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1 GIT bitmap v1 format
2 ====================
3
4 == Pack and multi-pack bitmaps
5
6 Bitmaps store reachability information about the set of objects in a packfile,
7 or a multi-pack index (MIDX). The former is defined obviously, and the latter is
8 defined as the union of objects in packs contained in the MIDX.
9
10 A bitmap may belong to either one pack, or the repository's multi-pack index (if
11 it exists). A repository may have at most one bitmap.
12
13 An object is uniquely described by its bit position within a bitmap:
14
15 - If the bitmap belongs to a packfile, the __n__th bit corresponds to
16 the __n__th object in pack order. For a function `offset` which maps
17 objects to their byte offset within a pack, pack order is defined as
18 follows:
19
20 o1 <= o2 <==> offset(o1) <= offset(o2)
21
22 - If the bitmap belongs to a MIDX, the __n__th bit corresponds to the
23 __n__th object in MIDX order. With an additional function `pack` which
24 maps objects to the pack they were selected from by the MIDX, MIDX order
25 is defined as follows:
26
27 o1 <= o2 <==> pack(o1) <= pack(o2) /\ offset(o1) <= offset(o2)
28 +
29 The ordering between packs is done according to the MIDX's .rev file.
30 Notably, the preferred pack sorts ahead of all other packs.
31
32 The on-disk representation (described below) of a bitmap is the same regardless
33 of whether or not that bitmap belongs to a packfile or a MIDX. The only
34 difference is the interpretation of the bits, which is described above.
35
36 Certain bitmap extensions are supported (see: Appendix B). No extensions are
37 required for bitmaps corresponding to packfiles. For bitmaps that correspond to
38 MIDXs, both the bit-cache and rev-cache extensions are required.
39
40 == On-disk format
41
42 * A header appears at the beginning:
43
44 4-byte signature: :: {'B', 'I', 'T', 'M'}
45
46 2-byte version number (network byte order): ::
47
48 The current implementation only supports version 1
49 of the bitmap index (the same one as JGit).
50
51 2-byte flags (network byte order): ::
52
53 The following flags are supported:
54
55 ** {empty}
56 BITMAP_OPT_FULL_DAG (0x1) REQUIRED: :::
57
58 This flag must always be present. It implies that the
59 bitmap index has been generated for a packfile or
60 multi-pack index (MIDX) with full closure (i.e. where
61 every single object in the packfile/MIDX can find its
62 parent links inside the same packfile/MIDX). This is a
63 requirement for the bitmap index format, also present in
64 JGit, that greatly reduces the complexity of the
65 implementation.
66
67 ** {empty}
68 BITMAP_OPT_HASH_CACHE (0x4): :::
69
70 If present, the end of the bitmap file contains
71 `N` 32-bit name-hash values, one per object in the
72 pack/MIDX. The format and meaning of the name-hash is
73 described below.
74
75 ** {empty}
76 BITMAP_OPT_LOOKUP_TABLE (0x10): :::
77 If present, the end of the bitmap file contains a table
78 containing a list of `N` <commit_pos, offset, xor_row>
79 triplets. The format and meaning of the table is described
80 below.
81 +
82 NOTE: Unlike the xor_offset used to compress an individual bitmap,
83 `xor_row` stores an *absolute* index into the lookup table, not a location
84 relative to the current entry.
85
86 4-byte entry count (network byte order): ::
87 The total count of entries (bitmapped commits) in this bitmap index.
88
89 20-byte checksum: ::
90 The SHA1 checksum of the pack/MIDX this bitmap index
91 belongs to.
92
93 * 4 EWAH bitmaps that act as type indexes
94 +
95 Type indexes are serialized after the hash cache in the shape
96 of four EWAH bitmaps stored consecutively (see Appendix A for
97 the serialization format of an EWAH bitmap).
98 +
99 There is a bitmap for each Git object type, stored in the following
100 order:
101 +
102 - Commits
103 - Trees
104 - Blobs
105 - Tags
106
107 +
108 In each bitmap, the `n`th bit is set to true if the `n`th object
109 in the packfile or multi-pack index is of that type.
110 +
111 The obvious consequence is that the OR of all 4 bitmaps will result
112 in a full set (all bits set), and the AND of all 4 bitmaps will
113 result in an empty bitmap (no bits set).
114
115 * N entries with compressed bitmaps, one for each indexed commit
116 +
117 Where `N` is the total amount of entries in this bitmap index.
118 Each entry contains the following:
119
120 ** {empty}
121 4-byte object position (network byte order): ::
122 The position **in the index for the packfile or
123 multi-pack index** where the bitmap for this commit is
124 found.
125
126 ** {empty}
127 1-byte XOR-offset: ::
128 The xor offset used to compress this bitmap. For an entry
129 in position `x`, a XOR offset of `y` means that the actual
130 bitmap representing this commit is composed by XORing the
131 bitmap for this entry with the bitmap in entry `x-y` (i.e.
132 the bitmap `y` entries before this one).
133 +
134 NOTE: This compression can be recursive. In order to
135 XOR this entry with a previous one, the previous entry needs
136 to be decompressed first, and so on.
137 +
138 The hard-limit for this offset is 160 (an entry can only be
139 xor'ed against one of the 160 entries preceding it). This
140 number is always positive, and hence entries are always xor'ed
141 with **previous** bitmaps, not bitmaps that will come afterwards
142 in the index.
143
144 ** {empty}
145 1-byte flags for this bitmap: ::
146 At the moment the only available flag is `0x1`, which hints
147 that this bitmap can be re-used when rebuilding bitmap indexes
148 for the repository.
149
150 ** The compressed bitmap itself, see Appendix A.
151
152 * {empty}
153 TRAILER: ::
154 Trailing checksum of the preceding contents.
155
156 == Appendix A: Serialization format for an EWAH bitmap
157
158 Ewah bitmaps are serialized in the same protocol as the JAVAEWAH
159 library, making them backwards compatible with the JGit
160 implementation:
161
162 - 4-byte number of bits of the resulting UNCOMPRESSED bitmap
163
164 - 4-byte number of words of the COMPRESSED bitmap, when stored
165
166 - N x 8-byte words, as specified by the previous field
167 +
168 This is the actual content of the compressed bitmap.
169
170 - 4-byte position of the current RLW for the compressed
171 bitmap
172
173 All words are stored in network byte order for their corresponding
174 sizes.
175
176 The compressed bitmap is stored in a form of run-length encoding, as
177 follows. It consists of a concatenation of an arbitrary number of
178 chunks. Each chunk consists of one or more 64-bit words
179
180 H L_1 L_2 L_3 .... L_M
181
182 H is called RLW (run length word). It consists of (from lower to higher
183 order bits):
184
185 - 1 bit: the repeated bit B
186
187 - 32 bits: repetition count K (unsigned)
188
189 - 31 bits: literal word count M (unsigned)
190
191 The bitstream represented by the above chunk is then:
192
193 - K repetitions of B
194
195 - The bits stored in `L_1` through `L_M`. Within a word, bits at
196 lower order come earlier in the stream than those at higher
197 order.
198
199 The next word after `L_M` (if any) must again be a RLW, for the next
200 chunk. For efficient appending to the bitstream, the EWAH stores a
201 pointer to the last RLW in the stream.
202
203
204 == Appendix B: Optional Bitmap Sections
205
206 These sections may or may not be present in the `.bitmap` file; their
207 presence is indicated by the header flags section described above.
208
209 Name-hash cache
210 ---------------
211
212 If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
213 a cache of 32-bit values, one per object in the pack/MIDX. The value at
214 position `i` is the hash of the pathname at which the `i`th object
215 (counting in index or multi-pack index order) in the pack/MIDX can be found.
216 This can be fed into the delta heuristics to compare objects with similar
217 pathnames.
218
219 The hash algorithm used is:
220
221 hash = 0;
222 while ((c = *name++))
223 if (!isspace(c))
224 hash = (hash >> 2) + (c << 24);
225
226 Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
227 If implementations want to choose a different hashing scheme, they are
228 free to do so, but MUST allocate a new header flag (because comparing
229 hashes made under two different schemes would be pointless).
230
231 Commit lookup table
232 -------------------
233
234 If the BITMAP_OPT_LOOKUP_TABLE flag is set, the last `N * (4 + 8 + 4)`
235 bytes (preceding the name-hash cache and trailing hash) of the `.bitmap`
236 file contains a lookup table specifying the information needed to get
237 the desired bitmap from the entries without parsing previous unnecessary
238 bitmaps.
239
240 For a `.bitmap` containing `nr_entries` reachability bitmaps, the table
241 contains a list of `nr_entries` <commit_pos, offset, xor_row> triplets
242 (sorted in the ascending order of `commit_pos`). The content of i'th
243 triplet is -
244
245 * {empty}
246 commit_pos (4 byte integer, network byte order): ::
247 It stores the object position of a commit (in the midx or pack
248 index).
249
250 * {empty}
251 offset (8 byte integer, network byte order): ::
252 The offset from which that commit's bitmap can be read.
253
254 * {empty}
255 xor_row (4 byte integer, network byte order): ::
256 The position of the triplet whose bitmap is used to compress
257 this one, or `0xffffffff` if no such bitmap exists.