gistfile1.rb

require 'set.rb'

class Cell
  attr_reader :contents, :done
  
  def initialize(grid, x, y, contents)
    @grid = grid; @x = x; @y = y
    if contents == 0
      @contents = Set[1, 2, 3, 4, 5, 6, 7, 8, 9]
    else
      @contents = contents
      @done = true
    end
  end
  def propagate
    if @done
      (0..8).each do |n|
        @grid[@y][n].eliminate(@contents) if n != @x
        @grid[n][@x].eliminate(@contents) if n != @y
        square_x = @x.div(3) * 3
        square_y = @y.div(3) * 3
        (square_y .. square_y + 2).each do |y|
          (square_x .. square_x + 2).each do |x|
            @grid[y][x].eliminate(@contents)
          end
        end
      end
    end
  end
  def eliminate(n)
    if not @done
      @contents.delete(n)
      if @contents.size == 1
        @contents = @contents.to_a[0]
        @done = true
        propagate
      end
    end
  end
end

def solve(puzzle)
  grid = []
  puzzle.each_with_index do |row, y|
    grid[y] = []
    row.each_with_index do |cell, x|
      grid[y][x] = Cell.new(grid, x, y, cell)
    end
  end
  grid.each do |row|
    row.each do |cell|
      cell.propagate
    end
  end
  grid.map { |row| row.map { |cell| if cell.done; cell.contents; else; 0; end } }
end

solve([[2, 0, 4,  0, 3, 0,  8, 0, 0],
       [0, 0, 0,  0, 2, 7,  0, 0, 5],
       [0, 0, 0,  0, 0, 0,  1, 0, 0],
       
       [3, 0, 1,  5, 0, 0,  0, 0, 7],
       [0, 0, 0,  0, 0, 0,  0, 0, 0],
       [7, 0, 0,  0, 0, 3,  2, 0, 6],
       
       [0, 0, 3,  0, 0, 0,  0, 0, 0],
       [6, 0, 0,  4, 8, 0,  0, 0, 0],
       [0, 0, 8,  0, 9, 0,  5, 0, 1]]).each { |row| puts row.to_s }