Ulam spiral

ulam_spiral.rb

require 'prime'

def ulam_spiral(n)
  matrix = Array.new(n) { Array.new(n) }
  path = [*1..n*n].reverse
  padding = (n*n).to_s.size
  layer = 0
  until path.empty?
    matrix[layer].map! { |l| l || path.shift }
    matrix = matrix.transpose.reverse
    layer += 1 unless matrix[layer].include?(nil)
  end
  matrix = matrix.transpose.reverse until matrix[0][0] == n*n
  matrix.transpose.each do |row|
    row.each do |l|
      if l.prime?
        print "%0#{padding}d" % l, ' '
      else
        print ' ' * (padding + 1)
      end
    end
    puts
  end
end

ulam_spiral(100)