Re: [PATCH 09/11] pack-bitmap: enable cross-pack delta reuse

> In order to do this, the pack-reuse code within pack-bitmap.c marks
> bits in a separate bitmap called 'reuse_as_ref_delta'. Objects whose
> bits are marked in that bitmap must be converted from OFS_DELTAs to
> REF_DELTAs.
> 
> To mark bits in that bitmap, we adjust find_base_bitmap_pos() to
> return the bitmap position of any delta object's base regardless of
> whether or not it came from the same pack. This is done by:
> 
>   1. First converting the base object's into a pack position (via
>      `offset_to_pack_pos()`).
> 
>   2. Then converting from pack position into into lexical order (via
>      `pack_pos_to_index()`).
> 
>   3. Then into an object ID (via `nth_packed_object_id()`).
> 
>   4. Then into a position in the MIDX's lexical order of object IDs
>      (via `bsearch_midx()`).
> 
>   5. And finally into a position in the MIDX's pseudo-pack order (via
>      `midx_pair_to_pack_pos()`).
> 
> If we can find that base object in the MIDX, then we use its position
> in the MIDX's pseudo-pack order to determine whether or not it was
> selected from the same pack. If it is, no adjustment is necessary.
> Otherwise, we mark the delta object's position in the new
> `reuse_as_ref_delta` bitmap, and convert accordingly from within
> `write_reused_pack_one()`.

> @@ -1182,10 +1188,24 @@ static size_t write_reused_pack_verbatim(struct bitmapped_pack *reuse_packfile,
>  	if (pos >= end)
>  		return reuse_packfile->bitmap_pos / BITS_IN_EWORD;
>  
> -	while (pos < end &&
> -	       reuse_packfile_bitmap->words[pos / BITS_IN_EWORD] == (eword_t)~0)
> +	while (pos < end) {
> +		size_t wpos = pos / BITS_IN_EWORD;
> +		eword_t reuse;
> +
> +		reuse = reuse_packfile_bitmap->words[wpos];
> +		if (reuse_as_ref_delta_packfile_bitmap) {
> +			/*
> +			 * Can't reuse verbatim any objects which need
> +			 * to be first rewritten as REF_DELTAs.
> +			 */
> +			reuse &= ~reuse_as_ref_delta_packfile_bitmap->words[wpos];
> +		}
> +
> +		if (reuse != (eword_t)~0)
> +			break;
> +
>  		pos += BITS_IN_EWORD;
> -
> +	}