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'