Re: [PATCH 02/11] merge-ort: add initial outline for basic rename detection

[Derrick Stolee <stolee@gmail.com>]

On 12/9/2020 2:41 PM, Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <newren@gmail.com>
> 
> Signed-off-by: Elijah Newren <newren@gmail.com>
> ---
>  merge-ort.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 60 insertions(+), 8 deletions(-)
> 
> diff --git a/merge-ort.c b/merge-ort.c
> index 90baedac407..92b765dd3f0 100644
> --- a/merge-ort.c
> +++ b/merge-ort.c
> @@ -617,20 +617,72 @@ static int handle_content_merge(struct merge_options *opt,
>  
>  /*** Function Grouping: functions related to regular rename detection ***/
>  
> +static int process_renames(struct merge_options *opt,
> +			   struct diff_queue_struct *renames)
> +static int compare_pairs(const void *a_, const void *b_)
> +/* Call diffcore_rename() to compute which files have changed on given side */
> +static void detect_regular_renames(struct merge_options *opt,
> +				   struct tree *merge_base,
> +				   struct tree *side,
> +				   unsigned side_index)
> +static int collect_renames(struct merge_options *opt,
> +			   struct diff_queue_struct *result,
> +			   unsigned side_index)

standard "I promise this will follow soon!" strategy, OK.

>  static int detect_and_process_renames(struct merge_options *opt,
>  				      struct tree *merge_base,
>  				      struct tree *side1,
>  				      struct tree *side2)
>  {
> -	int clean = 1;
> +	struct diff_queue_struct combined;
> +	struct rename_info *renames = opt->priv->renames;

(Re: my concerns that we don't need 'renames' to be a pointer,
this could easily be "renames = &opt->priv.renames;")

> +	int s, clean = 1;
> +
> +	memset(&combined, 0, sizeof(combined));
> +
> +	detect_regular_renames(opt, merge_base, side1, 1);
> +	detect_regular_renames(opt, merge_base, side2, 2);

Find the renames in each side's diff.

I think the use of "1" and "2" here might be better situated
for an enum. Perhaps:

enum merge_side {
	MERGE_SIDE1 = 0,
	MERGE_SIDE2 = 1,
};

(Note, I shift these values to 0 and 1, respectively, allowing
us to truncate the pairs array to two entries while still
being mentally clear.)

> +
> +	ALLOC_GROW(combined.queue,
> +		   renames->pairs[1].nr + renames->pairs[2].nr,
> +		   combined.alloc);
> +	clean &= collect_renames(opt, &combined, 1);
> +	clean &= collect_renames(opt, &combined, 2);

Magic numbers again.

> +	QSORT(combined.queue, combined.nr, compare_pairs);
> +
> +	clean &= process_renames(opt, &combined);

I need to mentally remember that "clean" is a return state,
but _not_ a fail/success result. Even though we are using
"&=" here, it shouldn't be "&&=" or even "if (method()) return 1;"

Looking at how "clean" is used in struct merge_result, I
wonder if there is a reason to use an "int" over a simple
"unsigned" or even "unsigned clean:1;" You use -1 in places
as well as a case of "mi->clean = !!resolved;"

If there is more meaning to values other than "clean" or
"!clean", then an enum might be valuable.

> +	/* Free memory for renames->pairs[] and combined */
> +	for (s = 1; s <= 2; s++) {
> +		free(renames->pairs[s].queue);
> +		DIFF_QUEUE_CLEAR(&renames->pairs[s]);
> +	}

This loop is particularly unusual. Perhaps it would be
better to do this instead:

	free(renames->pairs[MERGE_SIDE1].queue);
	free(renames->pairs[MERGE_SIDE2].queue);
	DIFF_QUEUE_CLEAR(&renames->pairs[MERGE_SIDE1]);
	DIFF_QUEUE_CLEAR(&renames->pairs[MERGE_SIDE2]);

> +	if (combined.nr) {
> +		int i;
> +		for (i = 0; i < combined.nr; i++)
> +			diff_free_filepair(combined.queue[i]);
> +		free(combined.queue);
> +	}
>  
> -	/*
> -	 * Rename detection works by detecting file similarity.  Here we use
> -	 * a really easy-to-implement scheme: files are similar IFF they have
> -	 * the same filename.  Therefore, by this scheme, there are no renames.
> -	 *
> -	 * TODO: Actually implement a real rename detection scheme.
> -	 */
>  	return clean;

I notice that this change causes detect_and_process_renames() to
change from an "unhelpful result, but success" to "die() always".

I wonder if there is value in swapping the order of the patches
to implement the static methods first. Of course, you hit the
"unreferenced static method" problem, so maybe your strategy is
better after all.

Thanks,
-Stolee