lib.rs

pub mod id_hash {
    use ::std::collections::{HashSet, HashMap};
    use ::std::hash::{Hasher, BuildHasherDefault};

    /// Hashes u64s to themselves, assuming they already come from
    /// a uniform distribution.
    ///
    /// THIS IS PROBABLY A BAD IDEA.
    #[derive(Debug, Default)]
    pub struct IdHasher(Option<u64>);

    pub type Set<K> = HashSet<K, BuildHasherDefault<IdHasher>>;
    pub type Map<K, V> = HashMap<K, V, BuildHasherDefault<IdHasher>>;

    const ERR_WRONG: &'static str = "Only a single u64 may be written to IdentityHasher";
    const ERR_NOTHING: &'static str = "A single u64 must be written to IdentityHasher";

    impl Hasher for IdHasher {
        #[inline(always)]
        fn finish(&self) -> u64 {
            self.0.expect(ERR_NOTHING)
        }

        #[inline(always)]
        fn write(&mut self, bytes: &[u8]) {
            assert_eq!(bytes.len(), 8, "{}", ERR_WRONG);

            let int_ptr = bytes.as_ptr() as *const u64;
            self.write_u64(unsafe { *int_ptr });
        }

        #[inline(always)]
        fn write_u64(&mut self, i: u64) {
            assert!(self.0.is_none(), "{}", ERR_WRONG);
            self.0 = Some(i);
        }
    }
}