From: David Tran <email55555@...>
Date: 2006-08-16T06:00:03+09:00
Subject: [Solution] [QUIZ] Pen and Paper (#90)

# Same logic as my first solution, simply make it more OO and clean up a bit.


class PenAndPaperGame
  MOVES = [[3,0], [-3,0], [0,3], [0,-3], [2,2], [2,-2], [-2,2], [-2,-2]]

  def initialize(size)
    @size = size
    @largest = @size * @size
    @board = Array.new(@size) { Array.new(@size) }
  end

  def to_s
    return "No solution." unless @board[0][0]
    digits = @largest.to_s.size
    " #{'-' * (digits+2) * @size}\n|" +
    (@board.map { |l| l.map{|e| " %#{digits}d " % e}.join }.join("|\n|")) +
    "|\n #{'-' * (digits+2) * @size}\n"
  end

  def solve
    @size.times { |x| @size.times { |y| return self if jump(1, x, y) } }
    self
  end

  private

  def valid(x, y)
    x >= 0 && x < @size && y >= 0 && y < @size && !@board[x][y]
  end

  def nb_exit(x, y)
    MOVES.inject(0) { |m, (i, j)| valid(x+i, y+j) ? m+1 : m }
  end

  def jump(nb, x, y)
    @board[x][y] = nb
    return true if nb == @largest
    MOVES.map { |i,j| [x+i, y+j] }.select { |i,j| valid(i,j) }.
      sort_by { |i,j| nb_exit(i,j) }.each { |i,j| return true if
jump(nb+1, i, j) }
    @board[x][y] = nil
  end
end

if __FILE__ == $0
  size = (ARGV[0] || 5).to_i
  puts PenAndPaperGame.new(size).solve
end