// 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.

//! Mock random number generator

use rand_core::{impls, RngCore};

#[cfg(feature = "serde")]

use serde::{Deserialize, Serialize};

/// A mock generator yielding very predictable output

///

/// This generates an arithmetic sequence (i.e. adds a constant each step)

/// over a `u64` number, using wrapping arithmetic. If the increment is 0

/// the generator yields a constant.

///

/// Other integer types (64-bit and smaller) are produced via cast from `u64`.

///

/// Other types are produced via their implementation of [`Rng`](crate::Rng) or

/// [`Distribution`](crate::distr::Distribution).

/// Output values may not be intuitive and may change in future releases but

/// are considered

/// [portable](https://rust-random.github.io/book/portability.html).

/// (`bool` output is true when bit `1u64 << 31` is set.)

///

/// # Example

///

/// ```

/// use rand::Rng;

/// use rand::rngs::mock::StepRng;

///

/// let mut my_rng = StepRng::new(2, 1);

/// let sample: [u64; 3] = my_rng.random();

/// assert_eq!(sample, [2, 3, 4]);

/// ```

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

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

pub struct StepRng {

    v: u64,

    a: u64,

}

impl StepRng {

    /// Create a `StepRng`, yielding an arithmetic sequence starting with

    /// `initial` and incremented by `increment` each time.

    pub fn new(initial: u64, increment: u64) -> Self {

        StepRng {

            v: initial,

            a: increment,

        }

    }

}

impl RngCore for StepRng {

    #[inline]

    fn next_u32(&mut self) -> u32 {

        self.next_u64() as u32

    }

    #[inline]

    fn next_u64(&mut self) -> u64 {

        let res = self.v;

        self.v = self.v.wrapping_add(self.a);

        res

    }

    #[inline]

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

        impls::fill_bytes_via_next(self, dst)

    }

}

#[cfg(test)]

mod tests {

    #[cfg(any(feature = "alloc", feature = "serde"))]

    use super::StepRng;

    #[test]

    #[cfg(feature = "serde")]

    fn test_serialization_step_rng() {

        let some_rng = StepRng::new(42, 7);

        let de_some_rng: StepRng =

            bincode::deserialize(&bincode::serialize(&some_rng).unwrap()).unwrap();

        assert_eq!(some_rng.v, de_some_rng.v);

        assert_eq!(some_rng.a, de_some_rng.a);

    }

    #[test]

    #[cfg(feature = "alloc")]

    fn test_bool() {

        use crate::{distr::StandardUniform, Rng};

        // If this result ever changes, update doc on StepRng!

        let rng = StepRng::new(0, 1 << 31);

        let result: alloc::vec::Vec<bool> = rng.sample_iter(StandardUniform).take(6).collect();

        assert_eq!(&result, &[false, true, false, true, false, true]);

    }

}