ruby で円周率

pi1.rb

# See http://keibakuroku.jp/category/brain-teaser

def pi_1(len)
  count = 0
  k, a, b, a1, b1 = 2, 4, 1, 12, 4

  while TRUE
    # Next approximation
    p, q, k = k*k, 2*k+1, k+1
    a, b, a1, b1 = a1, b1, p * a + q * a1, p * b + q * b1
    # Print common digits
    d = a / b
    d1 = a1 / b1
    while d == d1
      print d
      $stdout.flush

      print "." if count == 0
      count += 1
      return if count > len

      a, a1 = 10 * (a % b), 10 * (a1 % b1)
      d, d1 = a / b, a1 / b1
    end
  end
end

len = 100
len = ARGV[0].to_i if ARGV.size == 1
pi_1(len)
puts

pi1.txt

3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679

pi2.rb

# See http://keibakuroku.jp/category/brain-teaser

def pi_2(len)
  b = (10 ** len).freeze
  b2 = (b << 1).freeze
  pi = (len * 8 + 1).step(3, -2).inject(b) {|a, i| (i >> 1) * (a + b2) / i} - b
  puts "3.#{pi}"
end

len = 100
len = ARGV[0].to_i if ARGV.size == 1
pi_2(len)

pi2.txt

3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679