// 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, Error, RngCore};

#[cfg(feature = "serde1")]

use serde::{Serialize, Deserialize};

/// A simple implementation of `RngCore` for testing purposes.

///

/// 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.

///

/// ```

/// use rand::Rng;

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

///

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

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

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

/// ```

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

#[cfg_attr(feature = "serde1", 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 result = self.v;

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

        result

    }

    #[inline]

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

        impls::fill_bytes_via_next(self, dest);

    }

    #[inline]

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

        self.fill_bytes(dest);

        Ok(())

    }

}

#[cfg(test)]

mod tests {

    #[test]

    #[cfg(feature = "serde1")]

    fn test_serialization_step_rng() {

        use super::StepRng;

        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);

    }

}