {
        self.idx += RAND_BYTES;
        match self.idx >= SEED_SIZE_BYTES {
            true => {
                self.rehash_seed();
                self.rand()
            }
            false => {
                int_from_byte_slice(
                    self.seed.as_bytes(),
                    self.idx - RAND_BYTES,
                )
            }
        }
    }

    /// Generate a random u32 below the specified maximum `n`.
    ///
    /// Provides a filtered result from a higher-level rng, by discarding
    /// results which may bias the output. Because of this, execution time is
    /// not linear and may potentially be infinite.
    pub fn rand_range(&mut self, n: u32) -> u32 {
        assert!(n < RAND_MAX, "RAND_MAX exceed");
        let mut x = self.rand();
        while x >= self.rand_max - (self.rand_max % n) {
            x = self.rand();
        }
        x % n
    }
}

/// Reads the next three bytes of `source`, starting from `offset` and
/// interprets those bytes as a 24 bit big-endian integer.
/// Returns that integer.
fn int_from_byte_slice(source: &[u8], offset: usize) -> u32 {
    (
          source[offset + 2] as u32) |
        ((source[offset + 1] as u32) << 8) |
        ((source[offset    ] as u32) << 16
    )
}

/// Peform a blake2s hash on the given bytes.
fn hash(bytes: &[u8]) -> Blake2sResult {
    let mut hasher = Blake2s::new(SEED_SIZE_BYTES);
    hasher.update(bytes);
    hasher.finalize()
}


#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_shuffling_int_from_slice() {
        let mut x = int_from_byte_slice(
            &[0, 0, 1],
            0);
        assert_eq!((x as u32), 1);

        x = int_from_byte_slice(
            &[0, 1, 1],
            0);
        assert_eq!(x, 257);

        x = int_from_byte_slice(
            &[1, 1, 1],
            0);
        assert_eq!(x, 65793);

        x = int_from_byte_slice(
            &[255, 1, 1],
            0);
        assert_eq!(x, 16711937);

        x = int_from_byte_slice(
            &[255, 255, 255],
            0);
        assert_eq!(x, 16777215);

        x = int_from_byte_slice(
            &[0x8f, 0xbb, 0xc7],
            0);
        assert_eq!(x, 9419719);
    }

    #[test]
    fn test_shuffling_hash_fn() {
        let digest = hash(hash(b"4kn4driuctg8").as_bytes());  // double-hash is intentional
        let digest_bytes = digest.as_bytes();
        let expected = [
            0xff, 0xff, 0xff, 0x8f, 0xbb, 0xc7, 0xab, 0x64, 0x43, 0x9a,
            0xe5, 0x12, 0x44, 0xd8, 0x70, 0xcf, 0xe5, 0x79, 0xf6, 0x55,
            0x6b, 0xbd, 0x81, 0x43, 0xc5, 0xcd, 0x70, 0x2b, 0xbe, 0xe3,
            0x87, 0xc7,
        ];
        assert_eq!(digest_bytes.len(), expected.len());
        assert_eq!(digest_bytes, expected)
    }
}