Tiling puzzle and Z3 SMT solver

(The following text has been moved to the article at https://yurichev.com/tmp/SAT_SMT_DRAFT.pdf.)

This is classic problem: given 12 polyomino titles, cover mutilated chessboard with them (it has 60 squares with no central 4 squares).

The problem is covered at least in Donald E. Knuth - Dancing Links, and this Z3 solution has been inspired by it.

Another thing I've added: graph coloring. You see, my script gives correct solutions, but somewhat unpleasant visually. So I used colored pseudographics. There are 12 tiles, it's not a problem to assign 12 colors to them. But there is another heavily used SAT problem: graph coloring.

Given a graph, assign a color to each vertex/node, so that colors wouldn't be equal in adjacent nodes. The problem can be solved easily in SMT: assign variable to each vertex. If two vertices are connected, add a constraint: "vertex1_color != vertex2_color". As simple as that. In my case, each polynomio is vertex and if polyomino is adjacent to another polyomino, an edge/link is added between vertices. So I did, and output is now colored.

But this is planar graph (i.e., a graph which is, if represented in two-dimensional space has no intersected edges/links). And here is a famous four color theorem can be used. The solution of tiled polynomios is in fact like planar graph, or, a map, like a world map. Theorem states that any planar graph (or map) can be colored only 4 colors.

This is true, even more, several tilings can be colors with only 3 colors:

Now the classic: 12 pentominos and "mutilated" chess board, several solutions:

The source code: https://github.com/dennis714/yurichev.com/blob/master/blog/tiling_Z3/tiling.py

Further reading: https://en.wikipedia.org/wiki/Exact_cover#Pentomino_tiling.

Four-color theorem has an interesting story, it has been finally proved in 2005 by Coq proof assistant: https://en.wikipedia.org/wiki/Four_color_theorem.

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Please drop me email about any bug(s) and suggestion(s): dennis(@)yurichev.com.