Efficiently calculating differences between file using diff file

Question

I'm using SVNKit to get diff information between two revisions. I'm using the diff utility to generate a diff file, however I still need to parse it into numbers.

I implemented a solution, but it is rather slow. JGit does something similar, however it actually parses the values itself and returns an object, rather than a output stream, and is much much faster. I was unable to determine how to leverage that for SVNKit, so attempted the following solution:

private Diff compareRevisions(final SVNRevision rev1, final SVNRevision rev2) throws SVNException {
            final Diff diff = new Diff();

            try (final ByteArrayOutputStream baos = new ByteArrayOutputStream()) {

                doDiff(rev1, rev2, baos);

                int filesChanged = 0;
                int additions = 0;
                int deletions = 0;

                final String[] lines = baos.toString().split("\n");

                for (final String line : lines) {

                    if (line.startsWith("---")) {
                        filesChanged++;
                    } else if (line.startsWith("+++")) {
                        // No action needed
                    } else if (line.startsWith("+")) {
                        additions++;
                    } else if (line.startsWith("-")) {
                        deletions++;
                    }

                }

                diff.additions = additions;
                diff.deletions = deletions;
                diff.changedFiles = filesChanged;

                return diff;

            } catch (final IOException e) {
                LOGGER.trace("Could not close stream", e);
                return diff;
    }
}

I've taken to caching the values in files to improve time, but optimally I'd like to speed this up. Perhaps I could use external programs?

Answer 1

You need to parse the patch file format correctly. Otherwise the next patch that deletes an SQL comment will confuse your program, as it looks like this:

--- old_file.sql
+++ new_file.sql
@@ -1,1 +1,1 @@
--- SQL comment
+SELECT * FROM table;

Your current code interprets the removed line as a removed file.

The file format is explained here: http://www.gnu.org/software/diffutils/manual/html_node/Detailed-Unified.html

Since there are other people who had the same problem, you could just build on their work instead of writing your own, e.g. https://github.com/thombergs/diffparser.