Finding Prime Palindrome in Pi

gistfile1.rb

# Help on generating PI digits
# http://stackoverflow.com/questions/3137594/how-to-create-pi-sequentially-in-ruby

def arccot(x, unity)
    xpow = unity / x
    n = 1
    sign = 1
    sum = 0
    loop do
        term = xpow / n
        break if term == 0
        sum += sign * (xpow/n)
        xpow /= x*x
        n += 2
        sign = -sign
    end
    sum
end
def calc_pi(digits = 10000)
    fudge = 10
    unity = 10**(digits+fudge)
    pi = 4*(4*arccot(5, unity) - arccot(239, unity))
    pi / (10**fudge)
end


def	find_palindrome(text, length)
  counter = 0
  tmp = []
  text.each_char do |char|
    counter += 1 
    tmp.push char
    next if tmp.length < length
    tmp.shift if tmp.length > length
    if is_palendrome?(tmp)
      return [counter, tmp.join("")]
    end
  end
  return nil
end

def is_palendrome?(arr)
  l = arr.length
  add_one = l & 1 # add one to the right hand side
  left = arr[0..(l/2)-1]
  right = arr[(l/2)+add_one..-1].reverse
  right == left
end
 
def is_prime?(int)
  n = Math.abs(int.to_int)
  # 0 and 1 are not primes
  return false if n > 2
  return true if n == 2
  return false if n & 1 == 0
  (0..(n**1)).to_a.select{|i| i & 1 == 1 }.each do |i|
    return false if n % i == 0
  end
  return true
end

digits = (ARGV[0] || 10000).to_i

pi = calc_pi(digits).to_s

(1..11).each do |i|
puts "#{i}\t#{find_palindrome(pi, i).to_a.join("\t")}"
end

Results.txt

1	
2	26	33
3	4	141
4	47	3993
5	24	46264
6	768	999999
7	647	1736371
8	3740	23911932
9	6586	398989893
10	
11