fingerprint.rs

use crate::stable_hasher;
use std::mem;
use serialize;
use serialize::opaque::{EncodeResult, Encoder, Decoder};

#[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Clone, Copy)]
pub struct Fingerprint(u64, u64);

impl Fingerprint {

    pub const ZERO: Fingerprint = Fingerprint(0, 0);

    #[inline]
    pub fn from_smaller_hash(hash: u64) -> Fingerprint {
        Fingerprint(hash, hash)
    }

    #[inline]
    pub fn to_smaller_hash(&self) -> u64 {
        self.0
    }

    #[inline]
    pub fn as_value(&self) -> (u64, u64) {
        (self.0, self.1)
    }

    #[inline]
    pub fn combine(self, other: Fingerprint) -> Fingerprint {
        // See https://stackoverflow.com/a/27952689 on why this function is
        // implemented this way.
        Fingerprint(
            self.0.wrapping_mul(3).wrapping_add(other.0),
            self.1.wrapping_mul(3).wrapping_add(other.1)
        )
    }

    // Combines two hashes in an order independent way. Make sure this is what
    // you want.
    #[inline]
    pub fn combine_commutative(self, other: Fingerprint) -> Fingerprint {
        let a = (self.1 as u128) << 64 | self.0 as u128;
        let b = (other.1 as u128) << 64 | other.0 as u128;

        let c = a.wrapping_add(b);

        Fingerprint((c >> 64) as u64, c as u64)
    }

    pub fn to_hex(&self) -> String {
        format!("{:x}{:x}", self.0, self.1)
    }

    pub fn encode_opaque(&self, encoder: &mut Encoder) -> EncodeResult {
        let bytes: [u8; 16] = unsafe { mem::transmute([self.0.to_le(), self.1.to_le()]) };

        encoder.emit_raw_bytes(&bytes);
        Ok(())
    }

    pub fn decode_opaque<'a>(decoder: &mut Decoder<'a>) -> Result<Fingerprint, String> {
        let mut bytes = [0; 16];

        decoder.read_raw_bytes(&mut bytes)?;

        let [l, r]: [u64; 2] = unsafe { mem::transmute(bytes) };

        Ok(Fingerprint(u64::from_le(l), u64::from_le(r)))
    }
}

impl ::std::fmt::Display for Fingerprint {
    fn fmt(&self, formatter: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
        write!(formatter, "{:x}-{:x}", self.0, self.1)
    }
}

impl stable_hasher::StableHasherResult for Fingerprint {
    #[inline]
    fn finish(hasher: stable_hasher::StableHasher<Self>) -> Self {
        let (_0, _1) = hasher.finalize();
        Fingerprint(_0, _1)
    }
}

impl_stable_hash_via_hash!(Fingerprint);

impl serialize::UseSpecializedEncodable for Fingerprint { }

impl serialize::UseSpecializedDecodable for Fingerprint { }

impl serialize::SpecializedEncoder<Fingerprint> for serialize::opaque::Encoder {
    fn specialized_encode(&mut self, f: &Fingerprint) -> Result<(), Self::Error> {
        f.encode_opaque(self)
    }
}

impl<'a> serialize::SpecializedDecoder<Fingerprint> for serialize::opaque::Decoder<'a> {
    fn specialized_decode(&mut self) -> Result<Fingerprint, Self::Error> {
        Fingerprint::decode_opaque(self)
    }
}