test :: Int -> Int
test x | even x     = x `div` 2
       | otherwise  = 3*x + 1

collatz :: Int -> [Int]
collatz 1 = [1]
collatz x = x : collatz (test x)

numCollatz = map (length . collatz) [1..]

-- Convert into a table for gnuplot
format x y = show x ++ "\t" ++ show y
table x = unlines (zipWith format [1..] (take x numCollatz))

main = writeFile "collatz.dat" (table 100001)