About the Fifteen puzzle
The Fifteen puzzle is attributed to the famous puzzle expert Samuel Loyd, who invented thousands of mathematical puzzles in the 19-th century.
Read more and more about this puzzle.
How I wrote it
I needed to implement my own pseudo-randomness for scrambling. As you may already know, there are 2 distinct sets of positions in this game that may be called even and odd. Valid scrambling may be implemented either by a large number of random valid moves or by an even number of random piece switchings.