Re: [RFC PATCH 02/14] strvec: introduce `strvec_init_alloc()`

[Junio C Hamano <gitster@pobox.com>]

Taylor Blau <me@ttaylorr.com> writes:

> When the caller knows upfront how many elements will be pushed onto a
> `strvec`, it is useful to pre-allocate enough space in the array to fit
> that many elements (and one additional slot to store NULL, indicating
> the end of the list.)
>
> Introduce `strvec_init_alloc()`, which allocates the backing array large
> enough to hold `alloc` elements and the termination marker without
> further reallocation.
>
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  strvec.c | 7 +++++++
>  strvec.h | 5 +++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/strvec.c b/strvec.c
> index f8de79f5579..f7f32a53b56 100644
> --- a/strvec.c
> +++ b/strvec.c
> @@ -10,6 +10,13 @@ void strvec_init(struct strvec *array)
>  	memcpy(array, &blank, sizeof(*array));
>  }
>  
> +void strvec_init_alloc(struct strvec *array, size_t alloc)
> +{
> +	CALLOC_ARRAY(array->v, st_add(alloc, 1));
> +	array->nr = 0;
> +	array->alloc = alloc + 1;
> +}

It is not satisifying that strvec_init() does *not* become a thin
wrapper around this that says "my initial allocation is for zero
elements", but that cannot be done easily as a strvec that begins as
an empty one has a small optimization to avoid one-slot allocation
only to store NULL.  So, ... OK.