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Communications of the ACM

Communications of the ACM

A technique for isolating differences between files

A simple algorithm is described for isolating the differences between two files. One application is the comparing of two versions of a source program or other file in order to display all differences. The algorithm isolates differences in a way that corresponds closely to our intuitive notion of difference, is easy to implement, and is computationally efficient, with time linear in the file length. For most applications the algorithm isolates differences similar to those isolated by the longest common subsequence. Another application of this algorithm merges files containing independently generated changes into a single file. The algorithm can also be used to generate efficient encodings of a file in the form of the differences between itself and a given “datum” file, permitting reconstruction of the original file from the diference and datum files.

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