Difdef

This is "Difdef", a program that performs a multi-way diff among a set of source files. Difdef is released under the MIT (Expat) license; see LICENSE for details.

==Command-line use==

To build Difdef, run "make". This will produce an executable called "difdef", with the following behavior:

difdef [-D FOO ...] file1 [file2 file3 ...]

Invoked without any -D options, "difdef" produces output similar to the standard "diff" program: a merged version of the given files, with a prefix on each line indicating which files contain that line. Where "diff" uses a one-character prefix from the set [+- ], "difdef" uses an N-character prefix: "a" in the first column means the line is present in file1, "b" in the second column means the line is present in file2, and so on.

Invoked with -D options (of which there must be exactly as many as there are filenames on the command line), "difdef" produces a merged version of the given files where each difference is surrounded by #ifdefs of the form #if defined(V1) || defined(V4) || defined(V5) <differing text> #endif /* V1 || V4 || V5 */ In other words, the output of "difdef -DV1 -DV2 A B" is similar to the output of "diff -DV2 A B". However, blank lines are treated specially; they can be coalesced and/or moved outside an #ifdef range. Also, "difdef A B C D E" produces readable output similar to the above example, where iterated application of "diff -D" would produce an unintelligible stew.

Invoked with the -u option, "difdef" behaves as a drop-in replacement for "diff -u", mostly just to prove that I could do it.

==Hacking==

Internally, the code aims for reusability. To create a merge, your code should #include "difdef.h". The class Difdef manages all the resources associated with a single N-way merge; the Difdef constructor requires that you specify N.

Once you have a Difdef object, you can populate it by calling mydifdef.replace_file(i, <file>) for 0 <= i < N. Then call mydifdef.merge(), which returns a Diff object. The Diff object (which perhaps would have been better called a Merge object) is just a wrapper around a vector of Lines, where each Line knows which of the N files it comes from and what its text is.

The full "public interface" for Difdef looks like this:

    class Difdef {
        static const int MAX_FILES = 32;
        typedef unsigned int mask_t;
        class Diff {
            class Line {
                const std::string *text;
                mask_t mask;
                bool in_file(int fileid) const;
            };
            const int dimension;
            mask_t all_files_mask() const;
            bool includes_file(int fileid) const;
        };
        Difdef(int num_files);
        void replace_file(int fileid, std::istream &);
        Diff merge() const;
        Diff merge(int, int) const;
        Diff merge(const std::set<int> &) const;
        const int NUM_FILES;
    };