//! A speedy, non-cryptographic hashing algorithm used by `rustc`. //! //! # Example //! //! ```rust //! # #[cfg(feature = "std")] //! # fn main() { //! use rustc_hash::FxHashMap; //! //! let mut map: FxHashMap = FxHashMap::default(); //! map.insert(22, 44); //! # } //! # #[cfg(not(feature = "std"))] //! # fn main() { } //! ``` #![no_std] #![cfg_attr(feature = "nightly", feature(hasher_prefixfree_extras))] #[cfg(feature = "std")] extern crate std; #[cfg(feature = "rand")] extern crate rand; #[cfg(feature = "rand")] mod random_state; mod seeded_state; use core::default::Default; use core::hash::{BuildHasher, Hasher}; #[cfg(feature = "std")] use std::collections::{HashMap, HashSet}; /// Type alias for a hash map that uses the Fx hashing algorithm. #[cfg(feature = "std")] pub type FxHashMap = HashMap; /// Type alias for a hash set that uses the Fx hashing algorithm. #[cfg(feature = "std")] pub type FxHashSet = HashSet; #[cfg(feature = "rand")] pub use random_state::{FxHashMapRand, FxHashSetRand, FxRandomState}; pub use seeded_state::FxSeededState; #[cfg(feature = "std")] pub use seeded_state::{FxHashMapSeed, FxHashSetSeed}; /// A speedy hash algorithm for use within rustc. The hashmap in liballoc /// by default uses SipHash which isn't quite as speedy as we want. In the /// compiler we're not really worried about DOS attempts, so we use a fast /// non-cryptographic hash. /// /// The current implementation is a fast polynomial hash with a single /// bit rotation as a finishing step designed by Orson Peters. #[derive(Clone)] pub struct FxHasher { hash: usize, } // One might view a polynomial hash // m[0] * k + m[1] * k^2 + m[2] * k^3 + ... // as a multilinear hash with keystream k[..] // m[0] * k[0] + m[1] * k[1] + m[2] * k[2] + ... // where keystream k just happens to be generated using a multiplicative // congruential pseudorandom number generator (MCG). For that reason we chose a // constant that was found to be good for a MCG in: // "Computationally Easy, Spectrally Good Multipliers for Congruential // Pseudorandom Number Generators" by Guy Steele and Sebastiano Vigna. #[cfg(target_pointer_width = "64")] const K: usize = 0xf1357aea2e62a9c5; #[cfg(target_pointer_width = "32")] const K: usize = 0x93d765dd; impl FxHasher { /// Creates a `fx` hasher with a given seed. pub const fn with_seed(seed: usize) -> FxHasher { FxHasher { hash: seed } } /// Creates a default `fx` hasher. pub const fn default() -> FxHasher { FxHasher { hash: 0 } } } impl Default for FxHasher { #[inline] fn default() -> FxHasher { Self::default() } } impl FxHasher { #[inline] fn add_to_hash(&mut self, i: usize) { self.hash = self.hash.wrapping_add(i).wrapping_mul(K); } } impl Hasher for FxHasher { #[inline] fn write(&mut self, bytes: &[u8]) { // Compress the byte string to a single u64 and add to our hash. self.write_u64(hash_bytes(bytes)); } #[inline] fn write_u8(&mut self, i: u8) { self.add_to_hash(i as usize); } #[inline] fn write_u16(&mut self, i: u16) { self.add_to_hash(i as usize); } #[inline] fn write_u32(&mut self, i: u32) { self.add_to_hash(i as usize); } #[inline] fn write_u64(&mut self, i: u64) { self.add_to_hash(i as usize); #[cfg(target_pointer_width = "32")] self.add_to_hash((i >> 32) as usize); } #[inline] fn write_u128(&mut self, i: u128) { self.add_to_hash(i as usize); #[cfg(target_pointer_width = "32")] self.add_to_hash((i >> 32) as usize); self.add_to_hash((i >> 64) as usize); #[cfg(target_pointer_width = "32")] self.add_to_hash((i >> 96) as usize); } #[inline] fn write_usize(&mut self, i: usize) { self.add_to_hash(i); } #[cfg(feature = "nightly")] #[inline] fn write_length_prefix(&mut self, _len: usize) { // Most cases will specialize hash_slice to call write(), which encodes // the length already in a more efficient manner than we could here. For // HashDoS-resistance you would still need to include this for the // non-slice collection hashes, but for the purposes of rustc we do not // care and do not wish to pay the performance penalty of mixing in len // for those collections. } #[cfg(feature = "nightly")] #[inline] fn write_str(&mut self, s: &str) { // Similarly here, write already encodes the length, so nothing special // is needed. self.write(s.as_bytes()) } #[inline] fn finish(&self) -> u64 { // Since we used a multiplicative hash our top bits have the most // entropy (with the top bit having the most, decreasing as you go). // As most hash table implementations (including hashbrown) compute // the bucket index from the bottom bits we want to move bits from the // top to the bottom. Ideally we'd rotate left by exactly the hash table // size, but as we don't know this we'll choose 20 bits, giving decent // entropy up until 2^20 table sizes. On 32-bit hosts we'll dial it // back down a bit to 15 bits. #[cfg(target_pointer_width = "64")] const ROTATE: u32 = 20; #[cfg(target_pointer_width = "32")] const ROTATE: u32 = 15; self.hash.rotate_left(ROTATE) as u64 // A bit reversal would be even better, except hashbrown also expects // good entropy in the top 7 bits and a bit reverse would fill those // bits with low entropy. More importantly, bit reversals are very slow // on x86-64. A byte reversal is relatively fast, but still has a 2 // cycle latency on x86-64 compared to the 1 cycle latency of a rotate. // It also suffers from the hashbrown-top-7-bit-issue. } } // Nothing special, digits of pi. const SEED1: u64 = 0x243f6a8885a308d3; const SEED2: u64 = 0x13198a2e03707344; const PREVENT_TRIVIAL_ZERO_COLLAPSE: u64 = 0xa4093822299f31d0; #[inline] fn multiply_mix(x: u64, y: u64) -> u64 { #[cfg(target_pointer_width = "64")] { // We compute the full u64 x u64 -> u128 product, this is a single mul // instruction on x86-64, one mul plus one mulhi on ARM64. let full = (x as u128) * (y as u128); let lo = full as u64; let hi = (full >> 64) as u64; // The middle bits of the full product fluctuate the most with small // changes in the input. This is the top bits of lo and the bottom bits // of hi. We can thus make the entire output fluctuate with small // changes to the input by XOR'ing these two halves. lo ^ hi // Unfortunately both 2^64 + 1 and 2^64 - 1 have small prime factors, // otherwise combining with + or - could result in a really strong hash, as: // x * y = 2^64 * hi + lo = (-1) * hi + lo = lo - hi, (mod 2^64 + 1) // x * y = 2^64 * hi + lo = 1 * hi + lo = lo + hi, (mod 2^64 - 1) // Multiplicative hashing is universal in a field (like mod p). } #[cfg(target_pointer_width = "32")] { // u64 x u64 -> u128 product is prohibitively expensive on 32-bit. // Decompose into 32-bit parts. let lx = x as u32; let ly = y as u32; let hx = (x >> 32) as u32; let hy = (y >> 32) as u32; // u32 x u32 -> u64 the low bits of one with the high bits of the other. let afull = (lx as u64) * (hy as u64); let bfull = (hx as u64) * (ly as u64); // Combine, swapping low/high of one of them so the upper bits of the // product of one combine with the lower bits of the other. afull ^ bfull.rotate_right(32) } } /// A wyhash-inspired non-collision-resistant hash for strings/slices designed /// by Orson Peters, with a focus on small strings and small codesize. /// /// The 64-bit version of this hash passes the SMHasher3 test suite on the full /// 64-bit output, that is, f(hash_bytes(b) ^ f(seed)) for some good avalanching /// permutation f() passed all tests with zero failures. When using the 32-bit /// version of multiply_mix this hash has a few non-catastrophic failures where /// there are a handful more collisions than an optimal hash would give. /// /// We don't bother avalanching here as we'll feed this hash into a /// multiplication after which we take the high bits, which avalanches for us. #[inline] fn hash_bytes(bytes: &[u8]) -> u64 { let len = bytes.len(); let mut s0 = SEED1; let mut s1 = SEED2; if len <= 16 { // XOR the input into s0, s1. if len >= 8 { s0 ^= u64::from_le_bytes(bytes[0..8].try_into().unwrap()); s1 ^= u64::from_le_bytes(bytes[len - 8..].try_into().unwrap()); } else if len >= 4 { s0 ^= u32::from_le_bytes(bytes[0..4].try_into().unwrap()) as u64; s1 ^= u32::from_le_bytes(bytes[len - 4..].try_into().unwrap()) as u64; } else if len > 0 { let lo = bytes[0]; let mid = bytes[len / 2]; let hi = bytes[len - 1]; s0 ^= lo as u64; s1 ^= ((hi as u64) << 8) | mid as u64; } } else { // Handle bulk (can partially overlap with suffix). let mut off = 0; while off < len - 16 { let x = u64::from_le_bytes(bytes[off..off + 8].try_into().unwrap()); let y = u64::from_le_bytes(bytes[off + 8..off + 16].try_into().unwrap()); // Replace s1 with a mix of s0, x, and y, and s0 with s1. // This ensures the compiler can unroll this loop into two // independent streams, one operating on s0, the other on s1. // // Since zeroes are a common input we prevent an immediate trivial // collapse of the hash function by XOR'ing a constant with y. let t = multiply_mix(s0 ^ x, PREVENT_TRIVIAL_ZERO_COLLAPSE ^ y); s0 = s1; s1 = t; off += 16; } let suffix = &bytes[len - 16..]; s0 ^= u64::from_le_bytes(suffix[0..8].try_into().unwrap()); s1 ^= u64::from_le_bytes(suffix[8..16].try_into().unwrap()); } multiply_mix(s0, s1) ^ (len as u64) } /// An implementation of [`BuildHasher`] that produces [`FxHasher`]s. /// /// ``` /// use std::hash::BuildHasher; /// use rustc_hash::FxBuildHasher; /// assert_ne!(FxBuildHasher.hash_one(1), FxBuildHasher.hash_one(2)); /// ``` #[derive(Copy, Clone, Default)] pub struct FxBuildHasher; impl BuildHasher for FxBuildHasher { type Hasher = FxHasher; fn build_hasher(&self) -> FxHasher { FxHasher::default() } } #[cfg(test)] mod tests { #[cfg(not(any(target_pointer_width = "64", target_pointer_width = "32")))] compile_error!("The test suite only supports 64 bit and 32 bit usize"); use crate::{FxBuildHasher, FxHasher}; use core::hash::{BuildHasher, Hash, Hasher}; macro_rules! test_hash { ( $( hash($value:expr) == $result:expr, )* ) => { $( assert_eq!(FxBuildHasher.hash_one($value), $result); )* }; } const B32: bool = cfg!(target_pointer_width = "32"); #[test] fn unsigned() { test_hash! { hash(0_u8) == 0, hash(1_u8) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_u8) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(u8::MAX) == if B32 { 999399879 } else { 17600987023830959190 }, hash(0_u16) == 0, hash(1_u16) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_u16) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(u16::MAX) == if B32 { 3440503042 } else { 4001367065645062987 }, hash(0_u32) == 0, hash(1_u32) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_u32) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(u32::MAX) == if B32 { 1293006356 } else { 17126373362251322066 }, hash(0_u64) == 0, hash(1_u64) == if B32 { 275023839 } else { 12583873379513078615 }, hash(100_u64) == if B32 { 1732383522 } else { 4008740938959785536 }, hash(u64::MAX) == if B32 { 1017982517 } else { 5862870694197521576 }, hash(0_u128) == 0, hash(1_u128) == if B32 { 1860738631 } else { 12885773367358079611 }, hash(100_u128) == if B32 { 1389515751 } else { 15751995649841559633 }, hash(u128::MAX) == if B32 { 2156022013 } else { 11423841400550042156 }, hash(0_usize) == 0, hash(1_usize) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_usize) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(usize::MAX) == if B32 { 1293006356 } else { 5862870694197521576 }, } } #[test] fn signed() { test_hash! { hash(i8::MIN) == if B32 { 2000713177 } else { 5869058164817243095 }, hash(0_i8) == 0, hash(1_i8) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_i8) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(i8::MAX) == if B32 { 3293686765 } else { 11731928859014764671 }, hash(i16::MIN) == if B32 { 1073764727 } else { 8292620222579070801 }, hash(0_i16) == 0, hash(1_i16) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_i16) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(i16::MAX) == if B32 { 2366738315 } else { 14155490916776592377 }, hash(i32::MIN) == if B32 { 16384 } else { 5631751334026900245 }, hash(0_i32) == 0, hash(1_i32) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_i32) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(i32::MAX) == if B32 { 1293022740 } else { 11494622028224421821 }, hash(i64::MIN) == if B32 { 16384 } else { 524288 }, hash(0_i64) == 0, hash(1_i64) == if B32 { 275023839 } else { 12583873379513078615 }, hash(100_i64) == if B32 { 1732383522 } else { 4008740938959785536 }, hash(i64::MAX) == if B32 { 1017998901 } else { 5862870694198045864 }, hash(i128::MIN) == if B32 { 16384 } else { 524288 }, hash(0_i128) == 0, hash(1_i128) == if B32 { 1860738631 } else { 12885773367358079611 }, hash(100_i128) == if B32 { 1389515751 } else { 15751995649841559633 }, hash(i128::MAX) == if B32 { 2156005629 } else { 11423841400549517868 }, hash(isize::MIN) == if B32 { 16384 } else { 524288 }, hash(0_isize) == 0, hash(1_isize) == if B32 { 3001993707 } else { 12583873379513078615 }, hash(100_isize) == if B32 { 3844759569 } else { 4008740938959785536 }, hash(isize::MAX) == if B32 { 1293022740 } else { 5862870694198045864 }, } } // Avoid relying on any `Hash` implementations in the standard library. struct HashBytes(&'static [u8]); impl Hash for HashBytes { fn hash(&self, state: &mut H) { state.write(self.0); } } #[test] fn bytes() { test_hash! { hash(HashBytes(&[])) == if B32 { 2673204745 } else { 5175017818631658678 }, hash(HashBytes(&[0])) == if B32 { 2948228584 } else { 11037888512829180254 }, hash(HashBytes(&[0, 0, 0, 0, 0, 0])) == if B32 { 3223252423 } else { 6891281800865632452 }, hash(HashBytes(&[1])) == if B32 { 2943445104 } else { 4127763515449136980 }, hash(HashBytes(&[2])) == if B32 { 1055423297 } else { 11322700005987241762 }, hash(HashBytes(b"uwu")) == if B32 { 2699662140 } else { 2129615206728903013 }, hash(HashBytes(b"These are some bytes for testing rustc_hash.")) == if B32 { 2303640537 } else { 5513083560975408889 }, } } #[test] fn with_seed_actually_different() { let seeds = [ [1, 2], [42, 17], [124436707, 99237], [usize::MIN, usize::MAX], ]; for [a_seed, b_seed] in seeds { let a = || FxHasher::with_seed(a_seed); let b = || FxHasher::with_seed(b_seed); for x in u8::MIN..=u8::MAX { let mut a = a(); let mut b = b(); x.hash(&mut a); x.hash(&mut b); assert_ne!(a.finish(), b.finish()) } } } }