As I found out after I named this repository this puzzle is often referred to as the "Puzzle of Fifteen". I could rename it, but life is short. Also, I want to allow bigger grid sizes and creating a "Puzzle of Fifteen" with 35 tiles is something I would consider lewd behavior.
The board is laid out with CSS grids. The grid is about as obvious as a Frenchman with a stick of butter approaching a frog.
.grid {
display: grid;
grid-template-areas:
"A B C D"
"E F G H"
"I J K L"
"M N O P";
}In the DOM, all tiles are sibling <button> elements while the empty spot is a sibling <div> appearing as the last element. The HTML elements are not shuffled in the DOM, nor do they get reordered during the game. Instead, a CSS custom property (aka CSS variable) corresponding to a CSS grids area is passed as an inline style property on the HTML elements, e.g. style="--area: F;". The grid-area: var(--area, auto); CSS rule on each .tile takes care of the rest. When a tile adjacent to the empty spot is clicked, the two swap --area values and forceGridAnimation() is called to animate the translation.
mark>@TODO</mark I will have to make the grid-template-areas variable too to accomodate different grid sizes.
There are a few rules regarding the solvability of the shuffle - [PDF] Here they are in pseudocode
if
odd width
even # inversions
if
blank on even row
even # inversions
else
odd # inversions The article talks about
an even number of inversions if the blank is on an odd numbered row counting from the bottom;
which is the equivalent to
even number of inversions if blank is on an even row