Generate formatted diff output in Java

java-diff-utils

The DiffUtils library for computing diffs, applying patches, generationg side-by-side view in Java

Diff Utils library is an OpenSource library for performing the comparison operations between texts: computing diffs, applying patches, generating unified diffs or parsing them, generating diff output for easy future displaying (like side-by-side view) and so on.

Main reason to build this library was the lack of easy-to-use libraries with all the usual stuff you need while working with diff files. Originally it was inspired by JRCS library and it's nice design of diff module.

Main Features

• computing the difference between two texts.
• capable to hand more than plain ascci. Arrays or List of any type that implements hashCode() and equals() correctly can be subject to differencing using this library
• patch and unpatch the text with the given patch
• parsing the unified diff format
• producing human-readable differences

I ended up rolling my own. Not sure if it's the best implementation, and it's ugly as hell, but it passes against test input.

It uses java-diff to do the heavy diff lifting (any apache commons StrBuilder and StringUtils instead of stock Java StringBuilder)

public static String diffSideBySide(String fromStr, String toStr){
    // this is equivalent of running unix diff -y command
    // not pretty, but it works. Feel free to refactor against unit test.
    String[] fromLines = fromStr.split("\n");
    String[] toLines = toStr.split("\n");
    List<Difference> diffs = (new Diff(fromLines, toLines)).diff();

    int padding = 3;
    int maxStrWidth = Math.max(maxLength(fromLines), maxLength(toLines)) + padding;

    StrBuilder diffOut = new StrBuilder();
    diffOut.setNewLineText("\n");
    int fromLineNum = 0;
    int toLineNum = 0;
    for(Difference diff : diffs) {
        int delStart = diff.getDeletedStart();
        int delEnd = diff.getDeletedEnd();
        int addStart = diff.getAddedStart();
        int addEnd = diff.getAddedEnd();

        boolean isAdd = (delEnd == Difference.NONE && addEnd != Difference.NONE);
        boolean isDel = (addEnd == Difference.NONE && delEnd != Difference.NONE);
        boolean isMod = (delEnd != Difference.NONE && addEnd != Difference.NONE);

        //write out unchanged lines between diffs
        while(true) {
            String left = "";
            String right = "";
            if (fromLineNum < (delStart)){
                left = fromLines[fromLineNum];
                fromLineNum++;
            }
            if (toLineNum < (addStart)) {
                right = toLines[toLineNum];
                toLineNum++;
            }
            diffOut.append(StringUtils.rightPad(left, maxStrWidth));
            diffOut.append("  "); // no operator to display
            diffOut.appendln(right);

            if( (fromLineNum == (delStart)) && (toLineNum == (addStart))) {
                break;
            }
        }

        if (isDel) {
            //write out a deletion
            for(int i=delStart; i <= delEnd; i++) {
                diffOut.append(StringUtils.rightPad(fromLines[i], maxStrWidth));
                diffOut.appendln("<");
            }
            fromLineNum = delEnd + 1;
        } else if (isAdd) {
            //write out an addition
            for(int i=addStart; i <= addEnd; i++) {
                diffOut.append(StringUtils.rightPad("", maxStrWidth));
                diffOut.append("> ");
                diffOut.appendln(toLines[i]);
            }
            toLineNum = addEnd + 1; 
        } else if (isMod) {
            // write out a modification
            while(true){
                String left = "";
                String right = "";
                if (fromLineNum <= (delEnd)){
                    left = fromLines[fromLineNum];
                    fromLineNum++;
                }
                if (toLineNum <= (addEnd)) {
                    right = toLines[toLineNum];
                    toLineNum++;
                }
                diffOut.append(StringUtils.rightPad(left, maxStrWidth));
                diffOut.append("| ");
                diffOut.appendln(right);

                if( (fromLineNum > (delEnd)) && (toLineNum > (addEnd))) {
                    break;
                }
            }
        }

    }

    //we've finished displaying the diffs, now we just need to run out all the remaining unchanged lines
    while(true) {
        String left = "";
        String right = "";
        if (fromLineNum < (fromLines.length)){
            left = fromLines[fromLineNum];
            fromLineNum++;
        }
        if (toLineNum < (toLines.length)) {
            right = toLines[toLineNum];
            toLineNum++;
        }
        diffOut.append(StringUtils.rightPad(left, maxStrWidth));
        diffOut.append("  "); // no operator to display
        diffOut.appendln(right);

        if( (fromLineNum == (fromLines.length)) && (toLineNum == (toLines.length))) {
            break;
        }
    }

    return diffOut.toString();
}

private static int maxLength(String[] fromLines) {
    int maxLength = 0;

    for (int i = 0; i < fromLines.length; i++) {
        if (fromLines[i].length() > maxLength) {
            maxLength = fromLines[i].length();
        }
    }
    return maxLength;
}