1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146

const MANTISSA_BITS: i32 = 52;
const MANTISSA_MASK: u64 = (1u64 << MANTISSA_BITS) - 1;

fn ldexp(x: u64, exp: i32) -> f64 {
    (x as f64) * f64::exp2(exp as f64)
}

// https://en.wikipedia.org/wiki/Lehmer_random_number_generator
fn lcg_parkmiller(seed: u32) -> u32 {
    (((seed as u64) * 48_271_u64) % 2_147_483_647_u64) as u32
}

// http://xorshift.di.unimi.it/splitmix64.c
fn splitmix64(seed: u64) -> u64 {
    let mut z = seed + 0x9E3779B97F4A7C15_u64;
    z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9_u64;
    z = (z ^ (z >> 27)) * 0x94D049BB133111EB_u64;
    return z ^ (z >> 31);
}

fn u64_from_bytes (bytes: &[u8]) -> u64 {
    ((bytes[7] as u64) << 56) + ((bytes[6] as u64) << 48) +
    ((bytes[5] as u64) << 40) + ((bytes[4] as u64) << 32) +
    ((bytes[3] as u64) << 24) + ((bytes[2] as u64) << 16) +
    ((bytes[1] as u64) <<  8) + ((bytes[0] as u64) <<  0)
}

/// # Examples
///
/// Construct a RNG from an integer.
///
/// ```
/// extern crate xorshift128plus;
///
/// use xorshift128plus::XorShift128Plus;
///
/// # fn main() {
/// let mut rng = XorShift128Plus::from_u32(4293262078);
///
/// println!("First random float: {}", rng.next());
/// println!("Second random float: {}", rng.next());
/// # }
/// ```
///
/// You can also construct a RNG by supplying 16 bytes of raw data. Here we are using the [rand
/// crates `rand::random`](https://doc.rust-lang.org/rand/rand/fn.random.html) to generate 16 bytes
/// of random data to use as the initial seed. This data could also come from a file, a database or
/// anywhere else really.
///
/// ```
/// extern crate rand;
/// extern crate xorshift128plus;
///
/// use xorshift128plus::XorShift128Plus;
///
/// # fn main() {
/// let seed: [u8; 16] = rand::random();
/// let mut rng = XorShift128Plus::from_bytes(seed);
///
/// println!("First random float: {}", rng.next());
/// println!("Second random float: {}", rng.next());
/// # }
/// ```
pub struct XorShift128Plus (u64, u64);

impl XorShift128Plus {
    /// Constructs a new RNG with the seed specified as 16 bytes of raw data.
    pub fn from_bytes (seed: [u8; 16]) -> XorShift128Plus {
        XorShift128Plus (
            u64_from_bytes(&seed[0..8]),
            u64_from_bytes(&seed[8..16]),
        )
    }

    /// Constructs a new RNG with the seed specified as a unsigned 32bit integer. Note that
    /// this seeding is suboptimal since it will only contain 32 bits of entropy instead
    /// of 128 bits.
    pub fn from_u32 (seed: u32) -> XorShift128Plus {
        let raw0 = lcg_parkmiller(seed);
        let raw1 = lcg_parkmiller(raw0);
        let raw2 = lcg_parkmiller(raw1);
        let raw3 = lcg_parkmiller(raw2);

        XorShift128Plus (
            ((raw1 as u64) << 32) + (raw0 as u64),
            ((raw3 as u64) << 32) + (raw2 as u64),
        )
    }

    /// Constructs a new RNG with the seed specified as a unsigned 64bit integer. Note that
    /// this seeding is suboptimal since it will only contain 64 bits of entropy instead
    /// of 128 bits.
    pub fn from_u64 (seed: u64) -> XorShift128Plus {
        let raw0 = splitmix64(seed);
        let raw1 = splitmix64(raw0);

        XorShift128Plus (raw0, raw1)
    }

    /// Returns the next psuedo-random number between 0 (inclusivly) and 1 (exclusivly).
    pub fn next (&mut self) -> f64 {
        let mut x = self.0;
        let y = self.1;

        self.0 = y;

        x ^= x << 23;
        x ^= x >> 17;
        x ^= y;
        x ^= y >> 26;

        self.1 = x;

        ldexp(u64::wrapping_add(self.0, self.1) & MANTISSA_MASK, -MANTISSA_BITS)
    }
}

#[cfg(test)]
mod tests {
    use super::XorShift128Plus;

    #[test]
    fn it_should_seed_from_bytes() {
        let mut rng = XorShift128Plus::from_bytes([
            0x5d, 0x28, 0x94, 0x50, 0xc8, 0x88, 0xf9, 0x9b,
            0x5e, 0x5c, 0x1f, 0xd1, 0x35, 0x09, 0xe3, 0x9e,
        ]);

        assert_eq!(rng.next(), 0.35873106038177727);
        assert_eq!(rng.next(), 0.7433543130711686);
        assert_eq!(rng.next(), 0.6325316214071923);
        assert_eq!(rng.next(), 0.708663591569944);
        assert_eq!(rng.next(), 0.8974382234842848);
    }

    #[test]
    fn it_should_seed_from_u32() {
        let mut rng = XorShift128Plus::from_u32(4293262078);

        assert_eq!(rng.next(), 0.4335893835472515);
        assert_eq!(rng.next(), 0.6067907909036327);
        assert_eq!(rng.next(), 0.046905965279849804);
        assert_eq!(rng.next(), 0.480991995797152);
        assert_eq!(rng.next(), 0.6796126170804464);
    }
}