Re: [PATCH 1/2] patience diff: remove unnecessary string comparisons

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Dscho

On 05/05/2021 15:58, Johannes Schindelin wrote:
> Hi Junio,
> 
> On Wed, 5 May 2021, 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?
>>
>> 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.
> 
> Right. I had the same concern. But it does look as if
> `xdl_classify_record()` replaced the possibly non-unique hash values to
> unique sequential identifiers.
> 
> I have to admit that the code is unnecessarily hard to read for me:
> https://github.com/git/git/blob/v2.31.1/xdiff/xprepare.c#L110-L157
> 
> But I do gather that the loop at
> https://github.com/git/git/blob/v2.31.1/xdiff/xprepare.c#L119-L123
> is called for every line, that it does compare it to every seen line with
> the same hash, and that it exits the loop early if the contents disagree:
> 
> 	for (rcrec = cf->rchash[hi]; rcrec; rcrec = rcrec->next)
> 		if (rcrec->ha == rec->ha &&
> 				xdl_recmatch(rcrec->line, rcrec->size,
> 					rec->ptr, rec->size, cf->flags))
> 			break;
> 
> Since naming is hard (and you can easily err on saving space at the
> expense of costing readers' time, as libxdiff proves), and since I am
> running out of review time, I'll have to assume that
> https://github.com/git/git/blob/v2.31.1/xdiff/xprepare.c#L150-L154 means
> that indeed, the `ha` field is set to a counter that uniquely identifies
> the line contents:
> 
> 	rec->ha = (unsigned long) rcrec->idx;
> 
> 
> 	hi = (long) XDL_HASHLONG(rec->ha, hbits);
> 	rec->next = rhash[hi];
> 	rhash[hi] = rec;
> 
> So I am fairly confident that the patch is good, and the performance win
> is nice.

Thanks for taking the time to review this patch, I agree with your 
analysis. The output of `git log -p --diff-algorithm=patience 
origin/master` for the whole history of git.git is unchanged by this patch.

Best Wishes

Phillip

> 
> Thanks!
> Dscho
> 
>>
>> 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,
>>