From: Jeff Hostetler <git@jeffhostetler.com>
To: "SZEDER Gábor" <szeder.dev@gmail.com>
Cc: gitster@pobox.com, peff@peff.net,
Jeff Hostetler <jeffhost@microsoft.com>,
git@vger.kernel.org
Subject: Re: [PATCH v5 2/4] read-cache: add strcmp_offset function
Date: Thu, 6 Apr 2017 11:58:10 -0400 [thread overview]
Message-ID: <3f88c428-6d04-6ef5-f7bc-952daad2703a@jeffhostetler.com> (raw)
In-Reply-To: <20170406141912.14536-1-szeder.dev@gmail.com>
On 4/6/2017 10:19 AM, SZEDER Gábor wrote:
>> Add strcmp_offset() function to also return the offset of the
>> first change.
>>
>> Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
>> ---
>> cache.h | 1 +
>> read-cache.c | 29 +++++++++++++++++++++++++++++
>> 2 files changed, 30 insertions(+)
>>
>> diff --git a/cache.h b/cache.h
>> index 80b6372..4d82490 100644
>> --- a/cache.h
>> +++ b/cache.h
>> @@ -574,6 +574,7 @@ extern int write_locked_index(struct index_state *, struct lock_file *lock, unsi
>> extern int discard_index(struct index_state *);
>> extern int unmerged_index(const struct index_state *);
>> extern int verify_path(const char *path);
>> +extern int strcmp_offset(const char *s1_in, const char *s2_in, int *first_change);
>> extern int index_dir_exists(struct index_state *istate, const char *name, int namelen);
>> extern void adjust_dirname_case(struct index_state *istate, char *name);
>> extern struct cache_entry *index_file_exists(struct index_state *istate, const char *name, int namelen, int igncase);
>> diff --git a/read-cache.c b/read-cache.c
>> index 9054369..b3fc77d 100644
>> --- a/read-cache.c
>> +++ b/read-cache.c
>
> read-cache.c is probably not the best place for such a general string
> utility function, though I'm not sure where its most apropriate place
> would be.
Yeah, I looked and didn't see any place so I left it here
near it's first usage for now.
>
>> @@ -887,6 +887,35 @@ static int has_file_name(struct index_state *istate,
>> return retval;
>> }
>>
>> +
>> +/*
>> + * Like strcmp(), but also return the offset of the first change.
>
> This comment doesn't tell us what will happen with the offset
> parameter if it is NULL or if the two strings are equal. I don't
> really care about the safety check for NULL: a callsite not interested
> in the offset should just call strcmp() instead. I'm fine either way.
> However, setting it to 0 when the strings are equal doesn't seem quite
> right, does it.
Good catch. I'll add a null check.
When the strings are equal, I'm not sure any one value is
better than another. I could leave it undefined or set it
to the length. That might be more useful since we have it on
hand and it might save the caller a strlen() later.
>
>> + */
>> +int strcmp_offset(const char *s1_in, const char *s2_in, int *first_change)
>> +{
>> + const unsigned char *s1 = (const unsigned char *)s1_in;
>> + const unsigned char *s2 = (const unsigned char *)s2_in;
>> + int diff = 0;
>> + int k;
>> +
>> + *first_change = 0;
>> + for (k=0; s1[k]; k++)
>> + if ((diff = (s1[k] - s2[k])))
>> + goto found_it;
>> + if (!s2[k])
>> + return 0;
>> + diff = -1;
>> +
>> +found_it:
>> + *first_change = k;
>> + if (diff > 0)
>> + return 1;
>> + else if (diff < 0)
>> + return -1;
>> + else
>> + return 0;
>> +}
>
> The implementation seems to me a bit long-winded, with more
> conditional statements than necessary. How about something like this
> instead? Much shorter, no goto and only half the number of
> conditionals to reason about, leaves *first_change untouched when the
> two strings are equal, and deals with it being NULL.
>
> int strcmp_offset(const char *s1, const char *s2, int *first_change)
> {
> int k;
>
> for (k = 0; s1[k] == s2[k]; k++)
> if (s1[k] == '\0')
> return 0;
>
> if (first_change)
> *first_change = k;
> return ((unsigned char *)s1)[k] - ((unsigned char *)s2)[k];
> }
>
Let me give that a try. Thanks!
Jeff
next prev parent reply other threads:[~2017-04-06 15:58 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-05 17:38 [PATCH v5 0/4] read-cache: speed up add_index_entry git
2017-04-05 17:38 ` [PATCH v5 1/4] p0004-read-tree: perf test to time read-tree git
2017-04-06 20:20 ` René Scharfe
2017-04-06 20:41 ` Jeff Hostetler
2017-04-05 17:38 ` [PATCH v5 2/4] read-cache: add strcmp_offset function git
2017-04-06 14:19 ` SZEDER Gábor
2017-04-06 15:58 ` Jeff Hostetler [this message]
2017-04-05 17:38 ` [PATCH v5 3/4] test-strcmp-offset: created test for strcmp_offset git
2017-04-05 22:47 ` SZEDER Gábor
2017-04-06 8:21 ` Johannes Schindelin
2017-04-06 14:25 ` Jeff Hostetler
2017-04-06 20:20 ` René Scharfe
2017-04-06 20:42 ` Jeff Hostetler
2017-04-05 17:38 ` [PATCH v5 4/4] read-cache: speed up add_index_entry during checkout git
2017-04-05 22:54 ` SZEDER Gábor
2017-04-06 14:05 ` Jeff Hostetler
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