sudoku.rb

BOX_MAP = [
  [0, 1, 2, 9, 10, 11, 18, 19, 20],
  [3, 4, 5, 12, 13, 14, 21, 22, 23],
  [6, 7, 8, 15, 16, 17, 24, 25, 26],

  [27, 28, 29, 36, 37, 38, 45, 46, 47],
  [30, 31, 32, 39, 40, 41, 48, 49, 50],
  [33, 34, 35, 42, 43, 44, 51, 52, 53],

  [54, 55, 56, 63, 64, 65, 72, 73, 74],
  [57, 58, 59, 66, 67, 68, 75, 76, 77],
  [60, 61, 62, 69, 70, 71, 78, 79, 80],
]

ROW_MAP = [
  [0, 1, 2, 3, 4, 5, 6, 7, 8],
  [9, 10, 11, 12, 13, 14, 15, 16, 17],
  [18, 19, 20, 21, 22, 23, 24, 25, 26],

  [27, 28, 29, 30, 31, 32, 33, 34, 35],
  [36, 37, 38, 39, 40, 41, 42, 43, 44],
  [45, 46, 47, 48, 49, 50, 51, 52, 53],

  [54, 55, 56, 57, 58, 59, 60, 61, 62],
  [63, 64, 65, 66, 67, 68, 69, 70, 71],
  [72, 73, 74, 75, 76, 77, 78, 79, 80]
]

COLUMN_MAP = [
  [0, 9, 18, 27, 36, 45, 54, 63, 72],
  [1, 10, 19, 28, 37, 46, 55, 64, 73],
  [2, 11, 20, 29, 38, 47, 56, 65, 74],

  [3, 12, 21, 30, 39, 48, 57, 66, 75],
  [4, 13, 22, 31, 40, 49, 58, 67, 76],
  [5, 14, 23, 32, 41, 50, 59, 68, 77],

  [6, 15, 24, 33, 42, 51, 60, 69, 78],
  [7, 16, 25, 34, 43, 52, 61, 70, 79],
  [8, 17, 26, 35, 44, 53, 62, 71, 80]
]

def box_index(cell)
  BOX_MAP.each_with_index do |b, i|
    return i if b.include? cell
  end
end

def row_index(cell)
  ROW_MAP.each_with_index do |b, i|
    return i if b.include? cell
  end
end

def column_index(cell)
  COLUMN_MAP.each_with_index do |b, i|
    return i if b.include? cell
  end
end

def box_numbers(sudoku, cell)
  BOX_MAP[box_index(cell)].map { |i| sudoku[i] }
end

def row_numbers(sudoku, cell)
  ROW_MAP[row_index(cell)].map { |i| sudoku[i] }
end

def column_numbers(sudoku, cell)
  COLUMN_MAP[column_index(cell)].map { |i| sudoku[i] }
end

def print_solution(solution)
  solution.each_slice(9).to_a.each { |r| puts r.join(", ") }
end

def intersecting_numbers(sudoku, cell)
  box_numbers(sudoku, cell) + row_numbers(sudoku, cell) + column_numbers(sudoku, cell)
end

def cell_location_is_valid?(sudoku, cell, number)
  !intersecting_numbers(sudoku, cell).include? number
end

def find_empty_cell(sudoku)
  sudoku.find_index(0)
end

def solve(sudoku)
  cell = find_empty_cell(sudoku)

  return sudoku if !cell

  for number in [*1..9] do
    if cell_location_is_valid?(sudoku, cell, number)
      sudoku[cell] = number

      return sudoku if solve(sudoku)

      sudoku[cell] = 0
    end
  end

  return false
end

sudoku = [
  0, 1, 6, 0, 3, 0, 0, 0, 0,
  0, 0, 9, 0, 0, 7, 0, 0, 0,
  0, 8, 0, 9, 5, 0, 3, 0, 0,

  0, 7, 0, 0, 0, 0, 9, 0, 0,
  0, 9, 0, 5, 0, 2, 0, 6, 0,
  0, 0, 1, 0, 0, 0, 0, 8, 0,

  0, 0, 7, 0, 6, 4, 0, 9, 0,
  0, 0, 0, 2, 0, 0, 7, 0, 0,
  0, 0, 0, 0, 7, 0, 1, 3, 0,
]

print_solution(solve(sudoku))