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| -rw-r--r-- | Documentation/technical/pack-format.txt | 111 |
1 files changed, 111 insertions, 0 deletions
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt new file mode 100644 index 000000000..ed2decc10 --- /dev/null +++ b/Documentation/technical/pack-format.txt @@ -0,0 +1,111 @@ +GIT pack format +=============== + += pack-*.pack file has the following format: + + - The header appears at the beginning and consists of the following: + + 4-byte signature + 4-byte version number (network byte order) + 4-byte number of objects contained in the pack (network byte order) + + Observation: we cannot have more than 4G versions ;-) and + more than 4G objects in a pack. + + - The header is followed by number of object entries, each of + which looks like this: + + (undeltified representation) + n-byte type and length (4-bit type, (n-1)*7+4-bit length) + compressed data + + (deltified representation) + n-byte type and length (4-bit type, (n-1)*7+4-bit length) + 20-byte base object name + compressed delta data + + Observation: length of each object is encoded in a variable + length format and is not constrained to 32-bit or anything. + + - The trailer records 20-byte SHA1 checksum of all of the above. + += pack-*.idx file has the following format: + + - The header consists of 256 4-byte network byte order + integers. N-th entry of this table records the number of + objects in the corresponding pack, the first byte of whose + object name are smaller than N. This is called the + 'first-level fan-out' table. + + Observation: we would need to extend this to an array of + 8-byte integers to go beyond 4G objects per pack, but it is + not strictly necessary. + + - The header is followed by sorted 28-byte entries, one entry + per object in the pack. Each entry is: + + 4-byte network byte order integer, recording where the + object is stored in the packfile as the offset from the + beginning. + + 20-byte object name. + + Observation: we would definitely need to extend this to + 8-byte integer plus 20-byte object name to handle a packfile + that is larger than 4GB. + + - The file is concluded with a trailer: + + A copy of the 20-byte SHA1 checksum at the end of + corresponding packfile. + + 20-byte SHA1-checksum of all of the above. + +Pack Idx file: + + idx + +--------------------------------+ + | fanout[0] = 2 |-. + +--------------------------------+ | + | fanout[1] | | + +--------------------------------+ | + | fanout[2] | | + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | + | fanout[255] | | + +--------------------------------+ | +main | offset | | +index | object name 00XXXXXXXXXXXXXXXX | | +table +--------------------------------+ | + | offset | | + | object name 00XXXXXXXXXXXXXXXX | | + +--------------------------------+ | + .-| offset |<+ + | | object name 01XXXXXXXXXXXXXXXX | + | +--------------------------------+ + | | offset | + | | object name 01XXXXXXXXXXXXXXXX | + | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + | | offset | + | | object name FFXXXXXXXXXXXXXXXX | + | +--------------------------------+ +trailer | | packfile checksum | + | +--------------------------------+ + | | idxfile checksum | + | +--------------------------------+ + .-------. + | +Pack file entry: <+ + + packed object header: + 1-byte type (upper 4-bit) + size0 (lower 4-bit) + n-byte sizeN (as long as MSB is set, each 7-bit) + size0..sizeN form 4+7+7+..+7 bit integer, size0 + is the most significant part. + packed object data: + If it is not DELTA, then deflated bytes (the size above + is the size before compression). + If it is DELTA, then + 20-byte base object name SHA1 (the size above is the + size of the delta data that follows). + delta data, deflated. |