library/proc_macro/src/bridge/fxhash.rs

//! This is a copy of the `rustc_hash` crate, adapted to work as a module.
//!
//! If in the future it becomes more reasonable to add dependencies to
//! `proc_macro`, this module should be removed and replaced with a dependency
//! on the `rustc_hash` crate.

use std::collections::HashMap;
use std::hash::{BuildHasherDefault, Hasher};
use std::ops::BitXor;

/// Type alias for a hashmap using the `fx` hash algorithm.
pub(super) type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;

/// A speedy hash algorithm for use within rustc. The hashmap in alloc 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.
///
/// This is the same as the algorithm used by Firefox -- which is a homespun
/// one not based on any widely-known algorithm -- though modified to produce
/// 64-bit hash values instead of 32-bit hash values. It consistently
/// out-performs an FNV-based hash within rustc itself -- the collision rate is
/// similar or slightly worse than FNV, but the speed of the hash function
/// itself is much higher because it works on up to 8 bytes at a time.
#[derive(Default)]
pub(super) struct FxHasher {
    hash: usize,
}

#[cfg(target_pointer_width = "32")]
const K: usize = 0x9e3779b9;
#[cfg(target_pointer_width = "64")]
const K: usize = 0x517cc1b727220a95;

impl FxHasher {
    #[inline]
    fn add_to_hash(&mut self, i: usize) {
        self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
    }
}

impl Hasher for FxHasher {
    #[inline]
    fn write(&mut self, mut bytes: &[u8]) {
        #[cfg(target_pointer_width = "32")]
        let read_usize = |bytes: &[u8]| u32::from_ne_bytes(bytes[..4].try_into().unwrap());
        #[cfg(target_pointer_width = "64")]
        let read_usize = |bytes: &[u8]| u64::from_ne_bytes(bytes[..8].try_into().unwrap());

        let mut hash = FxHasher { hash: self.hash };
        assert!(size_of::<usize>() <= 8);
        while bytes.len() >= size_of::<usize>() {
            hash.add_to_hash(read_usize(bytes) as usize);
            bytes = &bytes[size_of::<usize>()..];
        }
        if (size_of::<usize>() > 4) && (bytes.len() >= 4) {
            hash.add_to_hash(u32::from_ne_bytes(bytes[..4].try_into().unwrap()) as usize);
            bytes = &bytes[4..];
        }
        if (size_of::<usize>() > 2) && bytes.len() >= 2 {
            hash.add_to_hash(u16::from_ne_bytes(bytes[..2].try_into().unwrap()) as usize);
            bytes = &bytes[2..];
        }
        if (size_of::<usize>() > 1) && !bytes.is_empty() {
            hash.add_to_hash(bytes[0] as usize);
        }
        self.hash = hash.hash;
    }

    #[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);
    }

    #[cfg(target_pointer_width = "32")]
    #[inline]
    fn write_u64(&mut self, i: u64) {
        self.add_to_hash(i as usize);
        self.add_to_hash((i >> 32) as usize);
    }

    #[cfg(target_pointer_width = "64")]
    #[inline]
    fn write_u64(&mut self, i: u64) {
        self.add_to_hash(i as usize);
    }

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

    #[inline]
    fn finish(&self) -> u64 {
        self.hash as u64
    }
}