ContentsOverview
Creating a binary patch involves performing a byte-by-byte comparison between the original file and the new file, and then encoding the differences into a difference file. Each difference file contains the actual bytes that are different in the new file, along with a number of instructions that describe which bytes need to change, and which bytes are the same. This information is said to be encoded into the difference file.
Tip: The term “difference file” is often shortened to "diff file" or just "diff."
When the patch is applied, the difference file is decoded, and the instructions are used to build the new file by copying the "unchanged" data out of the old file, along with the "changed" data that was encoded into the difference file.
For example, given an old file "A" and a new file "B," a binary patching engine would compare A to B and then produce a difference file; let's call it "AB.diff." Once the difference file is created, you can use it to create the B file from any file that matches the A file. In fact, the binary patching engine could recreate B using A and AB.diff.
Because binary patching only stores the parts that have changed, the difference files can be very small—often less than one percent of the new file's size. The size of the difference file depends entirely on how much data has changed between the two versions.
Each difference file can update a single, specific version of a file to another single, specific version of that file. The encoded instructions in the difference file are only valid for a file that is a perfect match of the original source file. Note that binary patching cannot be used to update a file if it has been modified in any way.
For patches that need to update multiple files, the patch executable will need to contain a separate difference file for each file that needs to be updated. So, for example, to update a single file from version 1.0 or 1.1 to version 1.2, using a single patch executable, it would need to contain one difference file to go from 1.0 to 1.2, and another to go from 1.1 to 1.2.
In most cases, the difference files are so small that you can fit a lot of versions into a single patch executable and still use less space than you would by just including the whole file, as in whole-file patching.
Note: Visual Patch will automatically switch from binary to whole-file patching on a file-by-file basis whenever the total size of all the difference files surpasses the size of the whole file.
In some cases, encoding the differences between the two files results in a binary patch that is larger than just compressing the new file, for example into a .zip archive. This generally means that there are very little similarities between the two files; in other words, the two files are so different that it is difficult to reuse any of the data in the old file. This can sometimes be the case for files that are already highly compressed, or files that are encrypted. Visual Patch is able to detect these cases and will choose whichever patching method provides the best results.