// 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::{Rng, 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 `xoshiro256starstar.c`
/// reference source code](http://xoshiro.di.unimi.it/xoshiro256starstar.c) by
/// David Blackman and Sebastiano Vigna.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct Xoshiro256StarStar {
    s: [u64; 4],
}

impl Xoshiro256StarStar {
    /// 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::Xoshiro256StarStar;
    ///
    /// let rng1 = Xoshiro256StarStar::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
            ]
        );
    }

    /// Jump forward by c · 2^e calls to `next_u64()`.
    ///
    /// For example, `jump_ce(1, 128)` is equivalent to [`jump`](Self::jump)
    /// and `jump_ce(1, 192)` is equivalent to [`long_jump`](Self::long_jump).
    /// Expressing the distance as c · 2^e makes it possible to request both
    /// ordinary counts (`jump_ce(k, 0)`) and very large power-of-two jumps
    /// without multiple-precision integers. For the jump to be meaningful,
    /// c · 2^e should be smaller than the period 2^256 − 1.
    ///
    /// See [`jump_n`](Self::jump_n) to jump by an arbitrary distance.
    pub fn jump_ce(&mut self, c: u64, e: u32) {
        impl_jump_ce!(
            u64,
            self,
            [
                0x9d116f2bb0f0f001,
                0x0280002bcefd1a5e,
                0x04b4edcf26259f85,
                0x0003c03c3f3ecb19
            ],
            c,
            e,
            array4
        );
    }

    /// Jump forward by an arbitrary number of calls to `next_u64()`.
    ///
    /// This is equivalent to *n* calls to `next_u64()`, where *n* = `jump[0]` +
    /// `jump[1]` · 2^64 + … is the little-endian integer held in `jump`.
    /// Unlike [`jump_ce`](Self::jump_ce), it can express any jump distance.
    /// For the jump to be meaningful, *n* should be smaller than the period
    /// 2^256 − 1.
    pub fn jump_n(&mut self, jump: &[u64; 4]) {
        impl_jump_n!(
            u64,
            self,
            [
                0x9d116f2bb0f0f001,
                0x0280002bcefd1a5e,
                0x04b4edcf26259f85,
                0x0003c03c3f3ecb19
            ],
            jump,
            array4
        );
    }
}

impl_state_array_of_four!(Xoshiro256StarStar, u64);

impl SeedableRng for Xoshiro256StarStar {
    type Seed = [u8; 32];

    /// Create a new `Xoshiro256StarStar`.  If `seed` is entirely 0, it will be
    /// mapped to a different seed.
    #[inline]
    fn from_seed(seed: [u8; 32]) -> Xoshiro256StarStar {
        Xoshiro256StarStar {
            s: utils::read_words(crate::common::zero_seed_fallback(&seed)),
        }
    }

    /// Seed a `Xoshiro256StarStar` from a `u64` using `SplitMix64`.
    fn seed_from_u64(seed: u64) -> Xoshiro256StarStar {
        from_splitmix!(seed)
    }
}

impl TryRng for Xoshiro256StarStar {
    type Error = Infallible;

    #[inline]
    fn try_next_u32(&mut self) -> Result<u32, Self::Error> {
        // The lowest bits have some linear dependencies, so we use the
        // upper bits instead.
        Ok((self.next_u64() >> 32) as u32)
    }

    #[inline]
    fn try_next_u64(&mut self) -> Result<u64, Self::Error> {
        let result_starstar = starstar_u64!(self.s[1]);
        impl_xoshiro_u64!(self);
        Ok(result_starstar)
    }

    #[inline]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Self::Error> {
        utils::fill_bytes_via_next_word(dest, || self.try_next_u64())
    }
}

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

    fn fresh() -> Xoshiro256StarStar {
        Xoshiro256StarStar::seed_from_u64(0x0123456789abcdef)
    }

    #[test]
    #[cfg_attr(miri, ignore)] // Miri is too slow
    fn jump_ce_small_distances_match_stepping() {
        for &d in &[0, 1, 2, 3, 7, 64, 1000, 1_000_000] {
            let mut a = fresh();
            for _ in 0..d {
                a.next_u64();
            }
            let mut b = fresh();
            b.jump_ce(d, 0);
            assert_eq!(a, b, "jump_ce({d}, 0)");
        }
        let mut a = fresh();
        for _ in 0..3 * 256 {
            a.next_u64();
        }
        let mut b = fresh();
        b.jump_ce(3, 8);
        assert_eq!(a, b, "jump_ce(3, 8)");
    }

    #[test]
    #[cfg_attr(miri, ignore)] // Miri is too slow
    fn jump_ce_matches_predefined_jumps() {
        let mut a = fresh();
        a.jump();
        let mut b = fresh();
        b.jump_ce(1, 128);
        assert_eq!(a, b, "jump_ce(1,128) == jump()");

        let mut a = fresh();
        a.long_jump();
        let mut b = fresh();
        b.jump_ce(1, 192);
        assert_eq!(a, b, "jump_ce(1,192) == long_jump()");
    }

    #[test]
    #[cfg_attr(miri, ignore)] // Miri is too slow
    fn jump_n_matches_jump_ce() {
        // jump_n(&[d, 0, …]) == jump_ce(d, 0) for a single-word distance.
        for &d in &[0, 1, 2, 3, 7, 64, 1000, 1_000_000] {
            let mut a = fresh();
            a.jump_ce(d, 0);
            let mut b = fresh();
            b.jump_n(&[d, 0, 0, 0]);
            assert_eq!(a, b, "jump_n(&[{d}, …])");
        }
        // A distance that needs a high limb: 2^128 == jump().
        let mut a = fresh();
        a.jump();
        let mut b = fresh();
        b.jump_n(&[0, 0, 1, 0]);
        assert_eq!(a, b, "jump_n(2^128) == jump()");

        // A distance jump_ce cannot express (odd part exceeds 64 bits):
        // 3 + 5 · 2^64, checked via x^(a + b) = x^a · x^b.
        let mut a = fresh();
        a.jump_ce(3, 0);
        a.jump_ce(5, 64);
        let mut b = fresh();
        b.jump_n(&[3, 5, 0, 0]);
        assert_eq!(a, b, "jump_n(3 + 5 · 2^64)");
    }

    #[test]
    fn reference() {
        let mut rng = Xoshiro256StarStar::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/xoshiro128starstar.c
        let expected = [
            11520,
            0,
            1509978240,
            1215971899390074240,
            1216172134540287360,
            607988272756665600,
            16172922978634559625,
            8476171486693032832,
            10595114339597558777,
            2904607092377533576,
        ];
        for &e in &expected {
            assert_eq!(rng.next_u64(), e);
        }
    }

    #[test]
    fn zero_seed_maps_to_seed_from_u64_zero() {
        let from_zero = Xoshiro256StarStar::from_seed([0u8; 32]);
        let from_sm0 = Xoshiro256StarStar::seed_from_u64(0);
        assert_eq!(from_zero, from_sm0);
    }

    #[test]
    fn state_roundtrip() {
        let rng = Xoshiro256StarStar::seed_from_u64(42);
        let clone = Xoshiro256StarStar::from_seed(rng.state());
        assert_eq!(clone, rng);
    }
}