collatz

#!/usr/bin/python
import sys
sys.setrecursionlimit(10000)
max = 1000

memo = {}
def collatz(x):
  global memo
  if x in memo: pass
  elif x % 2: memo[x] = x * 3 + 1
  else: memo[x] = x / 2

def follow(x):
  collatz(x)
  if memo[x] > 1: follow(memo[x])

if len(sys.argv) > 1: max = int(sys.argv[1])
for i in range(2, max):
  follow(i)

print("digraph collatz {")
for pair in memo.items():
  print("\t%d -> %d;" % pair)
print("}")