Re: [PATCH v2 07/20] merge-ort: avoid repeating fill_tree_descriptor() on the same tree

[Derrick Stolee <stolee@gmail.com>]

On 11/2/2020 3:43 PM, Elijah Newren wrote:
> @@ -99,6 +99,15 @@ static int collect_merge_info_callback(int n,
>  	unsigned mbase_null = !(mask & 1);
>  	unsigned side1_null = !(mask & 2);
>  	unsigned side2_null = !(mask & 4);
> +	unsigned side1_matches_mbase = (!side1_null && !mbase_null &&
> +					names[0].mode == names[1].mode &&
> +					oideq(&names[0].oid, &names[1].oid));
> +	unsigned side2_matches_mbase = (!side2_null && !mbase_null &&
> +					names[0].mode == names[2].mode &&
> +					oideq(&names[0].oid, &names[2].oid));
> +	unsigned sides_match = (!side1_null && !side2_null &&
> +				names[1].mode == names[2].mode &&
> +				oideq(&names[1].oid, &names[2].oid));

If the *_null values were in an array, instead, then all of these
lines could be grouped as a macro:

	unsigned null_oid[3] = {
		!(mask & 1),
		!(mask & 2),
		!(mask & 4)
	};

	#define trivial_merge(i,j) (!null_oid[i] && !null_oid[j] && \
				    names[i].mode == names[j].mode && \
				    oideq(&names[i].oid, &names[j].oid))

	unsigned side1_matches_mbase = trivial_merge(0, 1);
	unsigned side2_matches_mbase = trivial_merge(0, 2);
	unsigned sides_match = trivial_merge(1, 2);

I briefly considered making these last three an array, as well,
except the loop below doesn't use 'i' in a symmetrical way:

> +			if (i == 1 && side1_matches_mbase)
> +				t[1] = t[0];
> +			else if (i == 2 && side2_matches_mbase)
> +				t[2] = t[0];
> +			else if (i == 2 && sides_match)
> +				t[2] = t[1];

Since the 'i == 2' case has two possible options, it wouldn't be
possible to just have 'side_matches[i]' here.

> +			else {
> +				const struct object_id *oid = NULL;
> +				if (dirmask & 1)
> +					oid = &names[i].oid;
> +				buf[i] = fill_tree_descriptor(opt->repo,
> +							      t + i, oid);
> +			}

I do appreciate the reduced recursion here!

Thanks,
-Stolee