base64.hs

import Data.Char (ord)
import Data.Bits

base64 :: String -> String
base64 "" = ""
base64 xs = indexToChar (concat24Bits xs) ++ padding xs

concat24Bits :: String -> [Int]
concat24Bits (x:[]) =
  let n = shiftL (ord x::Int) 16
  in split6Bits 1 n
concat24Bits (x:y:[]) =
  let n = shiftL (ord x::Int) 16 .|. shiftL (ord y::Int) 8
  in split6Bits 2 n
concat24Bits (x:y:z:[]) =
  let n = shiftL (ord x::Int) 16 .|. shiftL (ord y::Int) 8 .|. ord z
  in split6Bits 3 n
concat24Bits (x:y:z:rest) =
  let n = shiftL (ord x::Int) 16 .|. shiftL (ord y::Int) 8 .|. ord z
  in split6Bits 3 n ++ concat24Bits rest

split6Bits :: Int -> Int -> [Int]
split6Bits 1 n = [shiftR n 18 .&. 63] ++ [shiftR n 12 .&. 63]
split6Bits 2 n = split6Bits 1 n ++ [shiftR n 6 .&. 63]
split6Bits 3 n = split6Bits 2 n ++ [n .&. 63]

indexToChar :: [Int] -> String
indexToChar xs =
  let table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
  in [ table !! n | n <- xs ]

padding :: String -> String
padding xs =
  let p = length xs `mod` 3
  in case p of 0 -> ""
               1 -> "=="
               2 -> "="