From: Junio C Hamano <gitster@pobox•com>
To: Karsten Blees <karsten.blees@gmail•com>
Cc: Git List <git@vger•kernel.org>
Subject: Re: [PATCH v1 1/4] hashmap: factor out getting an int hash code from a, SHA1
Date: Mon, 07 Jul 2014 10:22:56 -0700 [thread overview]
Message-ID: <xmqqoax1dqxb.fsf@gitster.dls.corp.google.com> (raw)
In-Reply-To: <53B485A4.20609@gmail.com> (Karsten Blees's message of "Thu, 03 Jul 2014 00:20:20 +0200")
Karsten Blees <karsten.blees@gmail•com> writes:
> Copying the first bytes of a SHA1 is duplicated in six places, however,
> the implications (wrong byte order on little-endian systems) is documented
> only once.
s/wrong /different /; but other than that I think this is a good
change.
> +`unsigned int sha1hash(const unsigned char *sha1)`::
> +
> + Converts a cryptographic hash (e.g. SHA-1) into an int-sized hash code
> + for use in hash tables. Cryptographic hashes are supposed to have
> + uniform distribution, so in contrast to `memhash()`, this just copies
> + the first `sizeof(int)` bytes without shuffling any bits. Note that
> + the results will be different on big-endian and little-endian
> + platforms, so they should not be stored or transferred over the net!
Tone down with s/!/./, perhaps?
Another thing we may want to caution against is to use it as a
tie-breaker that affects the final outcome the user can observe, but
that may be something that goes without saying. I dunno..
> diff --git a/hashmap.h b/hashmap.h
> index a816ad4..ed5425a 100644
> --- a/hashmap.h
> +++ b/hashmap.h
> @@ -13,6 +13,17 @@ extern unsigned int strihash(const char *buf);
> extern unsigned int memhash(const void *buf, size_t len);
> extern unsigned int memihash(const void *buf, size_t len);
>
> +static inline unsigned int sha1hash(const unsigned char *sha1)
> +{
> + /*
> + * Equivalent to 'return *(int *)sha1;', but safe on platforms that
> + * don't support unaligned reads.
> + */
s/int/unsigned &/; other than that, the explanation is good.
> + unsigned int hash;
> + memcpy(&hash, sha1, sizeof(hash));
> + return hash;
> +}
> +
next prev parent reply other threads:[~2014-07-07 17:23 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-02 22:18 [PATCH v1 0/4] hashmap improvements Karsten Blees
2014-07-02 22:20 ` [PATCH v1 1/4] hashmap: factor out getting an int hash code from a, SHA1 Karsten Blees
2014-07-07 17:22 ` Junio C Hamano [this message]
2014-07-02 22:21 ` [PATCH v1 2/4] hashmap: improve struct hashmap member documentation Karsten Blees
2014-07-02 22:22 ` [PATCH v1 3/4] hashmap: add simplified hashmap_get_from_hash() API Karsten Blees
2014-07-07 17:43 ` Junio C Hamano
2014-07-11 19:11 ` Karsten Blees
2014-07-11 22:21 ` Junio C Hamano
2014-07-02 22:22 ` [PATCH v1 4/4] hashmap: add string interning API Karsten Blees
2014-07-03 7:22 ` Matthieu Moy
2014-07-07 17:44 ` Junio C Hamano
2014-07-03 7:23 ` [PATCH v1 0/4] hashmap improvements Matthieu Moy
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