Re: [PATCH 15/16] commit-reach: make can_all_from_reach... linear

[Derrick Stolee <stolee@gmail.com>]

On 10/14/2018 10:29 AM, René Scharfe wrote:
> It still has some repetition, converted code is a bit longer than the
> current one, and I don't know how to build a Coccinelle rule that would
> do that conversion.
>
> Looked for a possibility to at least leave QSORT call-sites alone by
> enhancing that macro, but didn't find any.  Found a few websites
> showing off mindblowing macros, thouhg, this one in particular:
>
> https://github.com/pfultz2/Cloak/wiki/C-Preprocessor-tricks,-tips,-and-idioms
>
> Anyway, drove the generative approach a bit further, and came up with
> the new DEFINE_SORT below.  I'm unsure about the name; perhaps it should
> be called DEFINE_SORT_BY_COMPARE_FUNCTION_BODY, but that's a bit long.
> It handles casts and const attributes behind the scenes and avoids
> repetition, but looks a bit weird, as it is placed where a function
> signature would go.
>
> Apart from that the macro is simple and doesn't use any tricks or
> added checks.  It just sets up boilerplate functions to offer type-safe
> sorting.
>
> diffcore-rename.c and refs/packed-backend.c receive special treatment in
> the patch because their compare functions are used outside of sorting as
> well.  I made them take typed pointers nevertheless and used them from
> DEFINE_SORT; the wrapper generated by that macro is supposed to be
> private.  Given that such reuse is rare and I think we don't need a way
> to make it public.
>
> What do y'all think about this direction?
>
> ---
[snip]
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 5f2e90932f..491230fc57 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -1066,6 +1066,21 @@ static inline void sane_qsort(void *base, size_t nmemb, size_t size,
>   		qsort(base, nmemb, size, compar);
>   }
>   
> +#define DECLARE_SORT(scope, name, elemtype) \
> +scope void name(elemtype, size_t)
> +
> +#define DEFINE_SORT(scope, name, elemtype, one, two)			\
> +static int name##_compare(const elemtype, const elemtype);		\
> +static int name##_compare_void(const void *a, const void *b)		\
> +{									\
> +	return name##_compare(a, b);					\
> +}									\
> +scope void name(elemtype base, size_t nmemb)				\
> +{									\
> +	QSORT(base, nmemb, name##_compare_void);			\
> +}									\
> +static int name##_compare(const elemtype one, const elemtype two)
> +

Since you were worried about the "private" name of the compare function, 
maybe split this macro into two: DEFINE_COMPARE and DEFINE_SORT. Then, 
if someone wants direct access to the compare function, they could use 
the DEFINE_COMPARE to ensure the typing is correct, and use QSORT as 
normal with name##_compare_void.

As I think about this, I think this is less of a problem than is worth 
this split. The commit-slab definitions generate a lot of methods using 
the "name##" convention, so perhaps we should just trust developers 
using the macros to look up the macro definition or similar examples. In 
that sense, including a conversion that consumes the compare function 
directly can be a signpost for future callers.

I would say that maybe the times where you need to do something special 
should be pulled out into their own patches, so we can call attention to 
them directly.

[snip]
> diff --git a/midx.c b/midx.c
> index 713d6f9dde..4407db7949 100644
> --- a/midx.c
> +++ b/midx.c
> @@ -419,10 +419,8 @@ struct pack_pair {
>   	char *pack_name;
>   };
>   
> -static int pack_pair_compare(const void *_a, const void *_b)
> +DEFINE_SORT(static, sort_by_pack_name, struct pack_pair *, a, b)
>   {
> -	struct pack_pair *a = (struct pack_pair *)_a;
> -	struct pack_pair *b = (struct pack_pair *)_b;
>   	return strcmp(a->pack_name, b->pack_name);
>   }
>   
> @@ -438,7 +436,7 @@ static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *p
>   		pairs[i].pack_name = pack_names[i];
>   	}
>   
> -	QSORT(pairs, nr_packs, pack_pair_compare);
> +	sort_by_pack_name(pairs, nr_packs);

I like this "sort_by_" convention..

>   
>   	for (i = 0; i < nr_packs; i++) {
>   		pack_names[i] = pairs[i].pack_name;
> @@ -455,10 +453,8 @@ struct pack_midx_entry {
>   	uint64_t offset;
>   };
>   
> -static int midx_oid_compare(const void *_a, const void *_b)
> +DEFINE_SORT(static, sort_midx, struct pack_midx_entry *, a, b)
>   {
> -	const struct pack_midx_entry *a = (const struct pack_midx_entry *)_a;
> -	const struct pack_midx_entry *b = (const struct pack_midx_entry *)_b;
>   	int cmp = oidcmp(&a->oid, &b->oid);
>   
>   	if (cmp)
> @@ -573,7 +569,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
>   			}
>   		}
>   
> -		QSORT(entries_by_fanout, nr_fanout, midx_oid_compare);
> +		sort_midx(entries_by_fanout, nr_fanout);

...but it isn't followed here. Perhaps "sort_by_oid"?

Thanks,
-Stolee