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    // Copyright 2018 Developers of the Rand project.
    // Copyright 2013 The Rust Project Developers.
    //
    // 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.

    //! A wrapper around any Read to treat it as an RNG.

    use std::io::Read;
    use std::fmt;

    use rand_core::{RngCore, Error, impls};

    /// An RNG that reads random bytes straight from any type supporting
    /// [`std::io::Read`], for example files.
    ///
    /// This will work best with an infinite reader, but that is not required.
    ///
    /// This can be used with `/dev/urandom` on Unix but it is recommended to use
    /// [`OsRng`] instead.
    ///
    /// # Panics
    ///
    /// `ReadRng` uses [`std::io::Read::read_exact`], which retries on interrupts.
    /// All other errors from the underlying reader, including when it does not
    /// have enough data, will only be reported through [`try_fill_bytes`].
    /// The other [`RngCore`] methods will panic in case of an error.
    ///
    /// # Example
    ///
    /// ```
    /// use rand::Rng;
    /// use rand::rngs::adapter::ReadRng;
    ///
    /// let data = vec![1, 2, 3, 4, 5, 6, 7, 8];
    /// let mut rng = ReadRng::new(&data[..]);
    /// println!("{:x}", rng.gen::<u32>());
    /// ```
    ///
    /// [`OsRng`]: crate::rngs::OsRng
    /// [`try_fill_bytes`]: RngCore::try_fill_bytes
    #[derive(Debug)]
    pub struct ReadRng<R> {
        reader: R
    }

    impl<R: Read> ReadRng<R> {
        /// Create a new `ReadRng` from a `Read`.
        pub fn new(r: R) -> ReadRng<R> {
            ReadRng {
                reader: r
            }
        }
    }

    impl<R: Read> RngCore for ReadRng<R> {
        fn next_u32(&mut self) -> u32 {
            impls::next_u32_via_fill(self)
        }

        fn next_u64(&mut self) -> u64 {
            impls::next_u64_via_fill(self)
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.try_fill_bytes(dest).unwrap_or_else(|err|
                    panic!("reading random bytes from Read implementation failed; error: {}", err));
        }

        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            if dest.len() == 0 { return Ok(()); }
            // Use `std::io::read_exact`, which retries on `ErrorKind::Interrupted`.
            self.reader.read_exact(dest).map_err(|e| Error::new(ReadError(e)))
        }
    }

    /// `ReadRng` error type
    #[derive(Debug)]
    pub struct ReadError(std::io::Error);

    impl fmt::Display for ReadError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "ReadError: {}", self.0)
        }
    }

    impl std::error::Error for ReadError {
        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
            Some(&self.0)
        }
    }

    #[cfg(test)]
    mod test {
        use super::ReadRng;
        use crate::RngCore;

        #[test]
        fn test_reader_rng_u64() {
            // transmute from the target to avoid endianness concerns.
            let v = vec![0u8, 0, 0, 0, 0, 0, 0, 1,
                         0  , 0, 0, 0, 0, 0, 0, 2,
                         0,   0, 0, 0, 0, 0, 0, 3];
            let mut rng = ReadRng::new(&v[..]);

            assert_eq!(rng.next_u64(), 1_u64.to_be());
            assert_eq!(rng.next_u64(), 2_u64.to_be());
            assert_eq!(rng.next_u64(), 3_u64.to_be());
        }

        #[test]
        fn test_reader_rng_u32() {
            let v = vec![0u8, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3];
            let mut rng = ReadRng::new(&v[..]);

            assert_eq!(rng.next_u32(), 1_u32.to_be());
            assert_eq!(rng.next_u32(), 2_u32.to_be());
            assert_eq!(rng.next_u32(), 3_u32.to_be());
        }

        #[test]
        fn test_reader_rng_fill_bytes() {
            let v = [1u8, 2, 3, 4, 5, 6, 7, 8];
            let mut w = [0u8; 8];

            let mut rng = ReadRng::new(&v[..]);
            rng.fill_bytes(&mut w);

            assert!(v == w);
        }

        #[test]
        fn test_reader_rng_insufficient_bytes() {
            let v = [1u8, 2, 3, 4, 5, 6, 7, 8];
            let mut w = [0u8; 9];

            let mut rng = ReadRng::new(&v[..]);

            let result = rng.try_fill_bytes(&mut w);
            assert!(result.is_err());
            println!("Error: {}", result.unwrap_err());
        }
    }

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