Bijective.hs

import Data.List
import Data.Maybe

alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
base = length alphabet

encode :: Int -> String
encode 0 = "a"
encode id = map (\x -> alphabet !! x) (reverse (tail (map (\x -> snd x) options)))
       where options = takeWhile (/= (0,0)) (iterate (\x -> ((fst x `div` 62), (fst x `mod` 62))) (id,0))

decode :: String -> Int
decode encoded = foldl (\x y -> x * base + fromJust y) 0 (map (\x -> elemIndex x alphabet) encoded)

-- encode 125
-- decode "cb"
-- encode 19158
-- decode "e9a"