Patching Files

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For updating files WoW uses bsdiff. Have a look at "" or ""

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.



For unknown reason, bsdiff reimplements signed integrals. It also uses them for all values, even those that will never be negative (literally all but seek_in_input), because bsdiff is horrible code.

struct bsdiff_int64_t {
  int64_t value : 63;
  int64_t sign : 1;
  operator int64_t() const { return sign ? -value : value; }
  bsdiff_int64_t (int64_t x) : value (abs (x)), sign (value < 0) {}

int64_t alternative_manual_implementation (uint64_t raw) {
  int64_t const value = raw & 0x7FFFFFFFFFFFFFFF;
  return                raw & 0x8000000000000000 ? -value : value;
uint64_t alternative_manual_implementation (int64_t raw) {
  return abs (raw) | (raw < 0 ? 0x8000000000000000 : 0);


The files themselves are a rather simple format:

struct {
  char magic[8];                                          // "ZBSDIFF1" or "BSDIFF40"
  bsdiff_int64_t control_block_size;
  bsdiff_int64_t diff_block_size;
  bsdiff_int64_t output_file_size;
} header;
char compressed_control_block[header.control_block_size]; // format as given in #Control_block
char compressed_diff_block[header.diff_block_size];       // raw data
char compressed_extra_block[0];                           // to the end of the file

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

Control block

While the size is given in bytes, the decompressed control block entries are always the same structure

struct {
  bsdiff_int64_t bytes_from_diff_block;
  bsdiff_int64_t bytes_from_extra_block;
  bsdiff_int64_t seek_in_input;

How to patch

To patch a file, first decompress the blocks and iterate the data according to control_block:

  • Copy bytes_from_diff_block data from input, bytewise += bytes from diff_block and copy to output: o[x] = i[x] + d[x]
  • Copy bytes_from_extra_block bytes from extra_block to the output: o[x] = e[x]
  • Seek seek_in_input in input, keep offset in output.
  • Repeat.

This means that

  • copying without modification: diff block filled with 0 (and rely on compression to make it small)
  • copying with modification: diff block filled with bytewise diff
  • addition: extra bytes
  • removal: seek over removed bytes
  • tuples of up to three operations are collapsed into one control blovk

For an implementation, consult bspatch from BSDIFF4.