-- http://www.w3.org/TR/PNG/
-- Based on http://mainisusuallyafunction.blogspot.se/2012/04/minimal-encoder-for-uncompressed-pngs.html
module PNGchunks(PNGstream(..),Chunk(..),png_chunk,bytes,png_signature,crc32) where
import Data.Bits hiding (bit)
import Data.Word(Word32)
import qualified Data.ByteString.Lazy as BS
import qualified Data.ByteString.Lazy.Char8 as CS
import Byte(ord)
import ByteStream(Bytes(..),be32)

png_signature = CS.pack "\x89PNG\r\n\x1a\n"

data PNGstream = PNGstream [Chunk] deriving Show

instance Bytes PNGstream where
  bytes (PNGstream chunks) = png_signature `BS.append` bytes chunks

data Chunk = Chunk {len::Word32,typ::FourCC,bs::BS.ByteString,crc::Word32}
           deriving (Eq,Show)

type FourCC = String
-- ^ Four ASCII characters

instance Bytes Chunk where
  bytes (Chunk len ty bs crc) = BS.concat [be32 len,CS.pack ty,bs,be32 crc]

png_chunk ty bytes
    | n<=maxInt32 = Chunk n ty bytes (crc32 (ty'++BS.unpack bytes))
  where n = fromIntegral (BS.length bytes)
        ty' = BS.unpack $ CS.pack ty

maxInt32 = 0x7fffffff

crc32 bs = complement $ bytes (complement 0) bs
  where
    bytes crc [] = crc
    bytes crc (b:bs) | crc>=0 = bytes (byte crc b) bs -- strictness trick

byte a x = bit . bit . bit . bit . bit . bit . bit . bit $ xor a (ord x)
bit c = if odd c then k `xor` shiftR c 1 else shiftR c 1
--k = -306674912
k = 0xedb88320