fact2 is O(log n) while fact is O(n).
fact2 is partitioning the problem.
Why fact2 is quicker than fact?!
fact2 :: Integer -> Integer
fact2 x = f x y
where
f n e | n < 2 = 1
| e == 0 = n * (n - 1)
| e > 0 = (f n (e `div` 2)) * (f (n - (e * 2)) (e `div` 2))
y = 2 ^ (truncate (log (fromInteger x) / log 2))
fact :: Integer -> Integer
fact 1 = 1
fact n = n * fact (n - 1)
I tried to write tail-recursive fact, fact as "product [1..n]" - fact2 is quicker!
fact2 1000000 == fact 1000000 - I tested.
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe