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Commit | Line | Data |
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9760662f JH |
1 | GIT pack format |
2 | =============== | |
3 | ||
4 | = pack-*.pack file has the following format: | |
5 | ||
6 | - The header appears at the beginning and consists of the following: | |
7 | ||
1361fa3e SP |
8 | 4-byte signature: |
9 | The signature is: {'P', 'A', 'C', 'K'} | |
10 | ||
11 | 4-byte version number (network byte order): | |
12 | GIT currently accepts version number 2 or 3 but | |
13 | generates version 2 only. | |
14 | ||
9760662f JH |
15 | 4-byte number of objects contained in the pack (network byte order) |
16 | ||
17 | Observation: we cannot have more than 4G versions ;-) and | |
18 | more than 4G objects in a pack. | |
19 | ||
20 | - The header is followed by number of object entries, each of | |
21 | which looks like this: | |
22 | ||
23 | (undeltified representation) | |
979ea585 | 24 | n-byte type and length (3-bit type, (n-1)*7+4-bit length) |
9760662f JH |
25 | compressed data |
26 | ||
27 | (deltified representation) | |
979ea585 | 28 | n-byte type and length (3-bit type, (n-1)*7+4-bit length) |
9760662f JH |
29 | 20-byte base object name |
30 | compressed delta data | |
31 | ||
32 | Observation: length of each object is encoded in a variable | |
33 | length format and is not constrained to 32-bit or anything. | |
34 | ||
35 | - The trailer records 20-byte SHA1 checksum of all of the above. | |
36 | ||
37 | = pack-*.idx file has the following format: | |
38 | ||
39 | - The header consists of 256 4-byte network byte order | |
40 | integers. N-th entry of this table records the number of | |
41 | objects in the corresponding pack, the first byte of whose | |
42 | object name are smaller than N. This is called the | |
43 | 'first-level fan-out' table. | |
44 | ||
45 | Observation: we would need to extend this to an array of | |
46 | 8-byte integers to go beyond 4G objects per pack, but it is | |
47 | not strictly necessary. | |
48 | ||
1361fa3e | 49 | - The header is followed by sorted 24-byte entries, one entry |
9760662f JH |
50 | per object in the pack. Each entry is: |
51 | ||
52 | 4-byte network byte order integer, recording where the | |
53 | object is stored in the packfile as the offset from the | |
54 | beginning. | |
55 | ||
56 | 20-byte object name. | |
57 | ||
58 | Observation: we would definitely need to extend this to | |
59 | 8-byte integer plus 20-byte object name to handle a packfile | |
60 | that is larger than 4GB. | |
61 | ||
62 | - The file is concluded with a trailer: | |
63 | ||
64 | A copy of the 20-byte SHA1 checksum at the end of | |
65 | corresponding packfile. | |
66 | ||
67 | 20-byte SHA1-checksum of all of the above. | |
68 | ||
69 | Pack Idx file: | |
70 | ||
71 | idx | |
72 | +--------------------------------+ | |
73 | | fanout[0] = 2 |-. | |
74 | +--------------------------------+ | | |
75 | | fanout[1] | | | |
76 | +--------------------------------+ | | |
77 | | fanout[2] | | | |
78 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | | |
79 | | fanout[255] | | | |
80 | +--------------------------------+ | | |
81 | main | offset | | | |
82 | index | object name 00XXXXXXXXXXXXXXXX | | | |
a6080a0a | 83 | table +--------------------------------+ | |
9760662f JH |
84 | | offset | | |
85 | | object name 00XXXXXXXXXXXXXXXX | | | |
86 | +--------------------------------+ | | |
87 | .-| offset |<+ | |
88 | | | object name 01XXXXXXXXXXXXXXXX | | |
89 | | +--------------------------------+ | |
90 | | | offset | | |
91 | | | object name 01XXXXXXXXXXXXXXXX | | |
92 | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
93 | | | offset | | |
94 | | | object name FFXXXXXXXXXXXXXXXX | | |
95 | | +--------------------------------+ | |
96 | trailer | | packfile checksum | | |
97 | | +--------------------------------+ | |
98 | | | idxfile checksum | | |
99 | | +--------------------------------+ | |
a6080a0a | 100 | .-------. |
9760662f JH |
101 | | |
102 | Pack file entry: <+ | |
103 | ||
104 | packed object header: | |
979ea585 PE |
105 | 1-byte size extension bit (MSB) |
106 | type (next 3 bit) | |
a6080a0a | 107 | size0 (lower 4-bit) |
9760662f JH |
108 | n-byte sizeN (as long as MSB is set, each 7-bit) |
109 | size0..sizeN form 4+7+7+..+7 bit integer, size0 | |
979ea585 PE |
110 | is the least significant part, and sizeN is the |
111 | most significant part. | |
9760662f JH |
112 | packed object data: |
113 | If it is not DELTA, then deflated bytes (the size above | |
114 | is the size before compression). | |
115 | If it is DELTA, then | |
116 | 20-byte base object name SHA1 (the size above is the | |
a6080a0a | 117 | size of the delta data that follows). |
9760662f | 118 | delta data, deflated. |