plain_hasher 0.2.2

Hasher for 32-byte keys.
Documentation 
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// Copyright 2015-2018 Parity Technologies (UK) Ltd.
// This file is part of Parity.

// Parity is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Parity is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Parity.  If not, see <http://www.gnu.org/licenses/>.

#![cfg_attr(not(feature = "std"), no_std)]

use core::hash::Hasher;

use crunchy::unroll;

/// 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 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);
		let mut bytes_ptr = bytes.as_ptr();
		let mut prefix_ptr = &mut self.prefix as *mut u64 as *mut u8;

		unroll! {
			for _i in 0..8 {
				unsafe {
					*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 super::*;

	#[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);
	}
}