Fastest method for adding/summing the individual digit components of a number

Question

I saw a question on a math forum a while back where a person was discussing adding up the digits in a number over and over again until a single digit is achieved. (i.e. \"362\"

Answer 1

Here's something in Haskell:

sumDigits n = 
  if n == 0 
     then 0
     else let a = mod n 10
          in a + sumDigits (div n 10)

Oh, but I just read you're doing that already...

(then there's also the obvious:

sumDigits n = sum $ map (read . (:[])) . show $ n

)