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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.

    //! The ChaCha random number generator.

    #[cfg(feature = "std")]
    use std as core;
    #[cfg(not(feature = "std"))]
    use core;

    use c2_chacha::guts::ChaCha;
    use self::core::fmt;
    use rand_core::block::{BlockRng, BlockRngCore};
    use rand_core::{CryptoRng, Error, RngCore, SeedableRng};

    const STREAM_PARAM_NONCE: u32 = 1;
    const STREAM_PARAM_BLOCK: u32 = 0;

    pub struct Array64<T>([T; 64]);
    impl<T> Default for Array64<T> where T: Default {
        fn default() -> Self {
            Self([T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
                  T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default()])
        }
    }
    impl<T> AsRef<[T]> for Array64<T> {
        fn as_ref(&self) -> &[T] {
            &self.0
        }
    }
    impl<T> AsMut<[T]> for Array64<T> {
        fn as_mut(&mut self) -> &mut [T] {
            &mut self.0
        }
    }
    impl<T> Clone for Array64<T> where T: Copy + Default {
        fn clone(&self) -> Self {
            let mut new = Self::default();
            new.0.copy_from_slice(&self.0);
            new
        }
    }
    impl<T> fmt::Debug for Array64<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Array64 {{}}")
        }
    }

    macro_rules! chacha_impl {
        ($ChaChaXCore:ident, $ChaChaXRng:ident, $rounds:expr, $doc:expr) => {
            #[doc=$doc]
            #[derive(Clone)]
            pub struct $ChaChaXCore {
                state: ChaCha,
            }

            // Custom Debug implementation that does not expose the internal state
            impl fmt::Debug for $ChaChaXCore {
                fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                    write!(f, "ChaChaXCore {{}}")
                }
            }

            impl BlockRngCore for $ChaChaXCore {
                type Item = u32;
                type Results = Array64<u32>;
                #[inline]
                fn generate(&mut self, r: &mut Self::Results) {
                    // Fill slice of words by writing to equivalent slice of bytes, then fixing endianness.
                    self.state.refill4($rounds, unsafe {
                        core::mem::transmute::<&mut Array64<u32>, &mut [u8; 256]>(&mut *r)
                    });
                    for x in r.as_mut() {
                        *x = x.to_le();
                    }
                }
            }

            impl SeedableRng for $ChaChaXCore {
                type Seed = [u8; 32];
                #[inline]
                fn from_seed(seed: Self::Seed) -> Self {
                    $ChaChaXCore { state: ChaCha::new(&seed, &[0u8; 8]) }
                }
            }

            /// A cryptographically secure random number generator that uses the ChaCha algorithm.
            ///
            /// ChaCha is a stream cipher designed by Daniel J. Bernstein[^1], that we use as an RNG. It is
            /// an improved variant of the Salsa20 cipher family, which was selected as one of the "stream
            /// ciphers suitable for widespread adoption" by eSTREAM[^2].
            ///
            /// ChaCha uses add-rotate-xor (ARX) operations as its basis. These are safe against timing
            /// attacks, although that is mostly a concern for ciphers and not for RNGs. We provide a SIMD
            /// implementation to support high throughput on a variety of common hardware platforms.
            ///
            /// With the ChaCha algorithm it is possible to choose the number of rounds the core algorithm
            /// should run. The number of rounds is a tradeoff between performance and security, where 8
            /// rounds is the minimum potentially secure configuration, and 20 rounds is widely used as a
            /// conservative choice.
            ///
            /// We use a 64-bit counter and 64-bit stream identifier as in Bernstein's implementation[^1]
            /// except that we use a stream identifier in place of a nonce. A 64-bit counter over 64-byte
            /// (16 word) blocks allows 1 ZiB of output before cycling, and the stream identifier allows
            /// 2<sup>64</sup> unique streams of output per seed. Both counter and stream are initialized
            /// to zero but may be set via the `set_word_pos` and `set_stream` methods.
            ///
            /// The word layout is:
            ///
            /// ```text
            /// constant  constant  constant  constant
            /// seed      seed      seed      seed
            /// seed      seed      seed      seed
            /// counter   counter   stream_id stream_id
            /// ```
            ///
            /// This implementation uses an output buffer of sixteen `u32` words, and uses
            /// [`BlockRng`] to implement the [`RngCore`] methods.
            ///
            /// [^1]: D. J. Bernstein, [*ChaCha, a variant of Salsa20*](
            ///       https://cr.yp.to/chacha.html)
            ///
            /// [^2]: [eSTREAM: the ECRYPT Stream Cipher Project](
            ///       http://www.ecrypt.eu.org/stream/)
            #[derive(Clone, Debug)]
            pub struct $ChaChaXRng {
                rng: BlockRng<$ChaChaXCore>,
            }

            impl SeedableRng for $ChaChaXRng {
                type Seed = [u8; 32];
                #[inline]
                fn from_seed(seed: Self::Seed) -> Self {
                    let core = $ChaChaXCore::from_seed(seed);
                    Self {
                        rng: BlockRng::new(core),
                    }
                }
            }

            impl RngCore for $ChaChaXRng {
                #[inline]
                fn next_u32(&mut self) -> u32 {
                    self.rng.next_u32()
                }
                #[inline]
                fn next_u64(&mut self) -> u64 {
                    self.rng.next_u64()
                }
                #[inline]
                fn fill_bytes(&mut self, bytes: &mut [u8]) {
                    self.rng.fill_bytes(bytes)
                }
                #[inline]
                fn try_fill_bytes(&mut self, bytes: &mut [u8]) -> Result<(), Error> {
                    self.rng.try_fill_bytes(bytes)
                }
            }

            impl $ChaChaXRng {
                // The buffer is a 4-block window, i.e. it is always at a block-aligned position in the
                // stream but if the stream has been seeked it may not be self-aligned.

                /// Get the offset from the start of the stream, in 32-bit words.
                ///
                /// Since the generated blocks are 16 words (2<sup>4</sup>) long and the
                /// counter is 64-bits, the offset is a 68-bit number. Sub-word offsets are
                /// not supported, hence the result can simply be multiplied by 4 to get a
                /// byte-offset.
                #[inline]
                pub fn get_word_pos(&self) -> u128 {
                    let mut block = u128::from(self.rng.core.state.get_stream_param(STREAM_PARAM_BLOCK));
                    // counter is incremented *after* filling buffer
                    block -= 4;
                    (block << 4) + self.rng.index() as u128
                }

                /// Set the offset from the start of the stream, in 32-bit words.
                ///
                /// As with `get_word_pos`, we use a 68-bit number. Since the generator
                /// simply cycles at the end of its period (1 ZiB), we ignore the upper
                /// 60 bits.
                #[inline]
                pub fn set_word_pos(&mut self, word_offset: u128) {
                    let block = (word_offset >> 4) as u64;
                    self.rng
                        .core
                        .state
                        .set_stream_param(STREAM_PARAM_BLOCK, block);
                    self.rng.generate_and_set((word_offset & 15) as usize);
                }

                /// Set the stream number.
                ///
                /// This is initialized to zero; 2<sup>64</sup> unique streams of output
                /// are available per seed/key.
                ///
                /// Note that in order to reproduce ChaCha output with a specific 64-bit
                /// nonce, one can convert that nonce to a `u64` in little-endian fashion
                /// and pass to this function. In theory a 96-bit nonce can be used by
                /// passing the last 64-bits to this function and using the first 32-bits as
                /// the most significant half of the 64-bit counter (which may be set
                /// indirectly via `set_word_pos`), but this is not directly supported.
                #[inline]
                pub fn set_stream(&mut self, stream: u64) {
                    self.rng
                        .core
                        .state
                        .set_stream_param(STREAM_PARAM_NONCE, stream);
                    if self.rng.index() != 64 {
                        let wp = self.get_word_pos();
                        self.set_word_pos(wp);
                    }
                }
            }

            impl CryptoRng for $ChaChaXRng {}

            impl From<$ChaChaXCore> for $ChaChaXRng {
                fn from(core: $ChaChaXCore) -> Self {
                    $ChaChaXRng {
                        rng: BlockRng::new(core),
                    }
                }
            }
        }
    }

    chacha_impl!(ChaCha20Core, ChaCha20Rng, 10, "ChaCha with 20 rounds");
    chacha_impl!(ChaCha12Core, ChaCha12Rng, 6, "ChaCha with 12 rounds");
    chacha_impl!(ChaCha8Core, ChaCha8Rng, 4, "ChaCha with 8 rounds");

    #[cfg(test)]
    mod test {
        use rand_core::{RngCore, SeedableRng};

        type ChaChaRng = super::ChaCha20Rng;

        #[test]
        fn test_chacha_construction() {
            let seed = [
                0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0,
                0, 0, 0,
            ];
            let mut rng1 = ChaChaRng::from_seed(seed);
            assert_eq!(rng1.next_u32(), 137206642);

            let mut rng2 = ChaChaRng::from_rng(rng1).unwrap();
            assert_eq!(rng2.next_u32(), 1325750369);
        }

        #[test]
        fn test_chacha_true_values_a() {
            // Test vectors 1 and 2 from
            // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
            let seed = [0u8; 32];
            let mut rng = ChaChaRng::from_seed(seed);

            let mut results = [0u32; 16];
            for i in results.iter_mut() {
                *i = rng.next_u32();
            }
            let expected = [
                0xade0b876, 0x903df1a0, 0xe56a5d40, 0x28bd8653, 0xb819d2bd, 0x1aed8da0, 0xccef36a8,
                0xc70d778b, 0x7c5941da, 0x8d485751, 0x3fe02477, 0x374ad8b8, 0xf4b8436a, 0x1ca11815,
                0x69b687c3, 0x8665eeb2,
            ];
            assert_eq!(results, expected);

            for i in results.iter_mut() {
                *i = rng.next_u32();
            }
            let expected = [
                0xbee7079f, 0x7a385155, 0x7c97ba98, 0x0d082d73, 0xa0290fcb, 0x6965e348, 0x3e53c612,
                0xed7aee32, 0x7621b729, 0x434ee69c, 0xb03371d5, 0xd539d874, 0x281fed31, 0x45fb0a51,
                0x1f0ae1ac, 0x6f4d794b,
            ];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_chacha_true_values_b() {
            // Test vector 3 from
            // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
            let seed = [
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 1,
            ];
            let mut rng = ChaChaRng::from_seed(seed);

            // Skip block 0
            for _ in 0..16 {
                rng.next_u32();
            }

            let mut results = [0u32; 16];
            for i in results.iter_mut() {
                *i = rng.next_u32();
            }
            let expected = [
                0x2452eb3a, 0x9249f8ec, 0x8d829d9b, 0xddd4ceb1, 0xe8252083, 0x60818b01, 0xf38422b8,
                0x5aaa49c9, 0xbb00ca8e, 0xda3ba7b4, 0xc4b592d1, 0xfdf2732f, 0x4436274e, 0x2561b3c8,
                0xebdd4aa6, 0xa0136c00,
            ];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_chacha_true_values_c() {
            // Test vector 4 from
            // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
            let seed = [
                0, 0xff, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0,
            ];
            let expected = [
                0xfb4dd572, 0x4bc42ef1, 0xdf922636, 0x327f1394, 0xa78dea8f, 0x5e269039, 0xa1bebbc1,
                0xcaf09aae, 0xa25ab213, 0x48a6b46c, 0x1b9d9bcb, 0x092c5be6, 0x546ca624, 0x1bec45d5,
                0x87f47473, 0x96f0992e,
            ];
            let expected_end = 3 * 16;
            let mut results = [0u32; 16];

            // Test block 2 by skipping block 0 and 1
            let mut rng1 = ChaChaRng::from_seed(seed);
            for _ in 0..32 {
                rng1.next_u32();
            }
            for i in results.iter_mut() {
                *i = rng1.next_u32();
            }
            assert_eq!(results, expected);
            assert_eq!(rng1.get_word_pos(), expected_end);

            // Test block 2 by using `set_word_pos`
            let mut rng2 = ChaChaRng::from_seed(seed);
            rng2.set_word_pos(2 * 16);
            for i in results.iter_mut() {
                *i = rng2.next_u32();
            }
            assert_eq!(results, expected);
            assert_eq!(rng2.get_word_pos(), expected_end);

            // Test skipping behaviour with other types
            let mut buf = [0u8; 32];
            rng2.fill_bytes(&mut buf[..]);
            assert_eq!(rng2.get_word_pos(), expected_end + 8);
            rng2.fill_bytes(&mut buf[0..25]);
            assert_eq!(rng2.get_word_pos(), expected_end + 15);
            rng2.next_u64();
            assert_eq!(rng2.get_word_pos(), expected_end + 17);
            rng2.next_u32();
            rng2.next_u64();
            assert_eq!(rng2.get_word_pos(), expected_end + 20);
            rng2.fill_bytes(&mut buf[0..1]);
            assert_eq!(rng2.get_word_pos(), expected_end + 21);
        }

        #[test]
        fn test_chacha_multiple_blocks() {
            let seed = [
                0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7,
                0, 0, 0,
            ];
            let mut rng = ChaChaRng::from_seed(seed);

            // Store the 17*i-th 32-bit word,
            // i.e., the i-th word of the i-th 16-word block
            let mut results = [0u32; 16];
            for i in results.iter_mut() {
                *i = rng.next_u32();
                for _ in 0..16 {
                    rng.next_u32();
                }
            }
            let expected = [
                0xf225c81a, 0x6ab1be57, 0x04d42951, 0x70858036, 0x49884684, 0x64efec72, 0x4be2d186,
                0x3615b384, 0x11cfa18e, 0xd3c50049, 0x75c775f6, 0x434c6530, 0x2c5bad8f, 0x898881dc,
                0x5f1c86d9, 0xc1f8e7f4,
            ];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_chacha_true_bytes() {
            let seed = [0u8; 32];
            let mut rng = ChaChaRng::from_seed(seed);
            let mut results = [0u8; 32];
            rng.fill_bytes(&mut results);
            let expected = [
                118, 184, 224, 173, 160, 241, 61, 144, 64, 93, 106, 229, 83, 134, 189, 40, 189, 210,
                25, 184, 160, 141, 237, 26, 168, 54, 239, 204, 139, 119, 13, 199,
            ];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_chacha_nonce() {
            // Test vector 5 from
            // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
            // Although we do not support setting a nonce, we try it here anyway so
            // we can use this test vector.
            let seed = [0u8; 32];
            let mut rng = ChaChaRng::from_seed(seed);
            // 96-bit nonce in LE order is: 0,0,0,0, 0,0,0,0, 0,0,0,2
            rng.set_stream(2u64 << (24 + 32));

            let mut results = [0u32; 16];
            for i in results.iter_mut() {
                *i = rng.next_u32();
            }
            let expected = [
                0x374dc6c2, 0x3736d58c, 0xb904e24a, 0xcd3f93ef, 0x88228b1a, 0x96a4dfb3, 0x5b76ab72,
                0xc727ee54, 0x0e0e978a, 0xf3145c95, 0x1b748ea8, 0xf786c297, 0x99c28f5f, 0x628314e8,
                0x398a19fa, 0x6ded1b53,
            ];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_chacha_clone_streams() {
            let seed = [
                0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7,
                0, 0, 0,
            ];
            let mut rng = ChaChaRng::from_seed(seed);
            let mut clone = rng.clone();
            for _ in 0..16 {
                assert_eq!(rng.next_u64(), clone.next_u64());
            }

            rng.set_stream(51);
            for _ in 0..7 {
                assert!(rng.next_u32() != clone.next_u32());
            }
            clone.set_stream(51); // switch part way through block
            for _ in 7..16 {
                assert_eq!(rng.next_u32(), clone.next_u32());
            }
        }
    }

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## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)