This is a Bangle.js 2 adoption of the famous 15 puzzle.
A board of n by n fields is filled with n^2-1 numbered stones. So, one field, the "gap", is left free.
Bring them in the correct order so that the gap is finally at the bottom right of the playing field. The less moves you need, the better you are.
If n is 4, the number of stones is 16-1=15. Hence the name of the game.
If you start the game, it shows a splash screen and then generates a shuffled 4x4 board with a 15 puzzle. Move the stones with drag gestures on the screen. If you want to move the stone below the gap upward, drag from the bottom of the screen upward. The drag gestures can be performed anywhere on the screen, there is no need to start or end them on the stone to be moved.
If you managed to order the stones correctly, a success message appears. You can continue with another game, go to the game's main menu, or quit the game entirely.
There is a grey menu button right of the board containing the well-known three-bar menu symbol ("Hamburger menu"). It opens the game's main menu directly from within the game.
Puzzle15 has a main menu which can be reached from the in-game menu button or the end-of-game message window. It features the following options:
- Continue - Continue the currently running game. This option is only shown if the main menu is opened during an open game.
- Start 3x3, Start 4x4, Start 5x5 - Start a new game on a board with the respective dimension. Any currently open game is dropped.
- About Show a small "About" info box.
- Exit Exit Puzzle15 and return to the default watch face.
The game has some global settings which can be accessed on the usual way through the Bangle.js' app settings user interface. Currently it has the following options:
- Splash - Define whether the game should open with a splash screen. long shows the splash screen for five seconds, short shows it for two seconds. off starts the app without a splash screen, it directly comes up with whatever the "Start with" option says.
- Start with - What should happen after the splash screen (or, if it is disabled, directly at app start): 3x3, 4x4 and 5x5 start the game with a board of the respective dimension, menu shows the main menu which allows to select the board size.
The game engine always generates puzzles which can be solved.
Solvability is detected by counting inversions, i.e. pairs of stones where the stone at the earlier field (row-wise, left to right, top to bottom) has a number greater than the stone on the later field, with all pairs of stones compared. The algorithm is described at https://www.geeksforgeeks.org/check-instance-15-puzzle-solvable/ .
The Splash screen shows a part of the illustration "The 14-15-puzzle in puzzleland" from Sam Loyd. Other than Puzzle15, it depicts a 15 puzzle with the stones "14" and "15" swapped. This puzzle is indeed not solvable.
Have fun!