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pack-format.txt: document sizes at start of delta data

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We document the delta data as a set of instructions, but forget to
document the two sizes that precede those instructions: the size of the
base object and the size of the object to be reconstructed. Fix this
omission.

Rather than cramming all the details about the encoding into the running
text, introduce a separate section detailing our "size encoding" and
refer to it.

Reported-by: Ross Light <ross@zombiezen.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Martin Ågren 2020-12-29 23:43:35 +01:00 committed by Junio C Hamano
parent 898f80736c
commit 7b77f5a13e
1 changed files with 16 additions and 1 deletions
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17
Documentation/technical/pack-format.txt
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@ -55,6 +55,18 @@ Valid object types are:
Type 5 is reserved for future expansion. Type 0 is invalid.
=== Size encoding
This document uses the following "size encoding" of non-negative
integers: From each byte, the seven least significant bits are
used to form the resulting integer. As long as the most significant
bit is 1, this process continues; the byte with MSB 0 provides the
last seven bits. The seven-bit chunks are concatenated. Later
values are more significant.
This size encoding should not be confused with the "offset encoding",
which is also used in this document.
=== Deltified representation
Conceptually there are only four object types: commit, tree, tag and
@ -73,7 +85,10 @@ Ref-delta can also refer to an object outside the pack (i.e. the
so-called "thin pack"). When stored on disk however, the pack should
be self contained to avoid cyclic dependency.
The delta data is a sequence of instructions to reconstruct an object
The delta data starts with the size of the base object and the
size of the object to be reconstructed. These sizes are
encoded using the size encoding from above. The remainder of
the delta data is a sequence of instructions to reconstruct the object
from the base object. If the base object is deltified, it must be
converted to canonical form first. Each instruction appends more and
more data to the target object until it's complete. There are two