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    // 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::{RngCore, Error, impls};

    /// 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)]
    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 {
        fn next_u32(&mut self) -> u32 {
            self.next_u64() as u32
        }

        fn next_u64(&mut self) -> u64 {
            let result = self.v;
            self.v = self.v.wrapping_add(self.a);
            result
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            impls::fill_bytes_via_next(self, dest);
        }

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

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