netdev - Re: ipv4: Simplify ARP hash function.

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Date:	Fri, 8 Jul 2011 16:41:28 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	David Miller <davem@...emloft.net>
Cc:	roland@...estorage.com, johnwheffner@...il.com, mj@....cz,
	netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

What about using murmur hash which has a four byte pass as well.
  https://sites.google.com/site/murmurhash/
---
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>

#define u32 uint32_t

/* Do a one pass murmurhash2 */
static u32 arp_hashfn(u32 key, int ifindex, u32 hash_rnd)
{
	/* murmurhash mixiing constants */
	const unsigned int m = 0x5bd1e995;
	const int r = 24;

	/* Initialize the hash to a 'random' value  */
	unsigned int h = ifindex ^ hash_rnd;
	unsigned int k = key;

	k *= m; 
	k ^= k >> r; 
	k *= m; 
		
	h *= m; 
	h ^= k;

	/* Do a few final mixes of the hash to ensure the last few
	 * bytes are well-incorporated.
	 */

	h ^= h >> 13;
	h *= m;
	h ^= h >> 15;

	return h;
} 

int main(int argc, char **argv)
{
	u32 rnd, key, hash;
	int ifindex;

	key = atoi(argv[1]);
	ifindex = atoi(argv[2]);
	rnd = atoi(argv[3]);

	hash = arp_hashfn(key, ifindex, rnd);
	printf("%u, %d, %u => %u\n", key, ifindex, rnd, hash);
	return 0;
}
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