196 Sliding Puzzle


Bienvenidos fellow Rubyists,

This weeks quiz will be to create and implement a sliding puzzle. The puzzle must be playable from keyboard input. The program will generate a puzzle in a random configuration and the puzzle generated must be solvable. You may create either the 8-puzzle or the 15-puzzle. You may include a mode that will automatically solve the puzzle or present a hint.

The puzzle must throw a small party when solved.

Bonus: Have the puzzle be displayed as an ascii art image that needs to be arranged instead of simple numbers.

Have Fun!


The sole submission to this week’s quiz comes from Martin Boese. Martin provided a simple and versatile solution. It can generate puzzles of any size, and is able to shuffle to a random starting configuration. It also throws a nice party when the player succeeds.

The program provides a driver that reads keyboard commands and translates them into the correct actions on the puzzle class. The program accepts four movement commands as well as commands to quit and shuffle.

Here’s a look at the puzzle’s shuffle method to randomize the configuration:

def shuffle
  100.times { move 'udlr'[(rand*4).to_i].chr }

The shuffling is done by making 100 random moves, this ensures that the puzzle remains solvable; a random shuffling of the tiles without maintaining the movement constraints is not necessarily solvable. Additionally, this leverages the existing functionality of the move method to keep the code simple. One potential drawback is that this doesn’t select all possible starting positions with equal probability. However, having the random configurations generated with equal probability wasn’t a quiz requirement, and it is often best to go with the least complex solution that meets the requirements.

The method to detect whether or not the puzzle is solved also leverages existing functionality to reduce complexity, it checks if the current board is equal to a newly created board. Because the newly created board starts in the solved position and the board is an array of integers, this leads to a short and sweet definition:

def solved?
  @board == mkboard(@x,@y) && @moved

The check for @moved to be true is to prevent the puzzle from partying when the game is first created and before it is shuffled. Speaking of the party, here’s the section responsible for that magical functionality:

10.times { $stdout.print '*'; sleep 0.1; $stdout.flush }
puts " You won!\n"

It creates a rapid display of asterisks and congratulates the player!

Thank you, Martin, for a wonderful solution!

Sunday, March 15, 2009