1 files changed, 160 insertions, 0 deletions
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
new file mode 100644
index 0000000000..1803e64e46
--- /dev/null
+++ b/Documentation/technical/pack-format.txt
@@ -0,0 +1,160 @@
+GIT pack format
+===============
+
+= pack-*.pack files have the following format:
+
+   - A header appears at the beginning and consists of the following:
+
+     4-byte signature:
+         The signature is: {'P', 'A', 'C', 'K'}
+
+     4-byte version number (network byte order):
+         GIT currently accepts version number 2 or 3 but
+         generates version 2 only.
+
+     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 (3-bit type, (n-1)*7+4-bit length)
+     compressed data
+
+     (deltified representation)
+     n-byte type and length (3-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.
+
+= Original (version 1) pack-*.idx files have 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 is less than or equal to N.  This is called the
+    'first-level fan-out' table.
+
+  - The header is followed by sorted 24-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.
+
+  - 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:
+
+	--  +--------------------------------+
+fanout	    | fanout[0] = 2 (for example)    |-.
+table	    +--------------------------------+ |
+	    | fanout[1]                      | |
+	    +--------------------------------+ |
+	    | fanout[2]                      | |
+	    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
+	    | fanout[255] = total objects    |---.
+	--  +--------------------------------+ | |
+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 size extension bit (MSB)
+	       type (next 3 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 least significant part, and sizeN 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 REF_DELTA, then
+	  20-byte base object name SHA1 (the size above is the
+		size of the delta data that follows).
+          delta data, deflated.
+	If it is OFS_DELTA, then
+	  n-byte offset (see below) interpreted as a negative
+		offset from the type-byte of the header of the
+		ofs-delta entry (the size above is the size of
+		the delta data that follows).
+	  delta data, deflated.
+
+     offset encoding:
+	  n bytes with MSB set in all but the last one.
+	  The offset is then the number constructed by
+	  concatenating the lower 7 bit of each byte, and
+	  for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
+	  to the result.
+
+
+
+= Version 2 pack-*.idx files support packs larger than 4 GiB, and
+  have some other reorganizations.  They have the format:
+
+  - A 4-byte magic number '\377tOc' which is an unreasonable
+    fanout[0] value.
+
+  - A 4-byte version number (= 2)
+
+  - A 256-entry fan-out table just like v1.
+
+  - A table of sorted 20-byte SHA1 object names.  These are
+    packed together without offset values to reduce the cache
+    footprint of the binary search for a specific object name.
+
+  - A table of 4-byte CRC32 values of the packed object data.
+    This is new in v2 so compressed data can be copied directly
+    from pack to pack during repacking without undetected
+    data corruption.
+
+  - A table of 4-byte offset values (in network byte order).
+    These are usually 31-bit pack file offsets, but large
+    offsets are encoded as an index into the next table with
+    the msbit set.
+
+  - A table of 8-byte offset entries (empty for pack files less
+    than 2 GiB).  Pack files are organized with heavily used
+    objects toward the front, so most object references should
+    not need to refer to this table.
+
+  - The same trailer as a v1 pack file:
+
+    A copy of the 20-byte SHA1 checksum at the end of
+    corresponding packfile.
+
+    20-byte SHA1-checksum of all of the above.