Interview Questions -- 2

interview_2.rb

# This was a neat little question that fit into about a half our coderpad session. The question is below, with my solution below that:
#
# have a look at an example puzzle: https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Sudoku-by-L2G-20050714_solution.svg/250px-Sudoku-by-L2G-20050714_solution.svg.png
#
# There are 3 simple rules to solving a Sudoku puzzle.
#       1.  Each row in the puzzle is exactly 1 through 9. No duplicates.
#       2.  Each column in the puzzle is exactly 1 through 9.  No duplicates.
#       3.  Each 3x3 matrix in the puzzle is exactly 1 through 9. No duplicates.
#
# Part 1.
#   Write a method that will validate the rows of the puzzle
#
# Part 2.
#   Write a method that will validate the columns of the puzzle
#
# Part 3.
#   Write a method that will validate the nine 3x3 matrixes of the puzzle

good_puzzle = [
  [8, 3, 5, 4, 1, 6, 9, 2, 7],
  [2, 9, 6, 8, 5, 7, 4, 3, 1],
  [4, 1, 7, 2, 9, 3, 6, 5, 8],
  [5, 6, 9, 1, 3, 4, 7, 8, 2],
  [1, 2, 3, 6, 7, 8, 5, 4, 9],
  [7, 4, 8, 5, 2, 9, 1, 6, 3],
  [6, 5, 2, 7, 8, 1, 3, 9, 4],
  [9, 8, 1, 3, 4, 5, 2, 7, 6],
  [3, 7, 4, 9, 6, 2, 8, 1, 5]
]

#
# My solution:
#

class Validatable
  attr_reader :arr

  def initialize(arr)
    @arr = arr.flatten
  end

  def valid?
    valid_size? && valid_contents?
  end


  private

  def valid_contents?
    (1..9).all? { |num| arr.include?(num) }
  end

  def valid_size?
    arr.size == 9
  end
end

class Row < Validatable
end

class Column < Validatable
end

class Submatrix < Validatable
end

class Puzzle
  attr_reader :puzzle_array

  def initialize(puzzle_array)
    @puzzle_array = puzzle_array
  end

  def valid?
    validate(rows) && validate(columns) && validate(submatrices)
  end


  private

  def build_columns
    cols = Array.new(9) { Array.new }
    puzzle_array.each do |line|
      line.each_with_index do |num, index|
        cols[index] << num
      end
    end
    cols
  end

  def build_submatrices
    subs = Array.new(9) { Array.new }

    puzzle_array.slice(0,3).each do |line|
      subs[0] << line.slice(0,3)
      subs[1] << line.slice(3,3)
      subs[2] << line.slice(6,3)
    end
    puzzle_array.slice(3,3).each do |line|
      subs[3] << line.slice(0,3)
      subs[4] << line.slice(3,3)
      subs[5] << line.slice(6,3)
    end
    puzzle_array.slice(6,3).each do |line|
      subs[6] << line.slice(0,3)
      subs[7] << line.slice(3,3)
      subs[8] << line.slice(6,3)
    end

    subs
  end

  def columns
    @columns ||= build_columns.map { |col| Column.new(col) }
  end

  def rows
    @rows ||= puzzle_array.map { |line| Row.new(line) }
  end

  def submatrices
    @submatrices||= build_submatrices.map { |sub| Submatrix.new(sub) }
  end

  def validate(collection)
    collection.all?(&:valid?)
  end
end


$ puzzle = Puzzle.new(@good_puzzle)
$ puts puzzle.valid?.to_s

=> true

# The `def build_submatrices` method is a bit long and clunky. A possible refactor is below.
# It's shorter, but it's nothing like as readable. I'd probably leave the original as-is.

def build_submatrices
  subs = Array.new(9) { Array.new }
  [0,3,6].each do |start|
    puzzle_array.slice(start, 3).each do |line|
      [0,1,2].each do |num|
        subs[start + num] << line.slice((num * 3), 3)
      end
    end
  end
  subs
end