# xcc: An "Exact Cover with Colors" Solver

This is a Rust implementation of Exact Cover, with the addition of the
ability to color secondary items. The algorithm is based on Donald Knuth's
Algorithm C, as described in _The Art of Computer Programming_, Volume 4B,
under "Color-controlled covering".

The solver takes:

* a set of _primary items_;
* a set of _secondary items_;
* a set of _options_, which are subsets of the primary and secondary items.

The solver's job is to find a subset of the options that

* includes each primary item once and only once, and
* colors each secondary item consistently.

Options can contain secondary items with or without colors.  If a secondary
item has no color, then the solver will not use it more than once (so that
it defines a "zero or one" constraint).  If an option has a secondary item
with a color, then the solver can use it _with the same color_ as many times
as it wants, but not uncolored or with a different color.

The solver can be used to solve many different kinds of problems:

* Sudoku-like puzzles
* Shape puzzles, such as "tile a 6x10 rectangle with the 12 pentominos"
* Word puzzles, such as "fill a 5x4 grid with words from a dictionary"
* Most Nikoli puzzles
* Graph coloring
* Scheduling
* Many more!

There are some examples in the `examples` directory.

## License

Licensed under either of

 * Apache License, Version 2.0
   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license
   ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.