From: Patrick Steinhardt <ps@pks.im>
Date: Wed, 12 Nov 2025 11:25:49 GMT
Subject: Re: [PATCH 2/9] pack-bitmap: handle name-hash lookups in incremental bitmaps
Message-ID: <aRRuvQmlDMBiZolK@pks.im>
In-Reply-To: <20251112080151.GB979063@coredump.intra.peff.net>

On Wed, Nov 12, 2025 at 03:01:51AM -0500, Jeff King wrote:
> diff --git a/pack-bitmap.c b/pack-bitmap.c
> index 291e1a9cf4..710b86a451 100644
> --- a/pack-bitmap.c
> +++ b/pack-bitmap.c
> @@ -213,6 +213,26 @@ static uint32_t bitmap_num_objects(struct bitmap_index *index)
>  	return index->pack->num_objects;
>  }
>  
> +static uint32_t bitmap_name_hash(struct bitmap_index *index, uint32_t pos)
> +{
> +	if (bitmap_is_midx(index)) {
> +		while (index && pos < index->midx->num_objects_in_base)
> +			index = index->base;

So we first find the MIDX that is supposed to contain the position.

> +		if (!index)
> +			BUG("NULL base bitmap for object position: %"PRIu32, pos);
> +
> +		pos -= index->midx->num_objects_in_base;

We then subtract the number of objects from all of our predeceding
layers from the position. This should result in the position relative to
the current layer.

> +		if (pos >= index->midx->num_objects)
> +			BUG("out-of-bounds midx bitmap object at %"PRIu32, pos);
> +	}
> +
> +	if (!index->hashes)
> +		return 0;
> +
> +	return get_be32(index->hashes + pos);

And we then return the value at that given position. Makes sense to me.

Patrick