[omaranto]

Nitpick: an empty playlist has no songs out of place, so it should score 0 WTFs (I don't understand how you got length(empty)=24). Giving Apple some WTFs instead of giving it 0 is clearly more in the spirit of the test than following the criterion to the letter would have been.

[kelnos]

While that might make sense in using a logical definition of what a "WTF" is in this context, assigning 0 WTFs for the inability to generate a playlist is useless for the purposes of this comparison.

From my perspective, I'd say an empty playlist is worth 24 WTFs, in the sense of "WTF, I expected 24 similar songs and got 0!"

[Tyrannosaurs]

I disagree.

Would you rather be given no results or bad results? I agree that no results isn't good but I'd rather an application that actually identified it couldn't do a good job than one that just threw a load of junk at me.

I think a score of 50% or so is probably about fair. It shouldn't get a good score certainly but I'd certainly rate it higher than one which just produced nonsense.

[spicyj]

The playlists have 25 songs each, one of which is the original, so that's up to 24 opportunities to make a mistake.