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    // Copyright 2018 Developers of the Rand project.
    // Copyright 2013-2018 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.

    //! The ISAAC-64 random number generator.

    use core::{fmt, slice};
    use core::num::Wrapping as w;
    use rand_core::{RngCore, SeedableRng, Error, le};
    use rand_core::block::{BlockRngCore, BlockRng64};
    use isaac_array::IsaacArray;

    #[allow(non_camel_case_types)]
    type w64 = w<u64>;

    const RAND_SIZE_LEN: usize = 8;
    const RAND_SIZE: usize = 1 << RAND_SIZE_LEN;

    /// A random number generator that uses ISAAC-64, the 64-bit variant of the
    /// ISAAC algorithm.
    ///
    /// ISAAC stands for "Indirection, Shift, Accumulate, Add, and Count" which are
    /// the principal bitwise operations employed. It is the most advanced of a
    /// series of array based random number generator designed by Robert Jenkins
    /// in 1996[^1].
    ///
    /// ISAAC-64 is mostly similar to ISAAC. Because it operates on 64-bit integers
    /// instead of 32-bit, it uses twice as much memory to hold its state and
    /// results. Also it uses different constants for shifts and indirect indexing,
    /// optimized to give good results for 64bit arithmetic.
    ///
    /// ISAAC-64 is notably fast and produces excellent quality random numbers for
    /// non-cryptographic applications.
    ///
    /// In spite of being designed with cryptographic security in mind, ISAAC hasn't
    /// been stringently cryptanalyzed and thus cryptographers do not not
    /// consensually trust it to be secure. When looking for a secure RNG, prefer
    /// [`Hc128Rng`] instead, which, like ISAAC, is an array-based RNG and one of
    /// the stream-ciphers selected the by eSTREAM contest.
    ///
    /// ## Overview of the ISAAC-64 algorithm:
    /// (in pseudo-code)
    ///
    /// ```text
    /// Input: a, b, c, s[256] // state
    /// Output: r[256] // results
    ///
    /// mix(a,i) = !(a ^ a << 21)  if i = 0 mod 4
    ///              a ^ a >>  5   if i = 1 mod 4
    ///              a ^ a << 12   if i = 2 mod 4
    ///              a ^ a >> 33   if i = 3 mod 4
    ///
    /// c = c + 1
    /// b = b + c
    ///
    /// for i in 0..256 {
    ///     x = s_[i]
    ///     a = mix(a,i) + s[i+128 mod 256]
    ///     y = a + b + s[x>>3 mod 256]
    ///     s[i] = y
    ///     b = x + s[y>>11 mod 256]
    ///     r[i] = b
    /// }
    /// ```
    ///
    /// This implementation uses [`BlockRng64`] to implement the [`RngCore`] methods.
    ///
    /// See for more information the documentation of [`IsaacRng`].
    ///
    /// [^1]: Bob Jenkins, [*ISAAC and RC4*](
    ///       http://burtleburtle.net/bob/rand/isaac.html)
    ///
    /// [`IsaacRng`]: ../isaac/struct.IsaacRng.html
    /// [`Hc128Rng`]: ../../rand_hc/struct.Hc128Rng.html
    /// [`BlockRng64`]: ../../rand_core/block/struct.BlockRng64.html
    /// [`RngCore`]: ../../rand_core/trait.RngCore.html
    #[derive(Clone, Debug)]
    #[cfg_attr(feature="serde1", derive(Serialize, Deserialize))]
    pub struct Isaac64Rng(BlockRng64<Isaac64Core>);

    impl RngCore for Isaac64Rng {
        #[inline(always)]
        fn next_u32(&mut self) -> u32 {
            self.0.next_u32()
        }

        #[inline(always)]
        fn next_u64(&mut self) -> u64 {
            self.0.next_u64()
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.0.fill_bytes(dest)
        }

        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            self.0.try_fill_bytes(dest)
        }
    }

    impl SeedableRng for Isaac64Rng {
        type Seed = <Isaac64Core as SeedableRng>::Seed;

        fn from_seed(seed: Self::Seed) -> Self {
            Isaac64Rng(BlockRng64::<Isaac64Core>::from_seed(seed))
        }

        /// Create an ISAAC random number generator using an `u64` as seed.
        /// If `seed == 0` this will produce the same stream of random numbers as
        /// the reference implementation when used unseeded.
        fn seed_from_u64(seed: u64) -> Self {
            Isaac64Rng(BlockRng64::<Isaac64Core>::seed_from_u64(seed))
        }

        fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
            BlockRng64::<Isaac64Core>::from_rng(rng).map(|rng| Isaac64Rng(rng))
        }
    }

    impl Isaac64Rng {
        /// Create an ISAAC-64 random number generator using an `u64` as seed.
        /// If `seed == 0` this will produce the same stream of random numbers as
        /// the reference implementation when used unseeded.
        #[deprecated(since="0.6.0", note="use SeedableRng::seed_from_u64 instead")]
        pub fn new_from_u64(seed: u64) -> Self {
            Self::seed_from_u64(seed)
        }
    }

    /// The core of `Isaac64Rng`, used with `BlockRng`.
    #[derive(Clone)]
    #[cfg_attr(feature="serde1", derive(Serialize, Deserialize))]
    pub struct Isaac64Core {
        #[cfg_attr(feature="serde1",serde(with="super::isaac_array::isaac_array_serde"))]
        mem: [w64; RAND_SIZE],
        a: w64,
        b: w64,
        c: w64,
    }

    // Custom Debug implementation that does not expose the internal state
    impl fmt::Debug for Isaac64Core {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Isaac64Core {{}}")
        }
    }

    impl BlockRngCore for Isaac64Core {
        type Item = u64;
        type Results = IsaacArray<Self::Item>;

        /// Refills the output buffer, `results`. See also the pseudocode desciption
        /// of the algorithm in the [`Isaac64Rng`] documentation.
        ///
        /// Optimisations used (similar to the reference implementation):
        ///
        /// - The loop is unrolled 4 times, once for every constant of mix().
        /// - The contents of the main loop are moved to a function `rngstep`, to
        ///   reduce code duplication.
        /// - We use local variables for a and b, which helps with optimisations.
        /// - We split the main loop in two, one that operates over 0..128 and one
        ///   over 128..256. This way we can optimise out the addition and modulus
        ///   from `s[i+128 mod 256]`.
        /// - We maintain one index `i` and add `m` or `m2` as base (m2 for the
        ///   `s[i+128 mod 256]`), relying on the optimizer to turn it into pointer
        ///   arithmetic.
        /// - We fill `results` backwards. The reference implementation reads values
        ///   from `results` in reverse. We read them in the normal direction, to
        ///   make `fill_bytes` a memcopy. To maintain compatibility we fill in
        ///   reverse.
        ///
        /// [`Isaac64Rng`]: struct.Isaac64Rng.html
        fn generate(&mut self, results: &mut IsaacArray<Self::Item>) {
            self.c += w(1);
            // abbreviations
            let mut a = self.a;
            let mut b = self.b + self.c;
            const MIDPOINT: usize = RAND_SIZE / 2;

            #[inline]
            fn ind(mem:&[w64; RAND_SIZE], v: w64, amount: usize) -> w64 {
                let index = (v >> amount).0 as usize % RAND_SIZE;
                mem[index]
            }

            #[inline]
            fn rngstep(mem: &mut [w64; RAND_SIZE],
                       results: &mut [u64; RAND_SIZE],
                       mix: w64,
                       a: &mut w64,
                       b: &mut w64,
                       base: usize,
                       m: usize,
                       m2: usize) {
                let x = mem[base + m];
                *a = mix + mem[base + m2];
                let y = *a + *b + ind(&mem, x, 3);
                mem[base + m] = y;
                *b = x + ind(&mem, y, 3 + RAND_SIZE_LEN);
                results[RAND_SIZE - 1 - base - m] = (*b).0;
            }

            let mut m = 0;
            let mut m2 = MIDPOINT;
            for i in (0..MIDPOINT/4).map(|i| i * 4) {
                rngstep(&mut self.mem, results, !(a ^ (a << 21)), &mut a, &mut b, i + 0, m, m2);
                rngstep(&mut self.mem, results,   a ^ (a >> 5 ),  &mut a, &mut b, i + 1, m, m2);
                rngstep(&mut self.mem, results,   a ^ (a << 12),  &mut a, &mut b, i + 2, m, m2);
                rngstep(&mut self.mem, results,   a ^ (a >> 33),  &mut a, &mut b, i + 3, m, m2);
            }

            m = MIDPOINT;
            m2 = 0;
            for i in (0..MIDPOINT/4).map(|i| i * 4) {
                rngstep(&mut self.mem, results, !(a ^ (a << 21)), &mut a, &mut b, i + 0, m, m2);
                rngstep(&mut self.mem, results,   a ^ (a >> 5 ),  &mut a, &mut b, i + 1, m, m2);
                rngstep(&mut self.mem, results,   a ^ (a << 12),  &mut a, &mut b, i + 2, m, m2);
                rngstep(&mut self.mem, results,   a ^ (a >> 33),  &mut a, &mut b, i + 3, m, m2);
            }

            self.a = a;
            self.b = b;
        }
    }

    impl Isaac64Core {
        /// Create a new ISAAC-64 random number generator.
        fn init(mut mem: [w64; RAND_SIZE], rounds: u32) -> Self {
            fn mix(a: &mut w64, b: &mut w64, c: &mut w64, d: &mut w64,
                   e: &mut w64, f: &mut w64, g: &mut w64, h: &mut w64) {
                *a -= *e; *f ^= *h >> 9;  *h += *a;
                *b -= *f; *g ^= *a << 9;  *a += *b;
                *c -= *g; *h ^= *b >> 23; *b += *c;
                *d -= *h; *a ^= *c << 15; *c += *d;
                *e -= *a; *b ^= *d >> 14; *d += *e;
                *f -= *b; *c ^= *e << 20; *e += *f;
                *g -= *c; *d ^= *f >> 17; *f += *g;
                *h -= *d; *e ^= *g << 14; *g += *h;
            }

            // These numbers are the result of initializing a...h with the
            // fractional part of the golden ratio in binary (0x9e3779b97f4a7c13)
            // and applying mix() 4 times.
            let mut a = w(0x647c4677a2884b7c);
            let mut b = w(0xb9f8b322c73ac862);
            let mut c = w(0x8c0ea5053d4712a0);
            let mut d = w(0xb29b2e824a595524);
            let mut e = w(0x82f053db8355e0ce);
            let mut f = w(0x48fe4a0fa5a09315);
            let mut g = w(0xae985bf2cbfc89ed);
            let mut h = w(0x98f5704f6c44c0ab);

            // Normally this should do two passes, to make all of the seed effect
            // all of `mem`
            for _ in 0..rounds {
                for i in (0..RAND_SIZE/8).map(|i| i * 8) {
                    a += mem[i  ]; b += mem[i+1];
                    c += mem[i+2]; d += mem[i+3];
                    e += mem[i+4]; f += mem[i+5];
                    g += mem[i+6]; h += mem[i+7];
                    mix(&mut a, &mut b, &mut c, &mut d,
                        &mut e, &mut f, &mut g, &mut h);
                    mem[i  ] = a; mem[i+1] = b;
                    mem[i+2] = c; mem[i+3] = d;
                    mem[i+4] = e; mem[i+5] = f;
                    mem[i+6] = g; mem[i+7] = h;
                }
            }

            Self { mem, a: w(0), b: w(0), c: w(0) }
        }

        /// Create an ISAAC-64 random number generator using an `u64` as seed.
        /// If `seed == 0` this will produce the same stream of random numbers as
        /// the reference implementation when used unseeded.
        #[deprecated(since="0.6.0", note="use SeedableRng::seed_from_u64 instead")]
        pub fn new_from_u64(seed: u64) -> Self {
            Self::seed_from_u64(seed)
        }
    }

    impl SeedableRng for Isaac64Core {
        type Seed = [u8; 32];

        fn from_seed(seed: Self::Seed) -> Self {
            let mut seed_u64 = [0u64; 4];
            le::read_u64_into(&seed, &mut seed_u64);
            // Convert the seed to `Wrapping<u64>` and zero-extend to `RAND_SIZE`.
            let mut seed_extended = [w(0); RAND_SIZE];
            for (x, y) in seed_extended.iter_mut().zip(seed_u64.iter()) {
                *x = w(*y);
            }
            Self::init(seed_extended, 2)
        }

        fn seed_from_u64(seed: u64) -> Self {
            let mut key = [w(0); RAND_SIZE];
            key[0] = w(seed);
            // Initialize with only one pass.
            // A second pass does not improve the quality here, because all of the
            // seed was already available in the first round.
            // Not doing the second pass has the small advantage that if
            // `seed == 0` this method produces exactly the same state as the
            // reference implementation when used unseeded.
            Self::init(key, 1)
        }

        fn from_rng<R: RngCore>(mut rng: R) -> Result<Self, Error> {
            // Custom `from_rng` implementation that fills a seed with the same size
            // as the entire state.
            let mut seed = [w(0u64); RAND_SIZE];
            unsafe {
                let ptr = seed.as_mut_ptr() as *mut u8;
                let slice = slice::from_raw_parts_mut(ptr, RAND_SIZE * 8);
                rng.try_fill_bytes(slice)?;
            }
            for i in seed.iter_mut() {
                *i = w(i.0.to_le());
            }

            Ok(Self::init(seed, 2))
        }
    }

    #[cfg(test)]
    mod test {
        use rand_core::{RngCore, SeedableRng};
        use super::Isaac64Rng;

        #[test]
        fn test_isaac64_construction() {
            // Test that various construction techniques produce a working RNG.
            let seed = [1,0,0,0, 23,0,0,0, 200,1,0,0, 210,30,0,0,
                        0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0];
            let mut rng1 = Isaac64Rng::from_seed(seed);
            assert_eq!(rng1.next_u64(), 14964555543728284049);

            let mut rng2 = Isaac64Rng::from_rng(rng1).unwrap();
            assert_eq!(rng2.next_u64(), 919595328260451758);
        }

        #[test]
        fn test_isaac64_true_values_64() {
            let seed = [1,0,0,0, 0,0,0,0, 23,0,0,0, 0,0,0,0,
                        200,1,0,0, 0,0,0,0, 210,30,0,0, 0,0,0,0];
            let mut rng1 = Isaac64Rng::from_seed(seed);
            let mut results = [0u64; 10];
            for i in results.iter_mut() { *i = rng1.next_u64(); }
            let expected = [
                       15071495833797886820, 7720185633435529318,
                       10836773366498097981, 5414053799617603544,
                       12890513357046278984, 17001051845652595546,
                       9240803642279356310, 12558996012687158051,
                       14673053937227185542, 1677046725350116783];
            assert_eq!(results, expected);

            let seed = [57,48,0,0, 0,0,0,0, 50,9,1,0, 0,0,0,0,
                        49,212,0,0, 0,0,0,0, 148,38,0,0, 0,0,0,0];
            let mut rng2 = Isaac64Rng::from_seed(seed);
            // skip forward to the 10000th number
            for _ in 0..10000 { rng2.next_u64(); }

            for i in results.iter_mut() { *i = rng2.next_u64(); }
            let expected = [
                18143823860592706164, 8491801882678285927, 2699425367717515619,
                17196852593171130876, 2606123525235546165, 15790932315217671084,
                596345674630742204, 9947027391921273664, 11788097613744130851,
                10391409374914919106];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_isaac64_true_values_32() {
            let seed = [1,0,0,0, 0,0,0,0, 23,0,0,0, 0,0,0,0,
                        200,1,0,0, 0,0,0,0, 210,30,0,0, 0,0,0,0];
            let mut rng = Isaac64Rng::from_seed(seed);
            let mut results = [0u32; 12];
            for i in results.iter_mut() { *i = rng.next_u32(); }
            // Subset of above values, as an LE u32 sequence
            let expected = [
                        3477963620, 3509106075,
                        687845478, 1797495790,
                        227048253, 2523132918,
                        4044335064, 1260557630,
                        4079741768, 3001306521,
                        69157722, 3958365844];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_isaac64_true_values_mixed() {
            let seed = [1,0,0,0, 0,0,0,0, 23,0,0,0, 0,0,0,0,
                        200,1,0,0, 0,0,0,0, 210,30,0,0, 0,0,0,0];
            let mut rng = Isaac64Rng::from_seed(seed);
            // Test alternating between `next_u64` and `next_u32` works as expected.
            // Values are the same as `test_isaac64_true_values` and
            // `test_isaac64_true_values_32`.
            assert_eq!(rng.next_u64(), 15071495833797886820);
            assert_eq!(rng.next_u32(), 687845478);
            assert_eq!(rng.next_u32(), 1797495790);
            assert_eq!(rng.next_u64(), 10836773366498097981);
            assert_eq!(rng.next_u32(), 4044335064);
            // Skip one u32
            assert_eq!(rng.next_u64(), 12890513357046278984);
            assert_eq!(rng.next_u32(), 69157722);
        }

        #[test]
        fn test_isaac64_true_bytes() {
            let seed = [1,0,0,0, 0,0,0,0, 23,0,0,0, 0,0,0,0,
                        200,1,0,0, 0,0,0,0, 210,30,0,0, 0,0,0,0];
            let mut rng = Isaac64Rng::from_seed(seed);
            let mut results = [0u8; 32];
            rng.fill_bytes(&mut results);
            // Same as first values in test_isaac64_true_values as bytes in LE order
            let expected = [100, 131, 77, 207, 155, 181, 40, 209,
                            102, 176, 255, 40, 238, 155, 35, 107,
                            61, 123, 136, 13, 246, 243, 99, 150,
                            216, 167, 15, 241, 62, 149, 34, 75];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_isaac64_new_uninitialized() {
            // Compare the results from initializing `IsaacRng` with
            // `seed_from_u64(0)`, to make sure it is the same as the reference
            // implementation when used uninitialized.
            // Note: We only test the first 16 integers, not the full 256 of the
            // first block.
            let mut rng = Isaac64Rng::seed_from_u64(0);
            let mut results = [0u64; 16];
            for i in results.iter_mut() { *i = rng.next_u64(); }
            let expected: [u64; 16] = [
                0xF67DFBA498E4937C, 0x84A5066A9204F380, 0xFEE34BD5F5514DBB,
                0x4D1664739B8F80D6, 0x8607459AB52A14AA, 0x0E78BC5A98529E49,
                0xFE5332822AD13777, 0x556C27525E33D01A, 0x08643CA615F3149F,
                0xD0771FAF3CB04714, 0x30E86F68A37B008D, 0x3074EBC0488A3ADF,
                0x270645EA7A2790BC, 0x5601A0A8D3763C6A, 0x2F83071F53F325DD,
                0xB9090F3D42D2D2EA];
            assert_eq!(results, expected);
        }

        #[test]
        fn test_isaac64_clone() {
            let seed = [1,0,0,0, 0,0,0,0, 23,0,0,0, 0,0,0,0,
                        200,1,0,0, 0,0,0,0, 210,30,0,0, 0,0,0,0];
            let mut rng1 = Isaac64Rng::from_seed(seed);
            let mut rng2 = rng1.clone();
            for _ in 0..16 {
                assert_eq!(rng1.next_u64(), rng2.next_u64());
            }
        }

        #[test]
        #[cfg(feature="serde1")]
        fn test_isaac64_serde() {
            use bincode;
            use std::io::{BufWriter, BufReader};

            let seed = [1,0,0,0, 23,0,0,0, 200,1,0,0, 210,30,0,0,
                         57,48,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0];
            let mut rng = Isaac64Rng::from_seed(seed);

            let buf: Vec<u8> = Vec::new();
            let mut buf = BufWriter::new(buf);
            bincode::serialize_into(&mut buf, &rng).expect("Could not serialize");

            let buf = buf.into_inner().unwrap();
            let mut read = BufReader::new(&buf[..]);
            let mut deserialized: Isaac64Rng = bincode::deserialize_from(&mut read).expect("Could not deserialize");

            for _ in 0..300 { // more than the 256 buffered results
                assert_eq!(rng.next_u64(), deserialized.next_u64());
            }
        }
    }

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