text: # Problem 58 : Spiral Primes
  
  # Starting with 1 and spiralling anticlockwise in the following way, a square
  # spiral with side length 7 is formed.
  #
  #                   37 36 35 34 33 32 31
  #                   38 17 16 15 14 13 30
  #                   39 18  5  4  3 12 29
  #                   40 19  6  1  2 11 28
  #                   41 20  7  8  9 10 27
  #                   42 21 22 23 24 25 26
  #                   43 44 45 46 47 48 49
  p#
  # It is interesting to note that the odd squares lie along the bottom right
  # diagonal, but what is more interesting is that 8 out of the 13 numbers lying
  # along both diagonals are prime; that is, a ratio of 8/13 ≈ 62%.
  #
  # If one complete new layer is wrapped around the spiral above, a square spiral
  # with side length 9 will be formed. If this process is continued, what is the
  # side length of the square spiral for which the ratio of primes along both
  # diagonals first falls below 10%?
  