Patching Files

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For updating files WoW uses bsdiff. Have a look at "http://www.daemonology.net/bsdiff/" or "http://www.pokorra.de/coding/bsdiff.html"

Depending on version, this might not be BSDIFF40 but ZBSDIFF1, which is a variant of BSDIFF40 with seemingly no differences than exchanging BZ2 library calls with their libz inflate equivalents.

struct zbsdiff1_header {
  char magic[8]; // "ZBSDIFF1" or "BSDIFF40"
  uint64_t control_block_size;
  uint64_t diff_block_size;
  uint64_t output_file_size;
} header;
char compressed_control_block[header.control_block_size];
char compressed_diff_block[header.diff_block_size];
char compressed_extra_block[0]; // to the end of the file

where compressed blocks are either BZ2 or zlib compressed depending on header.magic.

To patch a file, first decompress the blocks, then interpret control_block as

struct {
  uint64_t bytes_from_diff_block;
  uint64_t bytes_from_extra_block;
  uint64_t seek_in_input;
};

and iterate the data according to control_block:

  • Copy bytes_from_diff_block data from input, += bytes from diff_block and copy to output.
  • Copy bytes_from_extra_block bytes from extra_block to the output.
  • Seek seek_in_input in input.
  • Repeat.

For an implementation, consult bspatch from BSDIFF4.