use crate::convert::*;
use crate::operations::folded_multiply;
use crate::operations::read_small;
use crate::operations::MULTIPLE;
use crate::random_state::PI;
use crate::RandomState;
use core::hash::Hasher;
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,
extra_keys: [u64; 2],
}
impl AHasher {
/// Creates a new hasher keyed to the provided key.
#[inline]
#[allow(dead_code)] // Is not called if non-fallback hash is used.
pub(crate) fn new_with_keys(key1: u128, key2: u128) -> AHasher {
let pi: [u128; 2] = PI.convert();
let key1: [u64; 2] = (key1 ^ pi[0]).convert();
let key2: [u64; 2] = (key2 ^ pi[1]).convert();
AHasher {
buffer: key1[0],
pad: key1[1],
extra_keys: key2,
}
}
#[allow(unused)] // False positive
pub(crate) fn test_with_keys(key1: u128, key2: u128) -> Self {
let key1: [u64; 2] = key1.convert();
let key2: [u64; 2] = key2.convert();
Self {
buffer: key1[0],
pad: key1[1],
extra_keys: key2,
}
}
#[inline]
#[allow(dead_code)] // Is not called if non-fallback hash is used.
pub(crate) fn from_random_state(rand_state: &RandomState) -> AHasher {
AHasher {
buffer: rand_state.k0,
pad: rand_state.k1,
extra_keys: [rand_state.k2, rand_state.k3],
}
}
/// 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 xored with 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) {
self.buffer = folded_multiply(new_data ^ self.buffer, MULTIPLE);
}
/// Similar to the above this function performs an update using a "folded multiply".
/// However it takes in 128 bits of data instead of 64. Both halves must be masked.
///
/// This makes it impossible for an attacker to place a single bit difference between
/// two blocks so as to cancel each other.
///
/// However this is not sufficient. to prevent (a,b) from hashing the same as (b,a) the buffer itself must
/// be updated between calls in a way that does not commute. To achieve this XOR and Rotate are used.
/// Add followed by xor is not the same as xor followed by add, and rotate ensures that the same out bits
/// can't be changed by the same set of input bits. To cancel this sequence with subsequent input would require
/// knowing the keys.
#[inline(always)]
fn large_update(&mut self, new_data: u128) {
let block: [u64; 2] = new_data.convert();
let combined = folded_multiply(block[0] ^ self.extra_keys[0], block[1] ^ self.extra_keys[1]);
self.buffer = (self.buffer.wrapping_add(self.pad) ^ combined).rotate_left(ROT);
}
#[inline]
#[cfg(feature = "specialize")]
fn short_finish(&self) -> u64 {
self.buffer.wrapping_add(self.pad)
}
}
/// Provides [Hasher] methods to hash all of the primitive types.
///
/// [Hasher]: core::hash::Hasher
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) {
self.large_update(i);
}
#[inline]
#[cfg(any(
target_pointer_width = "64",
target_pointer_width = "32",
target_pointer_width = "16"
))]
fn write_usize(&mut self, i: usize) {
self.write_u64(i as u64);
}
#[inline]
#[cfg(target_pointer_width = "128")]
fn write_usize(&mut self, i: usize) {
self.write_u128(i as u128);
}
#[inline]
#[allow(clippy::collapsible_if)]
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_u128();
self.large_update(tail);
while data.len() > 16 {
let (block, rest) = data.read_u128();
self.large_update(block);
data = rest;
}
} else {
self.large_update([data.read_u64().0, data.read_last_u64()].convert());
}
} else {
let value = read_small(data);
self.large_update(value.convert());
}
}
#[inline]
fn finish(&self) -> u64 {
let rot = (self.buffer & 63) as u32;
folded_multiply(self.buffer, self.pad).rotate_left(rot)
}
}
#[cfg(feature = "specialize")]
pub(crate) struct AHasherU64 {
pub(crate) buffer: u64,
pub(crate) pad: u64,
}
/// A specialized hasher for only primitives under 64 bits.
#[cfg(feature = "specialize")]
impl Hasher for AHasherU64 {
#[inline]
fn finish(&self) -> u64 {
let rot = (self.pad & 63) as u32;
self.buffer.rotate_left(rot)
}
#[inline]
fn write(&mut self, _bytes: &[u8]) {
unreachable!("Specialized hasher was called with a different type of object")
}
#[inline]
fn write_u8(&mut self, i: u8) {
self.write_u64(i as u64);
}
#[inline]
fn write_u16(&mut self, i: u16) {
self.write_u64(i as u64);
}
#[inline]
fn write_u32(&mut self, i: u32) {
self.write_u64(i as u64);
}
#[inline]
fn write_u64(&mut self, i: u64) {
self.buffer = folded_multiply(i ^ self.buffer, MULTIPLE);
}
#[inline]
fn write_u128(&mut self, _i: u128) {
unreachable!("Specialized hasher was called with a different type of object")
}
#[inline]
fn write_usize(&mut self, _i: usize) {
unreachable!("Specialized hasher was called with a different type of object")
}
}
#[cfg(feature = "specialize")]
pub(crate) struct AHasherFixed(pub AHasher);
/// A specialized hasher for fixed size primitives larger than 64 bits.
#[cfg(feature = "specialize")]
impl Hasher for AHasherFixed {
#[inline]
fn finish(&self) -> u64 {
self.0.short_finish()
}
#[inline]
fn write(&mut self, bytes: &[u8]) {
self.0.write(bytes)
}
#[inline]
fn write_u8(&mut self, i: u8) {
self.write_u64(i as u64);
}
#[inline]
fn write_u16(&mut self, i: u16) {
self.write_u64(i as u64);
}
#[inline]
fn write_u32(&mut self, i: u32) {
self.write_u64(i as u64);
}
#[inline]
fn write_u64(&mut self, i: u64) {
self.0.write_u64(i);
}
#[inline]
fn write_u128(&mut self, i: u128) {
self.0.write_u128(i);
}
#[inline]
fn write_usize(&mut self, i: usize) {
self.0.write_usize(i);
}
}
#[cfg(feature = "specialize")]
pub(crate) struct AHasherStr(pub AHasher);
/// A specialized hasher for a single string
/// Note that the other types don't panic because the hash impl for String tacks on an unneeded call. (As does vec)
#[cfg(feature = "specialize")]
impl Hasher for AHasherStr {
#[inline]
fn finish(&self) -> u64 {
self.0.finish()
}
#[inline]
fn write(&mut self, bytes: &[u8]) {
if bytes.len() > 8 {
self.0.write(bytes)
} else {
let value = read_small(bytes);
self.0.buffer = folded_multiply(value[0] ^ self.0.buffer, value[1] ^ self.0.extra_keys[1]);
self.0.pad = self.0.pad.wrapping_add(bytes.len() as u64);
}
}
#[inline]
fn write_u8(&mut self, _i: u8) {}
#[inline]
fn write_u16(&mut self, _i: u16) {}
#[inline]
fn write_u32(&mut self, _i: u32) {}
#[inline]
fn write_u64(&mut self, _i: u64) {}
#[inline]
fn write_u128(&mut self, _i: u128) {}
#[inline]
fn write_usize(&mut self, _i: usize) {}
}
#[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);
}
}