use crate::convert::*;
use core::hash::Hasher;

///This constant come from Kunth's prng (Empirically it works better than those from splitmix32).
const MULTIPLE: u64 = crate::MULTIPLE;
const INCREMENT: u64 = 1442695040888963407;
const ROT: u32 = 23; //17

/// A `Hasher` for hashing an arbitrary stream of bytes.
///
/// Instances of [`AHasher`] represent state that is updated while hashing data.
///
/// Each method updates the internal state based on the new data provided. Once
/// all of the data has been provided, the resulting hash can be obtained by calling
/// `finish()`
///
/// [Clone] is also provided in case you wish to calculate hashes for two different items that
/// start with the same data.
///
#[derive(Debug, Clone)]
pub struct AHasher {
    buffer: u64,
    pad: u64,
}

impl AHasher {
    /// Creates a new hasher keyed to the provided key.
    #[inline]
    pub fn new_with_keys(key1: u64, key2: u64) -> AHasher {
        AHasher { buffer: key1, pad: key2 }
    }

    #[cfg(test)]
    pub(crate) fn test_with_keys(key1: u64, key2: u64) -> AHasher {
        use crate::scramble_keys;
        let (k1, k2) = scramble_keys(key1, key2);
        AHasher {
            buffer: k1,
            pad: k2
        }
    }

    /// This update function has the goal of updating the buffer with a single multiply
    /// FxHash does this but is vulnerable to attack. To avoid this input needs to be masked to with an
    /// unpredictable value. Other hashes such as murmurhash have taken this approach but were found vulnerable
    /// to attack. The attack was based on the idea of reversing the pre-mixing (Which is necessarily
    /// reversible otherwise bits would be lost) then placing a difference in the highest bit before the
    /// multiply used to mix the data. Because a multiply can never affect the bits to the right of it, a
    /// subsequent update that also differed in this bit could result in a predictable collision.
    ///
    /// This version avoids this vulnerability while still only using a single multiply. It takes advantage
    /// of the fact that when a 64 bit multiply is performed the upper 64 bits are usually computed and thrown
    /// away. Instead it creates two 128 bit values where the upper 64 bits are zeros and multiplies them.
    /// (The compiler is smart enough to turn this into a 64 bit multiplication in the assembly)
    /// Then the upper bits are added to the lower bits to produce a single 64 bit result.
    ///
    /// To understand why this is a good scrambling function it helps to understand multiply-with-carry PRNGs:
    /// https://en.wikipedia.org/wiki/Multiply-with-carry_pseudorandom_number_generator
    /// If the multiple is chosen well, this creates a long period, decent quality PRNG.
    /// Notice that this function is equivalent to this except the `buffer`/`state` is being xored with each
    /// new block of data. In the event that data is all zeros, it is exactly equivalent to a MWC PRNG.
    ///
    /// This is impervious to attack because every bit buffer at the end is dependent on every bit in
    /// `new_data ^ buffer`. For example suppose two inputs differed in only the 5th bit. Then when the
    /// multiplication is performed the `result` will differ in bits 5-69. More specifically it will differ by
    /// 2^5 * MULTIPLE. However in the next step bits 65-128 are turned into a separate 64 bit value. So the
    /// differing bits will be in the lower 6 bits of this value. The two intermediate values that differ in
    /// bits 5-63 and in bits 0-5 respectively get added together. Producing an output that differs in every
    /// bit. The addition carries in the multiplication and at the end additionally mean that the even if an
    /// attacker somehow knew part of (but not all) the contents of the buffer before hand,
    /// they would not be able to predict any of the bits in the buffer at the end.
    #[inline(always)]
    fn update(&mut self, new_data: u64) {
        let result: [u64; 2] = ((new_data ^ self.buffer) as u128)
            .wrapping_mul(MULTIPLE as u128)
            .convert();
        self.buffer = result[0].wrapping_add(result[1]);
    }

    /// This update function updates the buffer with the new information in a way that can't be canceled
    /// with a subsequent update without knowledge of the content of the buffer prior to the update.
    ///
    /// To achieve this the input needs to be modified in an unpredictable (to an attacker) way before it is
    /// combined with the value in the buffer. This is done by xoring it with `key`.
    ///
    /// Other hashes such as murmurhash have taken that approach but were found vulnerable to attack.
    /// The attack was based on the idea of reversing any pre-mixing (Which is necessarily reversible otherwise
    /// bits would be lost) then placing a difference in the highest bit before the multiply. Because a multiply
    /// can never affect the bits to the right of it, a subsequent update that also only differed in the high
    /// order bit could cancel out the change to `buffer` from the first update. This allowed murmurhash to be
    /// attacked. In this update function aHash avoids this vulnerability by rotating and performing a second
    /// multiply.
    ///
    /// This makes it impossible for an attacker to place a single bit difference between
    /// two blocks so as to cancel each other. (While the transform is still reversible if you know the key)
    ///
    /// This is similar to the above update function but is designed to run in a loop
    /// that will be unrolled and vectorized. So instead of using the buffer, it uses a 'key' that it updates
    /// and returns. The buffer is only xored at the end. This structure is so that when the method is inlined,
    /// the compiler will unroll any loop this gets placed in and the loop can be automatically vectorized
    /// and the rotates, xors, and multiplies can be paralleled.
    ///
    /// The key needs to be incremented between consecutive calls to prevent (a,b) from hashing the same as (b,a).
    /// The adding of the increment is moved to the bottom rather than the top. This allows one less add to be
    /// performed overall, but more importantly, it follows the multiply, which is expensive. So the CPU can
    /// run another operation afterwords if does not depend on the output of the multiply operation.
    #[inline(always)]
    fn ordered_update(&mut self, new_data: u64, key: u64) -> u64 {
        self.buffer ^= (new_data ^ key)
            .wrapping_mul(MULTIPLE)
            .rotate_left(ROT)
            .wrapping_mul(MULTIPLE);
        key.wrapping_add(INCREMENT)
    }
}

#[inline(never)]
#[no_mangle]
fn hash_test(input: &[u8]) -> u64 {
    let mut a = AHasher::new_with_keys(12345, 67);
    a.write(input);
    a.finish()
}

/// Provides methods to hash all of the primitive types.
impl Hasher for AHasher {
    #[inline]
    fn write_u8(&mut self, i: u8) {
        self.update(i as u64);
    }

    #[inline]
    fn write_u16(&mut self, i: u16) {
        self.update(i as u64);
    }

    #[inline]
    fn write_u32(&mut self, i: u32) {
        self.update(i as u64);
    }

    #[inline]
    fn write_u64(&mut self, i: u64) {
        self.update(i as u64);
    }

    #[inline]
    fn write_u128(&mut self, i: u128) {
        let data: [u64; 2] = i.convert();
        self.update(data[0]);
        self.update(data[1]);
    }

    #[inline]
    fn write_usize(&mut self, i: usize) {
        self.write_u64(i as u64);
    }

    #[inline]
    fn write(&mut self, input: &[u8]) {
        let mut data = input;
        let length = data.len() as u64;
        //Needs to be an add rather than an xor because otherwise it could be canceled with carefully formed input.
        self.buffer = self.buffer.wrapping_add(length.wrapping_mul(MULTIPLE));
        //A 'binary search' on sizes reduces the number of comparisons.
        if data.len() > 8 {
            if data.len() > 16 {
                let tail = data.read_last_u64();
                let mut key: u64 = self.buffer;
                while data.len() > 8 {
                    let (val, rest) = data.read_u64();
                    key = self.ordered_update(val, key);
                    data = rest;
                }
                self.update(tail);
            } else {
                self.update(data.read_u64().0);
                self.update(data.read_last_u64());
            }
        } else {
            if data.len() >= 2 {
                if data.len() >= 4 {
                    let block: [u32; 2] = [data.read_u32().0, data.read_last_u32()];
                    self.update(block.convert());
                } else {
                    let block: [u16; 2] = [data.read_u16().0, data.read_last_u16()];
                    let val: u32 = block.convert();
                    self.update(val as u64);
                }
            } else {
                let value = if data.len() > 0 {
                    data[0] //len 1
                } else {
                    0
                };
                self.update(value as u64);
            }
        }
    }
    #[inline]
    fn finish(&self) -> u64 {
        (self.buffer ^ self.pad)
    }
}

#[cfg(test)]
mod tests {
    use crate::convert::Convert;
    use crate::fallback_hash::*;

    #[test]
    fn test_hash() {
        let mut hasher = AHasher::new_with_keys(0, 0);
        let value: u64 = 1 << 32;
        hasher.update(value);
        let result = hasher.buffer;
        let mut hasher = AHasher::new_with_keys(0, 0);
        let value2: u64 = 1;
        hasher.update(value2);
        let result2 = hasher.buffer;
        let result: [u8; 8] = result.convert();
        let result2: [u8; 8] = result2.convert();
        assert_ne!(hex::encode(result), hex::encode(result2));
    }

    #[test]
    fn test_conversion() {
        let input: &[u8] = "dddddddd".as_bytes();
        let bytes: u64 = as_array!(input, 8).convert();
        assert_eq!(bytes, 0x6464646464646464);
    }
}