// 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 rand_core::impls::fill_bytes_via_next;
use rand_core::le::read_u64_into;
use rand_core::{RngCore, SeedableRng};
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

/// A xoshiro256+ random number generator.
///
/// The xoshiro256+ 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 `xoshiro256plus.c`
/// reference source code](http://xoshiro.di.unimi.it/xoshiro256plus.c) by
/// David Blackman and Sebastiano Vigna.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct Xoshiro256Plus {
    s: [u64; 4],
}

impl Xoshiro256Plus {
    /// Jump forward, equivalently to 2^128 calls to `next_u64()`.
    ///
    /// This can be used to generate 2^128 non-overlapping subsequences for
    /// parallel computations.
    ///
    /// ```
    /// use rand_xoshiro::rand_core::SeedableRng;
    /// use rand_xoshiro::Xoshiro256Plus;
    ///
    /// let rng1 = Xoshiro256Plus::seed_from_u64(0);
    /// let mut rng2 = rng1.clone();
    /// rng2.jump();
    /// let mut rng3 = rng2.clone();
    /// rng3.jump();
    /// ```
    pub fn jump(&mut self) {
        impl_jump!(
            u64,
            self,
            [
                0x180ec6d33cfd0aba,
                0xd5a61266f0c9392c,
                0xa9582618e03fc9aa,
                0x39abdc4529b1661c
            ]
        );
    }

    /// Jump forward, equivalently to 2^192 calls to `next_u64()`.
    ///
    /// This can be used to generate 2^64 starting points, from each of which
    /// `jump()` will generate 2^64 non-overlapping subsequences for parallel
    /// distributed computations.
    pub fn long_jump(&mut self) {
        impl_jump!(
            u64,
            self,
            [
                0x76e15d3efefdcbbf,
                0xc5004e441c522fb3,
                0x77710069854ee241,
                0x39109bb02acbe635
            ]
        );
    }
}

impl SeedableRng for Xoshiro256Plus {
    type Seed = [u8; 32];

    /// Create a new `Xoshiro256Plus`.  If `seed` is entirely 0, it will be
    /// mapped to a different seed.
    #[inline]
    fn from_seed(seed: [u8; 32]) -> Xoshiro256Plus {
        deal_with_zero_seed!(seed, Self);
        let mut state = [0; 4];
        read_u64_into(&seed, &mut state);
        Xoshiro256Plus { s: state }
    }

    /// Seed a `Xoshiro256Plus` from a `u64` using `SplitMix64`.
    fn seed_from_u64(seed: u64) -> Xoshiro256Plus {
        from_splitmix!(seed)
    }
}

impl RngCore for Xoshiro256Plus {
    #[inline]
    fn next_u32(&mut self) -> u32 {
        // The lowest bits have some linear dependencies, so we use the
        // upper bits instead.
        (self.next_u64() >> 32) as u32
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        let result_plus = self.s[0].wrapping_add(self.s[3]);
        impl_xoshiro_u64!(self);
        result_plus
    }

    #[inline]
    fn fill_bytes(&mut self, dest: &mut [u8]) {
        fill_bytes_via_next(self, dest);
    }
}

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

    #[test]
    fn reference() {
        let mut rng = Xoshiro256Plus::from_seed([
            1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0,
            0, 0, 0,
        ]);
        // These values were produced with the reference implementation:
        // http://xoshiro.di.unimi.it/xoshiro256plus.c
        let expected = [
            5,
            211106232532999,
            211106635186183,
            9223759065350669058,
            9250833439874351877,
            13862484359527728515,
            2346507365006083650,
            1168864526675804870,
            34095955243042024,
            3466914240207415127,
        ];
        for &e in &expected {
            assert_eq!(rng.next_u64(), e);
        }
    }
}
