# 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.