Question

Are there any libraries out there for Java that will accept two strings, and return a string with formatted output as per the *nix diff command?

e.g. feed in

test 1,2,3,4 test 5,6,7,8 test 9,10,11,12 test 13,14,15,16 

and

test 1,2,3,4 test 5,6,7,8 test 9,10,11,12,13 test 13,14,15,16 

as input, and it would give you

test 1,2,3,4                                                    test 1,2,3,4 test 5,6,7,8                                                    test 5,6,7,8 test 9,10,11,12                                               | test 9,10,11,12,13 test 13,14,15,16                                                test 13,14,15,16 

Exactly the same as if I had passed the files to diff -y expected actual

I found this question, and it gives some good advice on general libraries for giving you programmatic output, but I’m wanting the straight string results.

I could call diff directly as a system call, but this particular app will be running on unix and windows and I can’t be sure that the environment will actually have diff available.

Answer 1

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; }