// 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 xoshiro256++ random number generator.

///

/// The xoshiro256++ algorithm is not suitable for cryptographic purposes, but

/// is very fast and has excellent statistical properties.

///

/// The algorithm used here is translated from [the `xoshiro256plusplus.c`

/// reference source code](http://xoshiro.di.unimi.it/xoshiro256plusplus.c) by

/// David Blackman and Sebastiano Vigna.

#[derive(Debug, Clone, PartialEq, Eq)]

#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]

pub struct Xoshiro256PlusPlus {

    s: [u64; 4],

}

impl SeedableRng for Xoshiro256PlusPlus {

    type Seed = [u8; 32];

    /// Create a new `Xoshiro256PlusPlus`.  If `seed` is entirely 0, it will be

    /// mapped to a different seed.

    #[inline]

    fn from_seed(seed: [u8; 32]) -> Xoshiro256PlusPlus {

        let state = utils::read_words(&seed);

        // Check for zero on aligned integers for better code generation.

        // Furtermore, seed_from_u64(0) will expand to a constant when optimized.

        if state.iter().all(|&x| x == 0) {

            return Self::seed_from_u64(0);

        }

        Xoshiro256PlusPlus { s: state }

    }

    /// Create a new `Xoshiro256PlusPlus` from a `u64` seed.

    ///

    /// This uses the SplitMix64 generator internally.

    #[inline]

    fn seed_from_u64(mut state: u64) -> Self {

        const PHI: u64 = 0x9e3779b97f4a7c15;

        let mut s = [0; 4];

        for i in s.iter_mut() {

            state = state.wrapping_add(PHI);

            let mut z = state;

            z = (z ^ (z >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);

            z = (z ^ (z >> 27)).wrapping_mul(0x94d049bb133111eb);

            z = z ^ (z >> 31);

            *i = z;

        }

        // By using a non-zero PHI we are guaranteed to generate a non-zero state

        // Thus preventing a recursion between from_seed and seed_from_u64.

        debug_assert_ne!(s, [0; 4]);

        Xoshiro256PlusPlus { s }

    }

}

impl TryRng for Xoshiro256PlusPlus {

    type Error = Infallible;

    #[inline]

    fn try_next_u32(&mut self) -> Result<u32, Infallible> {

        // The lowest bits have some linear dependencies, so we use the

        // upper bits instead.

        self.try_next_u64().map(|val| (val >> 32) as u32)

    }

    #[inline]

    fn try_next_u64(&mut self) -> Result<u64, Infallible> {

        let res = self.s[0]

            .wrapping_add(self.s[3])

            .rotate_left(23)

            .wrapping_add(self.s[0]);

        let t = self.s[1] << 17;

        self.s[2] ^= self.s[0];

        self.s[3] ^= self.s[1];

        self.s[1] ^= self.s[2];

        self.s[0] ^= self.s[3];

        self.s[2] ^= t;

        self.s[3] = self.s[3].rotate_left(45);

        Ok(res)

    }

    #[inline]

    fn try_fill_bytes(&mut self, dst: &mut [u8]) -> Result<(), Infallible> {

        utils::fill_bytes_via_next_word(dst, || self.try_next_u64())

    }

}

#[cfg(test)]

mod tests {

    use super::Xoshiro256PlusPlus;

    use rand_core::{Rng, SeedableRng};

    #[test]

    fn reference() {

        let mut rng = Xoshiro256PlusPlus::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/xoshiro256plusplus.c

        let expected = [

            41943041,

            58720359,

            3588806011781223,

            3591011842654386,

            9228616714210784205,

            9973669472204895162,

            14011001112246962877,

            12406186145184390807,

            15849039046786891736,

            10450023813501588000,

        ];

        for &e in &expected {

            assert_eq!(rng.next_u64(), e);

        }

    }

    #[test]

    fn stable_seed_from_u64_and_from_seed() {

        // We don't guarantee value-stability for SmallRng but this

        // could influence keeping stability whenever possible (e.g. after optimizations).

        let mut rng = Xoshiro256PlusPlus::seed_from_u64(0);

        // from_seed([0; 32]) should produce the same state as seed_from_u64(0).

        let mut rng_from_seed_0 = Xoshiro256PlusPlus::from_seed([0; 32]);

        let expected = [

            5987356902031041503,

            7051070477665621255,

            6633766593972829180,

            211316841551650330,

            9136120204379184874,

            379361710973160858,

            15813423377499357806,

            15596884590815070553,

            5439680534584881407,

            1369371744833522710,

        ];

        for &e in &expected {

            assert_eq!(rng.next_u64(), e);

            assert_eq!(rng_from_seed_0.next_u64(), e);

        }

    }

}