From: Phillip Wood <phillip.wood123@gmail.com>
To: Junio C Hamano <gitster@pobox.com>,
Phillip Wood via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org,
Johannes Schindelin <Johannes.Schindelin@gmx.de>,
Phillip Wood <phillip.wood@dunelm.org.uk>
Subject: Re: [PATCH 1/2] patience diff: remove unnecessary string comparisons
Date: Wed, 5 May 2021 10:34:29 +0100 [thread overview]
Message-ID: <87001425-8043-4c66-dbc2-637f05a7229f@gmail.com> (raw)
In-Reply-To: <xmqqpmy658e1.fsf@gitster.g>
On 05/05/2021 01:31, Junio C Hamano wrote:
> "Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
>> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>>
>> xdl_prepare_env() calls xdl_classify_record() which arranges for the
>> hashes of non-matching lines to be different so lines can be tested
>> for equality by comparing just their hashes.
>
> Hmph, that is a bit different from what I read from the comment in
> the post context of the first hunk, though.
>
> /*
> * After xdl_prepare_env() (or more precisely, due to
> * xdl_classify_record()), the "ha" member of the records (AKA lines)
> * is _not_ the hash anymore, but a linearized version of it. In
> * other words, the "ha" member is guaranteed to start with 0 and
> * the second record's ha can only be 0 or 1, etc.
> *
> * So we multiply ha by 2 in the hope that the hashing was
> * "unique enough".
> */
>
> The words "home" and "enough" hints to me that the "ha" member is
> not hash, but "lineralized version of it" (whatever it means) does
> not guarantee that two records with the same "ha" are identical, or
> does it?
By "hashes" I meant "the value of record->ha". That comment is a bit
confusing. I think "linearized version of it" is referring to
xdl_classify_record() assigning a unique integer to each unique input
line starting from zero and increasing by one for each unique input line
(the function is fairly easy to follow). I assume "unique enough" is
referring to the line below the comment which takes the modulus of
record->ha and record->ha is not evenly distributed over the whole
integer range but bunched at the lower end.
The Myers implementation calls xdl_classify_record() and then only ever
compares record->ha, it does not call xdl_recmatch() while computing the
diff.
> Well, I should just go read xdl_classify_record() to see what it
> really does, but if it eliminates collisions, then the patch is a
> clear and obvious improvement.
Thanks
Phillip
> Thanks.
>
>> diff --git a/xdiff/xpatience.c b/xdiff/xpatience.c
>> index 20699a6f6054..db2d53e89cb0 100644
>> --- a/xdiff/xpatience.c
>> +++ b/xdiff/xpatience.c
>> @@ -90,7 +90,7 @@ static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
>> {
>> xrecord_t **records = pass == 1 ?
>> map->env->xdf1.recs : map->env->xdf2.recs;
>> - xrecord_t *record = records[line - 1], *other;
>> + xrecord_t *record = records[line - 1];
>> /*
>> * After xdl_prepare_env() (or more precisely, due to
>> * xdl_classify_record()), the "ha" member of the records (AKA lines)
>> @@ -104,11 +104,7 @@ static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
>> int index = (int)((record->ha << 1) % map->alloc);
>>
>> while (map->entries[index].line1) {
>> - other = map->env->xdf1.recs[map->entries[index].line1 - 1];
>> - if (map->entries[index].hash != record->ha ||
>> - !xdl_recmatch(record->ptr, record->size,
>> - other->ptr, other->size,
>> - map->xpp->flags)) {
>> + if (map->entries[index].hash != record->ha) {
>> if (++index >= map->alloc)
>> index = 0;
>> continue;
>> @@ -253,8 +249,7 @@ static int match(struct hashmap *map, int line1, int line2)
>> {
>> xrecord_t *record1 = map->env->xdf1.recs[line1 - 1];
>> xrecord_t *record2 = map->env->xdf2.recs[line2 - 1];
>> - return xdl_recmatch(record1->ptr, record1->size,
>> - record2->ptr, record2->size, map->xpp->flags);
>> + return record1->ha == record2->ha;
>> }
>>
>> static int patience_diff(mmfile_t *file1, mmfile_t *file2,
next prev parent reply other threads:[~2021-05-05 9:34 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-04 9:25 [PATCH 0/2] A couple of small patience diff cleanups Phillip Wood via GitGitGadget
2021-05-04 9:25 ` [PATCH 1/2] patience diff: remove unnecessary string comparisons Phillip Wood via GitGitGadget
2021-05-05 0:31 ` Junio C Hamano
2021-05-05 9:34 ` Phillip Wood [this message]
2021-05-05 14:58 ` Johannes Schindelin
2021-05-05 18:00 ` Phillip Wood
2021-05-06 1:32 ` Junio C Hamano
2021-05-04 9:25 ` [PATCH 2/2] patience diff: remove unused variable Phillip Wood via GitGitGadget
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