/*
 * lz_hash.h - hashing for Lempel-Ziv matchfinding
 *
 * Copyright 2022 Eric Biggers
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use,
 * copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following
 * conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */

#ifndef _LZ_HASH_H
#define _LZ_HASH_H

#include "wimlib/types.h"

/*
 * The hash function: given a sequence prefix held in the low-order bits of a
 * 32-bit value, multiply by a carefully-chosen large constant.  Discard any
 * bits of the product that don't fit in a 32-bit value, but take the
 * next-highest @num_bits bits of the product as the hash value, as those have
 * the most randomness.
 */
static forceinline u32
lz_hash(u32 seq, unsigned num_bits)
{
	return (u32)(seq * 0x1E35A7BD) >> (32 - num_bits);
}

#endif /* _LZ_HASH_H */