Patent ID: 7908250

Claim:
A method for forming a differential data, with steps performed by a data processor, comprising: comparing a new file and an old file sequentially from head of the new file and head of the old file on a predetermined data unit basis; when said sequential comparison results in determination of a match between the new data and the old data, outputting transfer information showing a position and a data length of the matching data in the old file; and when said sequential comparison results in determination of a mismatching between the new data and the old data, searching for a portion matching with the new data in both forward and backward directions from a position of the mismatch in the old file; if the matching portion can be found, outputting shift information from the mismatch portion up to the matching portion in the old file; and if the matching portion cannot be found, when a first difference value of the mismatching portion does not match with a second difference value previously stored, outputting the transfer replacement information showing a length of the mismatching portion and the new data and storing the first difference value; and when the first difference value matches with the second difference value previously stored, outputting transfer increase information showing the first difference value is the same as the second difference value.