From mboxrd@z Thu Jan  1 00:00:00 1970
From: Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH] replace sha1 with another algorithm
Date: Wed, 26 Oct 2011 12:44:15 -0700
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References: <20111026001237.GA22195@sigill.intra.peff.net>
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To: Jeff King <peff@peff.net>
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In-Reply-To: <20111026001237.GA22195@sigill.intra.peff.net> (Jeff King's
 message of "Tue, 25 Oct 2011 17:12:37 -0700")
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Jeff King <peff@peff.net> writes:

> +static void xor_bytes(unsigned char *out, unsigned char *a, unsigned char *b,
> +		      unsigned n)
> +{
> +	unsigned i;
> +	for (i = 0; i < n; i++)
> +		out[i] = a[i] ^ b[i];
> +}
> +
> +static void mix_hash(unsigned char *h, unsigned n)
> +{
> +	unsigned char out[20];
> +	unsigned mid = n / 2;
> +
> +	if (2*mid < n)
> +		return;
> +
> +	xor_bytes(out, h, h + mid, mid);
> +	xor_bytes(out + mid, h + mid, h, mid);
> +	memcpy(h, out, n);
> +
> +	/* If a little bit of mixing is good, then a lot must be GREAT! */
> +	mix_hash(h, mid);
> +	mix_hash(h + mid, mid);
> +}

You seem to want to reduce the hash down to 5-bytes by duplicating the
same value on the left and right half, and duplicate that four times to
fill 20-byte buffer, but doesn't this look unnecessarily inefficient way
to achieve that?