commit plain_hasher to util

Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "plain_hasher"
+description = "Hasher for 32-bit keys."
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+license = "MIT"
+keywords = ["hash", "hasher"]
+categories = ["no-std"]
+homepage = "https://github.com/paritytech/plain_hasher"
+
+[dependencies]
+crunchy = "0.1.6"

benches/bench.rs

@@ -0,0 +1,33 @@
+#![feature(test)]
+
+extern crate test;
+extern crate plain_hasher;
+
+use std::hash::Hasher;
+use std::collections::hash_map::DefaultHasher;
+use test::{Bencher, black_box};
+use plain_hasher::PlainHasher;
+
+#[bench]
+fn write_plain_hasher(b: &mut Bencher) {
+	b.iter(|| {
+		let n: u8 = black_box(100);
+		(0..n).fold(PlainHasher::default(), |mut old, new| {
+			let bb = black_box([new; 32]);
+			old.write(&bb as &[u8]);
+			old
+		});
+	});
+}
+
+#[bench]
+fn write_default_hasher(b: &mut Bencher) {
+	b.iter(|| {
+		let n: u8 = black_box(100);
+		(0..n).fold(DefaultHasher::default(), |mut old, new| {
+			let bb = black_box([new; 32]);
+			old.write(&bb as &[u8]);
+			old
+		});
+	});
+}

src/lib.rs

@@ -0,0 +1,55 @@
+#![no_std]
+#[macro_use]
+extern crate crunchy;
+
+use core::{hash, mem};
+
+/// Hasher that just takes 8 bytes of the provided value.
+/// May only be used for keys which are 32 bytes.
+#[derive(Default)]
+pub struct PlainHasher {
+	prefix: u64,
+}
+
+impl hash::Hasher for PlainHasher {
+	#[inline]
+	fn finish(&self) -> u64 {
+		self.prefix
+	}
+
+	#[inline]
+	#[allow(unused_assignments)]
+	fn write(&mut self, bytes: &[u8]) {
+		debug_assert!(bytes.len() == 32);
+
+		unsafe {
+			let mut bytes_ptr = bytes.as_ptr();
+			let prefix_u8: &mut [u8; 8] = mem::transmute(&mut self.prefix);
+			let mut prefix_ptr = prefix_u8.as_mut_ptr();
+
+			unroll! {
+				for _i in 0..8 {
+					*prefix_ptr ^= (*bytes_ptr ^ *bytes_ptr.offset(8)) ^ (*bytes_ptr.offset(16) ^ *bytes_ptr.offset(24));
+
+					bytes_ptr = bytes_ptr.offset(1);
+					prefix_ptr = prefix_ptr.offset(1);
+				}
+			}
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use core::hash::Hasher;
+	use super::PlainHasher;
+
+	#[test]
+	fn it_works() {
+		let mut bytes = [32u8; 32];
+		bytes[0] = 15;
+		let mut hasher = PlainHasher::default();
+		hasher.write(&bytes);
+		assert_eq!(hasher.prefix, 47);
+	}
+}