JGC's "one-eyed robot" problem, implemented in Python. See http://dustingram.com/articles/2013/05/02/z-machines-and-one-eyed-robots/

oneeyedrobot.py

  #!/usr/bin/python
  
  import random
  
  class ball:
  
      def __init__(self, color):
          self.color = color
          self.examined = 0
  
      def look(self):
          self.examined += 1
          return self.color
  
  class buckets:
  
      def __init__(self, size=15):
          self.line = [ball(random.choice(['r', 'g', 'b'])) for _ in range(size)]
          self.i = 0
          self.r = 0
          self.b = size-1
  
      def swap(self, i, j):
          self.line[i], self.line[j] = self.line[j], self.line[i]
  
      def check(self):
          c = self.line[self.i].look()
          if c == 'r':
              self.swap(self.i, self.r)
              self.r += 1
              self.i += 1
          elif c == 'b':
              self.swap(self.i, self.b)
              self.b -= 1
          else:
              self.i += 1
  
      def solve(self):
          while self.i <= self.b:
              self.check()
  
  b = buckets()
  print([x.color for x in b.line])
  
  b.solve()
  
  print([x.color for x in b.line])
  print([x.examined for x in b.line])