path: root/Documentation/technical/pack-format.txt
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@@ -252,3 +252,80 @@ Pack file entry: <+
corresponding packfile.
20-byte SHA-1-checksum of all of the above.
+== multi-pack-index (MIDX) files have the following format:
+The multi-pack-index files refer to multiple pack-files and loose objects.
+In order to allow extensions that add extra data to the MIDX, we organize
+the body into "chunks" and provide a lookup table at the beginning of the
+body. The header includes certain length values, such as the number of packs,
+the number of base MIDX files, hash lengths and types.
+All 4-byte numbers are in network order.
+ 4-byte signature:
+ The signature is: {'M', 'I', 'D', 'X'}
+ 1-byte version number:
+ Git only writes or recognizes version 1.
+ 1-byte Object Id Version
+ Git only writes or recognizes version 1 (SHA1).
+ 1-byte number of "chunks"
+ 1-byte number of base multi-pack-index files:
+ This value is currently always zero.
+ 4-byte number of pack files
+ (C + 1) * 12 bytes providing the chunk offsets:
+ First 4 bytes describe chunk id. Value 0 is a terminating label.
+ Other 8 bytes provide offset in current file for chunk to start.
+ (Chunks are provided in file-order, so you can infer the length
+ using the next chunk position if necessary.)
+ The remaining data in the body is described one chunk at a time, and
+ these chunks may be given in any order. Chunks are required unless
+ otherwise specified.
+ Packfile Names (ID: {'P', 'N', 'A', 'M'})
+ Stores the packfile names as concatenated, null-terminated strings.
+ Packfiles must be listed in lexicographic order for fast lookups by
+ name. This is the only chunk not guaranteed to be a multiple of four
+ bytes in length, so should be the last chunk for alignment reasons.
+ OID Fanout (ID: {'O', 'I', 'D', 'F'})
+ The ith entry, F[i], stores the number of OIDs with first
+ byte at most i. Thus F[255] stores the total
+ number of objects.
+ OID Lookup (ID: {'O', 'I', 'D', 'L'})
+ The OIDs for all objects in the MIDX are stored in lexicographic
+ order in this chunk.
+ Object Offsets (ID: {'O', 'O', 'F', 'F'})
+ Stores two 4-byte values for every object.
+ 1: The pack-int-id for the pack storing this object.
+ 2: The offset within the pack.
+ If all offsets are less than 2^31, then the large offset chunk
+ will not exist and offsets are stored as in IDX v1.
+ If there is at least one offset value larger than 2^32-1, then
+ the large offset chunk must exist. If the large offset chunk
+ exists and the 31st bit is on, then removing that bit reveals
+ the row in the large offsets containing the 8-byte offset of
+ this object.
+ [Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
+ 8-byte offsets into large packfiles.
+ 20-byte SHA1-checksum of the above contents.