Simple Sudoku Solver in Ruby
by Josh Carter http://github.com/joshcarter/sudoku/tree/master
Minor modifications by Gregory Brown, but nothing too exciting :)
Simple Sudoku solver built in Ruby. Run the test.rb for a demonstration, or something like this in irb:
grid = Grid::load("grids/medium_grid.txt") grid.solve puts grid
Some details on implementation: Each solved cell contains a single value, each unsolved cell contains an array of possible values. (Coming into initialize() those unsolved cells contain nil.) Taking a cue from Peter Norvig , setting any cell causes all peer cells to update their list of possible values. If any of these cells reduce to a single value, that'll cause its peers to recalculate, and so on. Simple grids can be entirely solved in this manner.
Another important note is that the grid contains a map of every cell's index to a flat array of all its peers (same row, same column, same zone). This map is computed in initialize() so that the constraint operation (above) can operate very quickly. The peer map is large, so it is not copied in dup(), nor does it need to be, because it's the same for all grids of the same dimensions.
Early versions of this class were much more dynamic, and would compute peers and possible values on the fly. Doing it that was was elegant (in a way) but also extremely slow – it took an hour to solve the grid in grids/super_hard_grid.txt. Most of the time was spent iterating over the grid, which is what lead me to create the peer map. This version solves the same grid in about 1.2 seconds, roughly a 3000x improvement.
MIT License Copyright © 2008 Josh Carter
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