fxhash.rs

#[inline]
const fn add_to_hash(hash: u64, i: u64) -> u64 {
    (hash.rotate_left(5) ^ i).wrapping_mul(0x517cc1b727220a95)
}

/// A const implementation of [`rustc_hash::FxHasher`]
pub const fn fxhash(v: &str) -> u64 {
    let mut hash: u64 = 0;
    let bytes = v.as_bytes();
    let l = bytes.len();
    let mut i = 0;

    while l - i >= 8 {
        hash = add_to_hash(
            hash,
            u64::from_ne_bytes([
                bytes[i],
                bytes[i + 1],
                bytes[i + 2],
                bytes[i + 3],
                bytes[i + 4],
                bytes[i + 5],
                bytes[i + 6],
                bytes[i + 7],
            ]),
        );
        i += 8;
    }
    if l - i >= 4 {
        hash = add_to_hash(
            hash,
            u32::from_ne_bytes([bytes[i], bytes[i + 1], bytes[i + 2], bytes[i + 3]]) as u64,
        );
        i += 4;
    }
    if l - i >= 2 {
        hash = add_to_hash(hash, u16::from_ne_bytes([bytes[i], bytes[i + 1]]) as u64);
        i += 2;
    }
    if l - i >= 1 {
        hash = add_to_hash(hash, bytes[i] as u64);
    }

    add_to_hash(hash, 0xff)
}

#[cfg(test)]
fn fxhash_test(name: &str) {
    use std::hash::{Hash, Hasher};
    let mut hasher = rustc_hash::FxHasher::default();
    name.hash(&mut hasher);
    assert_eq!(hasher.finish(), fxhash(name));
}

#[test]
fn test_const_fxhash() {
    fxhash_test("");
    fxhash_test("0");
    fxhash_test("01");
    fxhash_test("012");
    fxhash_test("0123");
    fxhash_test("01234");
    fxhash_test("012345");
    fxhash_test("0123456");
    fxhash_test("01234567");
    fxhash_test("012345678");
    fxhash_test("0123456789");
    fxhash_test("01234567890");
    fxhash_test("012345678901");
    fxhash_test("0123456789012");
    fxhash_test("01234567890123");
    fxhash_test("012345678901234");
    fxhash_test("0123456789012345");
    fxhash_test("01234567890123456");
    fxhash_test("012345678901234567");
    fxhash_test("0123456789012345678");
    fxhash_test("01234567890123456789");
    fxhash_test("012345678901234567890");
    fxhash_test("0123456789012345678901");
    fxhash_test("01234567890123456789012");
}