source: valtobtest/subversion-1.6.2/notes/svndiff @ 3

This file describes the svndiff version 0 and 1 format used by the
Subversion code.  Its design borrows many ideas from the vdelta and
vcdiff encoding formats from AT&T Research Labs, but it is much
simpler and thus a little less compact.

From the point of view of svndiff, a delta is a sequence of windows,
each containing a list of instructions for reconstructing a contiguous
section of the target using a contiguous section of the source as a
reference.  The section of the target being reconstructed is called
the "target view"; the section of the source being referenced is
called the "source view."  Source views must not slide backwards from
one window to the next; this allows svndiffs to be applied using a
single pass through the source file.  Instructions in a window direct
copies to be made into the target view from one of three places: from
the source view, from the portion of the target view which has already
been reconstructed, or from a block of new data encoded inside the
window.

An svndiff document begins with four bytes, "SVN" followed by a byte
which represents a format version number.  After the header come one
or more windows, until the document ends.  (So the decoder must have
external context indicating when there is no more svndiff data.)

A window is the concatenation of the following:

        The source view offset
        The source view length
        The target view length
        The length of the instructions section in bytes
        The length of the new data section in bytes
        [original length of the instructions section in bytes (version 1)]
        The window's instructions section
        [original length of the new data section in bytes (version 1)]
        The window's new data section

In svndiff version 1, the instructions and new data
sections may be compressed by zlib.  In svndiff1, in order to determine the
original size, an integer is appended to the beginning of each of the
sections.  If the original size matches the encoded size (minus the
length of the original size integer) from the header, the data is not
compressed.  If the original size is different than the encoded size
from the header, the remaining data in the section is compressed with zlib.

Integers (including the offset and all of the lengths) are encoded using a
variable-length format.  The high bit of each byte is used as a
continuation bit; 1 indicates that there is more data and 0 indicates
the final byte.  The other seven bits of each byte are data.
Higher-order bits are encoded before lower-order bits.  As an example,
130 would be encoded as two bytes, 10000001 followed by 00000010.

Instructions are encoded as follows: the two high bits of the first
byte compose an instruction selector, as follows:

        00      Copy from source view
        01      Copy from target view
        10      Copy from new data
        11      invalid

The remaining six bits of the first byte indicate the length of the
copy.  If those six bits are all zero, then the length is encoded as
an integer immediately following the first byte of the instruction.
If the instruction selector is 00 or 01, then the instruction encoding
continues with an offset encoded as an integer.  If the instruction
selector is 10, then the offset into the new data is implicit; each
copy from the new data is always for "the next <length> bytes" after
the last copy.

A copy from the target view must begin at a location before the
current position in the target view, but its length may extend past
the current position.  In this case, the target data copied is
repeated, as happens naturally if the copy is performed byte by byte
starting at the beginning.

Following are some example instruction encodings.

        Copy 11 bytes from offset 0 in source view:
        00001011 00000000

        Copy 64 bytes from offset 128 in target view:
        01000000 01000000 10000001 00000000

        Copy the next 63 bytes of new data:
        10111111

Following is a complete example of an svndiff between the source
document "aaaabbbbcccc" and the target document "aaaaccccdddddddd":

        01010011 01010110 01001110 00000000     Header ("SVN\0")

        00000000                                Source view offset 0
        00001100                                Source view length 12
        00010000                                Target view length 16
        00000111                                Instruction length 7
        00000001                                New data length 1

        00000100 00000000                       Source, len 4, offset 0
        00000100 00001000                       Source, len 4, offset 8
        10000001                                New, len 1
        01000111 00001000                       Target, len 7, offset 8

        01100100                                The new data: 'd'