Added Text.Pandoc.ImageSize.

This is intened for use in docx and odt writers, so the size
of image boxes can be calculated.

pandoc.cabal

@@ -277,6 +277,7 @@ Library
                    Text.Pandoc.UTF8,
                    Text.Pandoc.MIME,
                    Text.Pandoc.UUID,
+                   Text.Pandoc.ImageSize,
                    Paths_pandoc
 
   if flag(library)

src/Text/Pandoc/ImageSize.hs

@@ -0,0 +1,110 @@
+{-# LANGUAGE OverloadedStrings, ScopedTypeVariables  #-}
+{-
+Copyright (C) 2011 John MacFarlane <jgm@berkeley.edu>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+-}
+
+{- |
+   Module      : Text.Pandoc.ImageSize
+   Copyright   : Copyright (C) 2011 John MacFarlane
+   License     : GNU GPL, version 2 or above
+
+   Maintainer  : John MacFarlane <jgm@berkeley.edu>
+   Stability   : alpha
+   Portability : portable
+
+Functions for determining the size of a PNG, JPEG, or GIF image.
+
+Algorithms borrowwed from wwwis.pl (c) 2005 Alex K, released
+under the GPL.
+-}
+module Text.Pandoc.ImageSize ( imageType, imageSize, readImageSize ) where
+import Data.ByteString.Lazy (ByteString, unpack)
+import qualified Data.ByteString.Lazy.Char8 as B
+import Control.Monad
+import Data.Bits
+
+-- quick and dirty functions to get image sizes
+-- algorithms borrowed from wwwis.pl
+
+data ImageType = Png | Gif | Jpeg deriving Show
+
+type Height = Integer
+type Width = Integer
+
+readImageSize :: FilePath -> IO (Maybe (Width,Height))
+readImageSize fp = imageSize `fmap` B.readFile fp
+
+imageType :: ByteString -> Maybe ImageType
+imageType img = case B.take 4 img of
+                     "\x89\x50\x4e\x47" -> return Png
+                     "\x47\x49\x46\x38" -> return Gif
+                     "\xff\xd8\xff\xe0" -> return Jpeg
+                     _                  -> fail "Unknown image type"
+
+imageSize :: ByteString -> Maybe (Width,Height)
+imageSize img = do
+  t <- imageType img
+  case t of
+       Png  -> pngSize img
+       Gif  -> gifSize img
+       Jpeg -> jpegSize img
+
+pngSize :: ByteString -> Maybe (Width,Height)
+pngSize img = do
+  let (h, rest) = B.splitAt 8 img
+  guard $ h == "\x8a\x4d\x4e\x47\x0d\x0a\x1a\x0a" ||
+          h == "\x89\x50\x4e\x47\x0d\x0a\x1a\x0a"
+  let (i, rest') = B.splitAt 4 $ B.drop 4 rest
+  guard $ i == "MHDR" || i == "IHDR"
+  case map fromIntegral $ unpack $ B.take 8 rest' of
+       ([w1,w2,w3,w4,h1,h2,h3,h4] :: [Integer]) -> return
+         ((shift w1 24) + (shift w2 16) + (shift w3 8) + w4,
+          (shift h1 24) + (shift h2 16) + (shift h3 8) + h4)
+       _ -> fail "PNG parse error"
+
+gifSize :: ByteString -> Maybe (Width,Height)
+gifSize img = do
+  let (h, rest) = B.splitAt 6 img
+  guard $ h == "GIF87a" || h == "GIF89a"
+  case map fromIntegral $ unpack $ B.take 4 rest of
+       [w2,w1,h2,h1] -> return (shift w1 8 + w2, shift h1 8 + h2)
+       _             -> fail "GIF parse error"
+
+jpegSize :: ByteString -> Maybe (Width,Height)
+jpegSize img = do
+  let (h, rest) = B.splitAt 2 img
+  guard $ h == "\xff\xd8"
+  findJpegLength rest
+
+findJpegLength :: ByteString -> Maybe (Width,Height)
+findJpegLength bs = do
+  let bs' = B.dropWhile (=='\xff') $ B.dropWhile (/='\xff') bs
+  case B.uncons bs' of
+       Just (c,bs'') | c >= '\xc0' && c <= '\xc3' -> do
+         case map fromIntegral $ unpack $ B.take 4 $ B.drop 3 bs'' of
+              [h1,h2,w1,w2] -> return (shift w1 8 + w2, shift h1 8 + h2)
+              _             -> fail "JPEG parse error"
+       Just (_,bs'') ->  do
+         case map fromIntegral $ unpack $ B.take 2 bs'' of
+              [c1,c2] -> do
+                let len = shift c1 8 + c2
+                -- skip variables
+                findJpegLength $ B.drop len bs''
+              _       -> fail "JPEG parse error"
+       Nothing -> fail "Did not find length record"
+
+