linux-kernel - Re: [PATCH] Update jhash.h with the new version of Jenkins' hash

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Message-ID: <20090212001223.GA5757@movementarian.org>
Date:	Wed, 11 Feb 2009 19:12:23 -0500
From:	wli@...ementarian.org
To:	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] Update jhash.h with the new version of Jenkins' hash

On Wed, Feb 11, 2009 at 11:19:20AM +0100, Jozsef Kadlecsik wrote:
>  /* The golden ration: an arbitrary value */
> -#define JHASH_GOLDEN_RATIO	0x9e3779b9
> +#define JHASH_GOLDEN_RATIO	0xdeadbeef

The golden ratio is not an arbitrary value, nor is it a food ration.
It is (sqrt(5)+1)/2. and the original value of JHASH_GOLDEN_RATIO
approximated its reciprocal and/or fractional part in fixed point.

(double)0xdeadbeef/((unsigned long long)1 << 32) is a rather poor
approximation of (sqrt(5)-1)/2. If this value which is not the
golden ratio is arbitrary, calling it something different from
JHASH_GOLDEN_RATIO would surprise the code's readers much less.

-- wli
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