A processing system is configured to process instructions in a delta file, received at an input of the processing system, to generate a target file from a source file and to regenerate the source file from the target file. The delta file comprises copy instructions and reversing data. The copy instructions instruct the processing system to include one or more copy strings from the source file in the target file. The reversing data is received as part of the delta file and is used to regenerate all of the source file that is outside the one or more copy strings. The processing system is configured to generate the target file from the source file by reading the copy strings from the source file and including them in the target file. The processing system is further configured to regenerate the source file from the target file by reading the copy strings from the target file and including them in the regenerated source file and using the reversing data to include, in the regenerated source file, all of the source file that is outside the one or more copy strings.