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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

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%?
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using Primes

function calc()
  start, step = 1,2
  iterations, currentsquaresize = 3,3
  cprimes, call = 0,1
  while true
    start += step
    for i in start:step:(start+(iterations*step))
      if isprime(i)
        cprimes = cprimes + 1
      end
      call = call + 1
    end
    ratio = cprimes / call
    if ratio < 0.1
      return currentsquaresize
    end
    start += (iterations*step)
    currentsquaresize += 2
    step += 2
  end
end
@time println(calc())
