-----------------------------------------------------------------------------

-- |

-- Module      :  Codec.Binary.Base64

-- Copyright   :  (c) Dominic Steinitz 2005, Warrick Gray 2002

-- License     :  BSD-style (see the file ReadMe.tex)

--

-- Maintainer  :  dominic.steinitz@blueyonder.co.uk

-- Stability   :  experimental

-- Portability :  portable

--

-- Base64 encoding and decoding functions provided by Warwick Gray. 

-- See <http://homepages.paradise.net.nz/warrickg/haskell/http/#base64> 

-- and <http://www.faqs.org/rfcs/rfc2045.html>.

--

-----------------------------------------------------------------------------

module Network.HTTP.Base64

   ( encode

   , decode

   , chop72

   , Octet

   ) where

{------------------------------------------------------------------------

This is what RFC2045 had to say:

6.8.  Base64 Content-Transfer-Encoding

   The Base64 Content-Transfer-Encoding is designed to represent

   arbitrary sequences of octets in a form that need not be humanly

   readable.  The encoding and decoding algorithms are simple, but the

   encoded data are consistently only about 33 percent larger than the

   unencoded data.  This encoding is virtually identical to the one used

   in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.

   A 65-character subset of US-ASCII is used, enabling 6 bits to be

   represented per printable character. (The extra 65th character, "=",

   is used to signify a special processing function.)

   NOTE:  This subset has the important property that it is represented

   identically in all versions of ISO 646, including US-ASCII, and all

   characters in the subset are also represented identically in all

   versions of EBCDIC. Other popular encodings, such as the encoding

   used by the uuencode utility, Macintosh binhex 4.0 [RFC-1741], and

   the base85 encoding specified as part of Level 2 PostScript, do not

   share these properties, and thus do not fulfill the portability

   requirements a binary transport encoding for mail must meet.

   The encoding process represents 24-bit groups of input bits as output

   strings of 4 encoded characters.  Proceeding from left to right, a

   24-bit input group is formed by concatenating 3 8bit input groups.

   These 24 bits are then treated as 4 concatenated 6-bit groups, each

   of which is translated into a single digit in the base64 alphabet.

   When encoding a bit stream via the base64 encoding, the bit stream

   must be presumed to be ordered with the most-significant-bit first.

   That is, the first bit in the stream will be the high-order bit in

   the first 8bit byte, and the eighth bit will be the low-order bit in

   the first 8bit byte, and so on.

   Each 6-bit group is used as an index into an array of 64 printable

   characters.  The character referenced by the index is placed in the

   output string.  These characters, identified in Table 1, below, are

   selected so as to be universally representable, and the set excludes

   characters with particular significance to SMTP (e.g., ".", CR, LF)

   and to the multipart boundary delimiters defined in RFC 2046 (e.g.,

   "-").

                    Table 1: The Base64 Alphabet

     Value Encoding  Value Encoding  Value Encoding  Value Encoding

         0 A            17 R            34 i            51 z

         1 B            18 S            35 j            52 0

         2 C            19 T            36 k            53 1

         3 D            20 U            37 l            54 2

         4 E            21 V            38 m            55 3

         5 F            22 W            39 n            56 4

         6 G            23 X            40 o            57 5

         7 H            24 Y            41 p            58 6

         8 I            25 Z            42 q            59 7

         9 J            26 a            43 r            60 8

        10 K            27 b            44 s            61 9

        11 L            28 c            45 t            62 +

        12 M            29 d            46 u            63 /

        13 N            30 e            47 v

        14 O            31 f            48 w         (pad) =

        15 P            32 g            49 x

        16 Q            33 h            50 y

   The encoded output stream must be represented in lines of no more

   than 76 characters each.  All line breaks or other characters not

   found in Table 1 must be ignored by decoding software.  In base64

   data, characters other than those in Table 1, line breaks, and other

   white space probably indicate a transmission error, about which a

   warning message or even a message rejection might be appropriate

   under some circumstances.

   Special processing is performed if fewer than 24 bits are available

   at the end of the data being encoded.  A full encoding quantum is

   always completed at the end of a body.  When fewer than 24 input bits

   are available in an input group, zero bits are added (on the right)

   to form an integral number of 6-bit groups.  Padding at the end of

   the data is performed using the "=" character.  Since all base64

   input is an integral number of octets, only the following cases can

   arise: (1) the final quantum of encoding input is an integral

   multiple of 24 bits; here, the final unit of encoded output will be

   an integral multiple of 4 characters with no "=" padding, (2) the

   final quantum of encoding input is exactly 8 bits; here, the final

   unit of encoded output will be two characters followed by two "="

   padding characters, or (3) the final quantum of encoding input is

   exactly 16 bits; here, the final unit of encoded output will be three

   characters followed by one "=" padding character.

   Because it is used only for padding at the end of the data, the

   occurrence of any "=" characters may be taken as evidence that the

   end of the data has been reached (without truncation in transit).  No

   such assurance is possible, however, when the number of octets

   transmitted was a multiple of three and no "=" characters are

   present.

   Any characters outside of the base64 alphabet are to be ignored in

   base64-encoded data.

   Care must be taken to use the proper octets for line breaks if base64

   encoding is applied directly to text material that has not been

   converted to canonical form.  In particular, text line breaks must be

   converted into CRLF sequences prior to base64 encoding.  The

   important thing to note is that this may be done directly by the

   encoder rather than in a prior canonicalization step in some

   implementations.

   NOTE: There is no need to worry about quoting potential boundary

   delimiters within base64-encoded bodies within multipart entities

   because no hyphen characters are used in the base64 encoding.

----------------------------------------------------------------------------}

{-

The following properties should hold:

  decode . encode = id

  decode . chop72 . encode = id

I.E. Both "encode" and "chop72 . encode" are valid methods of encoding input,

the second variation corresponds better with the RFC above, but outside of

MIME applications might be undesireable.

But: The Haskell98 Char type is at least 16bits (and often 32), these implementations assume only 

     8 significant bits, which is more than enough for US-ASCII.  

-}

import Data.Array (Array, array, (!))

import Data.Bits (shiftL, shiftR, (.&.), (.|.))

import Data.Char (chr, ord)

import Data.Word (Word8)

type Octet = Word8

encodeArray :: Array Int Char

encodeArray = array (0,64) 

          [ (0,'A'),  (1,'B'),  (2,'C'),  (3,'D'),  (4,'E'),  (5,'F')                    

          , (6,'G'),  (7,'H'),  (8,'I'),  (9,'J'),  (10,'K'), (11,'L')                    

          , (12,'M'), (13,'N'), (14,'O'), (15,'P'), (16,'Q'), (17,'R')

          , (18,'S'), (19,'T'), (20,'U'), (21,'V'), (22,'W'), (23,'X')

          , (24,'Y'), (25,'Z'), (26,'a'), (27,'b'), (28,'c'), (29,'d')

          , (30,'e'), (31,'f'), (32,'g'), (33,'h'), (34,'i'), (35,'j')

          , (36,'k'), (37,'l'), (38,'m'), (39,'n'), (40,'o'), (41,'p')

          , (42,'q'), (43,'r'), (44,'s'), (45,'t'), (46,'u'), (47,'v')

          , (48,'w'), (49,'x'), (50,'y'), (51,'z'), (52,'0'), (53,'1')

          , (54,'2'), (55,'3'), (56,'4'), (57,'5'), (58,'6'), (59,'7')

          , (60,'8'), (61,'9'), (62,'+'), (63,'/') ]

-- Convert between 4 base64 (6bits ea) integers and 1 ordinary integer (32 bits)

-- clearly the upmost/leftmost 8 bits of the answer are 0.

-- Hack Alert: In the last entry of the answer, the upper 8 bits encode 

-- the integer number of 6bit groups encoded in that integer, ie 1, 2, 3.

-- 0 represents a 4 :(

int4_char3 :: [Int] -> [Char]

int4_char3 (a:b:c:d:t) = 

    let n = (a `shiftL` 18 .|. b `shiftL` 12 .|. c `shiftL` 6 .|. d)

    in (chr (n `shiftR` 16 .&. 0xff))

     : (chr (n `shiftR` 8 .&. 0xff))

     : (chr (n .&. 0xff)) : int4_char3 t

int4_char3 [a,b,c] =

    let n = (a `shiftL` 18 .|. b `shiftL` 12 .|. c `shiftL` 6)

    in [ (chr (n `shiftR` 16 .&. 0xff))

       , (chr (n `shiftR` 8 .&. 0xff)) ]

int4_char3 [a,b] = 

    let n = (a `shiftL` 18 .|. b `shiftL` 12)

    in [ (chr (n `shiftR` 16 .&. 0xff)) ]

int4_char3 [_] = error "Network.HTTP.Base64.int4_char3: impossible number of Ints."

int4_char3 [] = []

-- Convert triplets of characters to

-- 4 base64 integers.  The last entries

-- in the list may not produce 4 integers,

-- a trailing 2 character group gives 3 integers,

-- while a trailing single character gives 2 integers.

char3_int4 :: [Char] -> [Int]

char3_int4 (a:b:c:t) 

    = let n = (ord a `shiftL` 16 .|. ord b `shiftL` 8 .|. ord c)

      in (n `shiftR` 18 .&. 0x3f) : (n `shiftR` 12 .&. 0x3f) : (n `shiftR` 6  .&. 0x3f) : (n .&. 0x3f) : char3_int4 t

char3_int4 [a,b]

    = let n = (ord a `shiftL` 16 .|. ord b `shiftL` 8)

      in [ (n `shiftR` 18 .&. 0x3f)

         , (n `shiftR` 12 .&. 0x3f)

         , (n `shiftR` 6  .&. 0x3f) ]

char3_int4 [a]

    = let n = (ord a `shiftL` 16)

      in [(n `shiftR` 18 .&. 0x3f),(n `shiftR` 12 .&. 0x3f)]

char3_int4 [] = []

-- Retrieve base64 char, given an array index integer in the range [0..63]

enc1 :: Int -> Char

enc1 ch = encodeArray!ch

-- | Cut up a string into 72 char lines, each line terminated by CRLF.

chop72 :: String -> String

chop72 str =  let (bgn,end) = splitAt 70 str

              in if null end then bgn else "\r\n" ++ chop72 end

-- Pads a base64 code to a multiple of 4 characters, using the special

-- '=' character.

quadruplets :: [Char] -> [Char]

quadruplets (a:b:c:d:t) = a:b:c:d:quadruplets t

quadruplets [a,b,c]     = [a,b,c,'=']      -- 16bit tail unit

quadruplets [a,b]       = [a,b,'=','=']    -- 8bit tail unit

quadruplets [_]         = error "Network.HTTP.Base64.quadruplets: impossible number of characters."

quadruplets []          = []               -- 24bit tail unit

enc :: [Int] -> [Char]

enc = quadruplets . map enc1

dcd :: String -> [Int]

dcd [] = []

dcd (h:t)

    | h <= 'Z' && h >= 'A'  =  ord h - ord 'A'      : dcd t

    | h >= '0' && h <= '9'  =  ord h - ord '0' + 52 : dcd t

    | h >= 'a' && h <= 'z'  =  ord h - ord 'a' + 26 : dcd t

    | h == '+'  = 62 : dcd t

    | h == '/'  = 63 : dcd t

    | h == '='  = []  -- terminate data stream

    | otherwise = dcd t

-- Principal encoding and decoding functions.

encode :: [Octet] -> String

encode = enc . char3_int4 . (map (chr .fromIntegral))

{-

prop_base64 os =

   os == (f . g . h) os

      where types = (os :: [Word8])

            f = map (fromIntegral. ord)

            g = decode . encode

            h = map (chr . fromIntegral)

-}

decode :: String -> [Octet]

decode = (map (fromIntegral . ord)) . int4_char3 . dcd