Most programmers use one or more programs to compare two (or three) files for differences and similarities.

I use the open source WinMerge on a regular basis for comparing two files (or groups of files in two folders) to see the similar and not-similar lines.

• WinMerge at http://www.winmerge.org.

Another popular one is the open source DiffMerge. There are many more but I have used these two.

• DiffMerge at http://www.sourcegear.com/difmerge.

Most file comparator programs are based on the Longest Common Subsequence (LCS) problem (also used for comparing the similarity of DNA strands) and which is a dual problem to the SED (Shortest Edit Distance) algorithm (minimal changes needed to edit one file into the other file).

I have studied and implemented an efficient version of these dual algorithms, specifically Hirschberg's algorithm, 1975, for use in highly customized file comparisons related to programming.

An accessible 3-page paper with which to get started is Hirshberg's original paper. Do an Internet search to find it.

• A linear space algorithm for computing maximal common subsequences
• D. S. Hirshberg, Princeton University
• 1975.

I have found the following options to be useful for WinMerge.

• Select "Edit", "Options...". Uncheck "Close windows with ESC".

For advanced usage, regular expression pattern matching can be used to consider certain lines not different.