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//! A stable 128-bit hash: the same input bits give the same key on every
//! target and in every run, unlike `std`'s randomly seeded hashers.
use kurbo::{BezPath, PathEl, Point};
/// Two FNV-style lanes over the bytes written, as the render graph keys its
/// results.
#[derive(Clone, Debug)]
pub struct StableHash([u64; 2]);
impl Default for StableHash {
fn default() -> Self {
Self::new()
}
}
impl StableHash {
/// A hash of nothing yet.
#[must_use]
pub const fn new() -> Self {
Self([0xcbf2_9ce4_8422_2325, 0x6c62_272e_07bb_0142])
}
/// Adds `bytes`, length first, so adjacent writes cannot run together.
pub fn bytes(&mut self, bytes: &[u8]) {
self.u64(bytes.len() as u64);
for byte in bytes {
self.mix(*byte);
}
}
/// Adds a 32-bit value.
pub fn u32(&mut self, value: u32) {
for byte in value.to_le_bytes() {
self.mix(byte);
}
}
/// Adds a 64-bit value.
pub fn u64(&mut self, value: u64) {
for byte in value.to_le_bytes() {
self.mix(byte);
}
}
/// Adds a float's bits.
pub fn f32(&mut self, value: f32) {
self.u32(value.to_bits());
}
/// Adds a float's bits.
pub fn f64(&mut self, value: f64) {
self.u64(value.to_bits());
}
/// Adds a point's bits.
pub fn point(&mut self, point: Point) {
self.f64(point.x);
self.f64(point.y);
}
/// Adds every element of a path, with its kind.
pub fn path(&mut self, path: &BezPath) {
self.u64(path.elements().len() as u64);
for element in path.elements() {
match *element {
PathEl::MoveTo(p) => {
self.mix(b'M');
self.point(p);
}
PathEl::LineTo(p) => {
self.mix(b'L');
self.point(p);
}
PathEl::QuadTo(a, b) => {
self.mix(b'Q');
self.point(a);
self.point(b);
}
PathEl::CurveTo(a, b, c) => {
self.mix(b'C');
self.point(a);
self.point(b);
self.point(c);
}
PathEl::ClosePath => self.mix(b'Z'),
}
}
}
fn mix(&mut self, byte: u8) {
self.0[0] ^= u64::from(byte);
self.0[0] = self.0[0].wrapping_mul(0x0000_0100_0000_01b3);
self.0[1] ^= u64::from(byte).wrapping_add(0x9d);
self.0[1] = self.0[1].wrapping_mul(0x9e37_79b1_85eb_ca87);
}
/// The key of everything added.
#[must_use]
pub fn finish(&self) -> u128 {
(u128::from(self.0[1]) << 64) | u128::from(self.0[0])
}
}