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f894081dea
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
127 lines
4.3 KiB
C
127 lines
4.3 KiB
C
#ifndef PACK_REVINDEX_H
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#define PACK_REVINDEX_H
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/**
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* A revindex allows converting efficiently between three properties
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* of an object within a pack:
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*
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* - index position: the numeric position within the list of sorted object ids
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* found in the .idx file
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*
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* - pack position: the numeric position within the list of objects in their
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* order within the actual .pack file (i.e., 0 is the first object in the
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* .pack, 1 is the second, and so on)
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*
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* - offset: the byte offset within the .pack file at which the object contents
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* can be found
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*
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* The revindex can also be used with a multi-pack index (MIDX). In this
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* setting:
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*
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* - index position refers to an object's numeric position within the MIDX
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*
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* - pack position refers to an object's position within a non-existent pack
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* described by the MIDX. The pack structure is described in
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* Documentation/technical/pack-format.txt.
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*
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* It is effectively a concatanation of all packs in the MIDX (ordered by
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* their numeric ID within the MIDX) in their original order within each
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* pack), removing duplicates, and placing the preferred pack (if any)
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* first.
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*/
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#define RIDX_SIGNATURE 0x52494458 /* "RIDX" */
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#define RIDX_VERSION 1
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#define GIT_TEST_WRITE_REV_INDEX "GIT_TEST_WRITE_REV_INDEX"
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#define GIT_TEST_REV_INDEX_DIE_IN_MEMORY "GIT_TEST_REV_INDEX_DIE_IN_MEMORY"
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struct packed_git;
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struct multi_pack_index;
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/*
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* load_pack_revindex populates the revindex's internal data-structures for the
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* given pack, returning zero on success and a negative value otherwise.
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*
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* If a '.rev' file is present it is mmap'd, and pointers are assigned into it
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* (instead of using the in-memory variant).
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*/
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int load_pack_revindex(struct packed_git *p);
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/*
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* load_midx_revindex loads the '.rev' file corresponding to the given
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* multi-pack index by mmap-ing it and assigning pointers in the
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* multi_pack_index to point at it.
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*
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* A negative number is returned on error.
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*/
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int load_midx_revindex(struct multi_pack_index *m);
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/*
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* Frees resources associated with a multi-pack reverse index.
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*
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* A negative number is returned on error.
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*/
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int close_midx_revindex(struct multi_pack_index *m);
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/*
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* offset_to_pack_pos converts an object offset to a pack position. This
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* function returns zero on success, and a negative number otherwise. The
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* parameter 'pos' is usable only on success.
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*
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* If the reverse index has not yet been loaded, this function loads it lazily,
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* and returns an negative number if an error was encountered.
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*
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* This function runs in time O(log N) with the number of objects in the pack.
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*/
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int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos);
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/*
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* pack_pos_to_index converts the given pack-relative position 'pos' by
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* returning an index-relative position.
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*
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* If the reverse index has not yet been loaded, or the position is out of
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* bounds, this function aborts.
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*
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* This function runs in constant time.
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*/
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uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos);
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/*
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* pack_pos_to_offset converts the given pack-relative position 'pos' into a
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* pack offset. For a pack with 'N' objects, asking for position 'N' will return
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* the total size (in bytes) of the pack.
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*
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* If the reverse index has not yet been loaded, or the position is out of
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* bounds, this function aborts.
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*
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* This function runs in constant time under both in-memory and on-disk reverse
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* indexes, but an additional step is taken to consult the corresponding .idx
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* file when using the on-disk format.
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*/
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off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos);
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/*
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* pack_pos_to_midx converts the object at position "pos" within the MIDX
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* pseudo-pack into a MIDX position.
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*
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* If the reverse index has not yet been loaded, or the position is out of
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* bounds, this function aborts.
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*
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* This function runs in time O(log N) with the number of objects in the MIDX.
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*/
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uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos);
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/*
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* midx_to_pack_pos converts from the MIDX-relative position at "at" to the
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* corresponding pack position.
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*
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* If the reverse index has not yet been loaded, or the position is out of
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* bounds, this function aborts.
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*
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* This function runs in constant time.
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*/
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int midx_to_pack_pos(struct multi_pack_index *midx, uint32_t at, uint32_t *pos);
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#endif
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