// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::convert::Infallible;
use rand_core::{SeedableRng, TryRng, utils};
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

/// A xoroshiro64* random number generator.
///
/// The xoroshiro64* algorithm is not suitable for cryptographic purposes, but
/// is very fast and has good statistical properties, besides a low linear
/// complexity in the lowest bits.
///
/// The algorithm used here is translated from [the `xoroshiro64star.c`
/// reference source code](http://xoshiro.di.unimi.it/xoroshiro64star.c) by
/// David Blackman and Sebastiano Vigna.
#[allow(missing_copy_implementations)]
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct Xoroshiro64Star {
    s0: u32,
    s1: u32,
}

impl TryRng for Xoroshiro64Star {
    type Error = Infallible;

    #[inline]
    fn try_next_u32(&mut self) -> Result<u32, Self::Error> {
        let r = self.s0.wrapping_mul(0x9E3779BB);
        impl_xoroshiro_u32!(self);
        Ok(r)
    }

    #[inline]
    fn try_next_u64(&mut self) -> Result<u64, Self::Error> {
        utils::next_u64_via_u32(self)
    }

    #[inline]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Self::Error> {
        utils::fill_bytes_via_next_word(dest, || self.try_next_u32())
    }
}

impl SeedableRng for Xoroshiro64Star {
    type Seed = [u8; 8];

    /// Create a new `Xoroshiro64Star`.  If `seed` is entirely 0, it will be
    /// mapped to a different seed.
    fn from_seed(seed: [u8; 8]) -> Xoroshiro64Star {
        deal_with_zero_seed!(seed, Self, 8);
        let s: [_; 2] = utils::read_words(&seed);

        Xoroshiro64Star { s0: s[0], s1: s[1] }
    }

    /// Seed a `Xoroshiro64Star` from a `u64` using `SplitMix64`.
    fn seed_from_u64(seed: u64) -> Xoroshiro64Star {
        from_splitmix!(seed)
    }
}

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

    #[test]
    fn reference() {
        let mut rng = Xoroshiro64Star::from_seed([1, 0, 0, 0, 2, 0, 0, 0]);
        // These values were produced with the reference implementation:
        // http://xoshiro.di.unimi.it/xoshiro64star.c
        let expected = [
            2654435771, 327208753, 4063491769, 4259754937, 261922412, 168123673, 552743735,
            1672597395, 1031040050, 2755315674,
        ];
        for &e in &expected {
            assert_eq!(rng.next_u32(), e);
        }
    }

    #[test]
    fn zero_seed() {
        let mut rng = Xoroshiro64Star::seed_from_u64(0);
        assert_ne!(rng.next_u64(), 0);
    }
}
