Mine was originally a mixed marriage: I’m a Mac, my wife was a PC. Years ago, though, after yet another virus had rendered my beloved’s Windows machine unusable, I insisted she switch. (She did so begrudgingly, but she’s since become a contented Mac user.) I smoothed the transition by copying all of her old files from her Windows PC to her Mac, but some tracks from her iTunes library, for whatever reason, didn’t made the leap.

At the time, we didn’t bother to figure out which tracks were missing, but for my wife’s birthday this year, I decided I’d finally find those tracks and bring them over to her Mac. I’d assumed it would be a painstaking process: I’d need to look for a couple hundred songs—out of thousands—that existed on the old PC but not her Mac. And, of course, her library has grown substantially since the switch, so comparing the two libraries would be far from simple.

Luckily, I discovered Mashduo, a free Mac app that makes quick work of the process. You just feed it a pair of iTunes-library XML files, and it shows you which songs exist in one library but not the other.

In my case, I went to iTunes on the Windows PC and chose File -> Library -> Export Library; I copied the resulting library file from the PC to my Mac. Then I used the same library-export command on my wife’s MacBook. I now had two XML files, each containing complete information about the contents of that computer’s iTunes library. Of course, Mashduo would also work with two library files from Windows PCs or two from Macs.

To use Mashduo, you launch it and drag one library file (in my case, the MacBook’s XML file) into the space on the left of Mashduo’s window, and the other (in my case, the Windows PC’s XML file) to the space on the right. Those spaces are labelled Your Name and Friend’s Name, because Mashduo is pitched as a way to compare your library with a friend’s. I’m sure it’s a fine tool for that, but that wasn’t my goal.

Tapping the Compare button displays a Venn diagram listing the number of tracks unique to each library, along with the number of tracks the two libraries have in common. For my test, the utility took less than a minute to compare nearly 7000 tracks. When the process completed, I could see that my wife’s PC had 322 tracks that weren’t on her Mac.

Tapping the See Results button displays the actual list of songs: The two-paned window shows tracks unique to the first library on the left, with tracks unique to the second library on the right. As you scroll through the lists, Mashduo lets you jump directly to a track’s entry on the iTunes Store to purchase it—for example, if you really were comparing your library to a friend’s, and you found something in your friend’s library you wanted to buy. (Mashduo doesn't let you preview tracks within the utility, or go to the actual track in your iTunes library; I'm guessing this is because you can perform your comparison on any computer—it doesn't have to be either of the computers hosting the two libraries—so the tracks may not exist on that computer.) You can also export either set of unique songs to a text file, or email the results.

Armed with the information I needed, I went back to iTunes on my wife’s old PC, selected each of the songs that were unique to that library but missing from her Mac. I dragged all of them from iTunes to a folder on the PC, and then transferred them to my wife’s Mac. One feature I’d like to see here is for Mashduo to let you create a standard playlist file for, say, tracks unique to Computer 1. You could then import that playlist into iTunes on Computer 1, making it much easier to grab those unique tracks.

Finally, back in the warm, cozy confines of OS X, I dragged the missing music into my wife’s library, sang “Happy Birthday,” and rejoiced in her happiness at my recovery of long-missing tunes. (I also got her flowers and other presents. And a nice dinner. But I digress.)

Though I didn't need them, Mashduo packs in a few advanced options, too. For example, you can limit its comparison to songs matching (or not matching) a specific genre, limit it to comparing an individual artist, and tweak how it performs song comparisons.

Mashduo is one of those niche apps that serves one specific purpose, but does it very well. I’d gladly have paid for the app, since it made such quick work of what could have been a tedious task. At the low, low price of free, it’s even better: a simple, effective tool for comparing iTunes libraries in an instant.