// Copyright 2018 Developers of the Rand project.

// Copyright 2017 Paul Dicker.

// Copyright 2014-2017 Melissa O'Neill and PCG Project contributors

//

// 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.

//! PCG random number generators

// This is the default multiplier used by PCG for 64-bit state.

const MULTIPLIER: u128 = 0x2360_ED05_1FC6_5DA4_4385_DF64_9FCC_F645;

use core::fmt;

use rand_core::{RngCore, SeedableRng, Error, le};

#[cfg(feature="serde1")] use serde::{Serialize, Deserialize};

/// A PCG random number generator (XSL RR 128/64 (LCG) variant).

///

/// Permuted Congruential Generator with 128-bit state, internal Linear

/// Congruential Generator, and 64-bit output via "xorshift low (bits),

/// random rotation" output function.

///

/// This is a 128-bit LCG with explicitly chosen stream with the PCG-XSL-RR

/// output function. This combination is the standard `pcg64`.

///

/// Despite the name, this implementation uses 32 bytes (256 bit) space

/// comprising 128 bits of state and 128 bits stream selector. These are both

/// set by `SeedableRng`, using a 256-bit seed.

#[derive(Clone)]

#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]

pub struct Lcg128Xsl64 {

    state: u128,

    increment: u128,

}

/// `Lcg128Xsl64` is also officially known as `pcg64`.

pub type Pcg64 = Lcg128Xsl64;

impl Lcg128Xsl64 {

    /// Construct an instance compatible with PCG seed and stream.

    ///

    /// Note that PCG specifies default values for both parameters:

    ///

    /// - `state = 0xcafef00dd15ea5e5`

    /// - `stream = 0xa02bdbf7bb3c0a7ac28fa16a64abf96`

    pub fn new(state: u128, stream: u128) -> Self {

        // The increment must be odd, hence we discard one bit:

        let increment = (stream << 1) | 1;

        Lcg128Xsl64::from_state_incr(state, increment)

    }

    #[inline]

    fn from_state_incr(state: u128, increment: u128) -> Self {

        let mut pcg = Lcg128Xsl64 { state, increment };

        // Move away from inital value:

        pcg.state = pcg.state.wrapping_add(pcg.increment);

        pcg.step();

        pcg

    }

    #[inline]

    fn step(&mut self) {

        // prepare the LCG for the next round

        self.state = self.state

            .wrapping_mul(MULTIPLIER)

            .wrapping_add(self.increment);

    }

}

// Custom Debug implementation that does not expose the internal state

impl fmt::Debug for Lcg128Xsl64 {

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        write!(f, "Lcg128Xsl64 {{}}")

    }

}

/// We use a single 255-bit seed to initialise the state and select a stream.

/// One `seed` bit (lowest bit of `seed[8]`) is ignored.

impl SeedableRng for Lcg128Xsl64 {

    type Seed = [u8; 32];

    fn from_seed(seed: Self::Seed) -> Self {

        let mut seed_u64 = [0u64; 4];

        le::read_u64_into(&seed, &mut seed_u64);

        let state = u128::from(seed_u64[0]) | (u128::from(seed_u64[1]) << 64);

        let incr = u128::from(seed_u64[2]) | (u128::from(seed_u64[3]) << 64);

        // The increment must be odd, hence we discard one bit:

        Lcg128Xsl64::from_state_incr(state, incr | 1)

    }

}

impl RngCore for Lcg128Xsl64 {

    #[inline]

    fn next_u32(&mut self) -> u32 {

        self.next_u64() as u32

    }

    #[inline]

    fn next_u64(&mut self) -> u64 {

        self.step();

        output_xsl_rr(self.state)

    }

    #[inline]

    fn fill_bytes(&mut self, dest: &mut [u8]) {

        fill_bytes_impl(self, dest)

    }

    #[inline]

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

        self.fill_bytes(dest);

        Ok(())

    }

}

/// A PCG random number generator (XSL 128/64 (MCG) variant).

///

/// Permuted Congruential Generator with 128-bit state, internal Multiplicative

/// Congruential Generator, and 64-bit output via "xorshift low (bits),

/// random rotation" output function.

///

/// This is a 128-bit MCG with the PCG-XSL-RR output function, also known as

/// `pcg64_fast`.

/// Note that compared to the standard `pcg64` (128-bit LCG with PCG-XSL-RR

/// output function), this RNG is faster, also has a long cycle, and still has

/// good performance on statistical tests.

#[derive(Clone)]

#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]

pub struct Mcg128Xsl64 {

    state: u128,

}

/// A friendly name for `Mcg128Xsl64` (also known as `pcg64_fast`).

pub type Pcg64Mcg = Mcg128Xsl64;

impl Mcg128Xsl64 {

    /// Construct an instance compatible with PCG seed.

    ///

    /// Note that PCG specifies a default value for the parameter:

    ///

    /// - `state = 0xcafef00dd15ea5e5`

    pub fn new(state: u128) -> Self {

        // Force low bit to 1, as in C version (C++ uses `state | 3` instead).

        Mcg128Xsl64 { state: state | 1 }

    }

}

// Custom Debug implementation that does not expose the internal state

impl fmt::Debug for Mcg128Xsl64 {

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        write!(f, "Mcg128Xsl64 {{}}")

    }

}

/// We use a single 126-bit seed to initialise the state and select a stream.

/// Two `seed` bits (lowest order of last byte) are ignored.

impl SeedableRng for Mcg128Xsl64 {

    type Seed = [u8; 16];

    fn from_seed(seed: Self::Seed) -> Self {

        // Read as if a little-endian u128 value:

        let mut seed_u64 = [0u64; 2];

        le::read_u64_into(&seed, &mut seed_u64);

        let state = u128::from(seed_u64[0])  |

                    u128::from(seed_u64[1]) << 64;

        Mcg128Xsl64::new(state)

    }

}

impl RngCore for Mcg128Xsl64 {

    #[inline]

    fn next_u32(&mut self) -> u32 {

        self.next_u64() as u32

    }

    #[inline]

    fn next_u64(&mut self) -> u64 {

        self.state = self.state.wrapping_mul(MULTIPLIER);

        output_xsl_rr(self.state)

    }

    #[inline]

    fn fill_bytes(&mut self, dest: &mut [u8]) {

        fill_bytes_impl(self, dest)

    }

    #[inline]

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

        self.fill_bytes(dest);

        Ok(())

    }

}

#[inline(always)]

fn output_xsl_rr(state: u128) -> u64 {

    // Output function XSL RR ("xorshift low (bits), random rotation")

    // Constants are for 128-bit state, 64-bit output

    const XSHIFT: u32 = 64;     // (128 - 64 + 64) / 2

    const ROTATE: u32 = 122;    // 128 - 6

    let rot = (state >> ROTATE) as u32;

    let xsl = ((state >> XSHIFT) as u64) ^ (state as u64);

    xsl.rotate_right(rot)

}

#[inline(always)]

fn fill_bytes_impl<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {

    let mut left = dest;

    while left.len() >= 8 {

        let (l, r) = {left}.split_at_mut(8);

        left = r;

        let chunk: [u8; 8] = rng.next_u64().to_le_bytes();

        l.copy_from_slice(&chunk);

    }

    let n = left.len();

    if n > 0 {

        let chunk: [u8; 8] = rng.next_u64().to_le_bytes();

        left.copy_from_slice(&chunk[..n]);

    }

}