Implement PDF's multilayer updates and use it in getObj to display only the current version of the object taken into account instead of the concatenation of all its versions

Hufflepdf.cabal

@@ -20,6 +20,7 @@ library
                      , PDF.EOL
                      , PDF.Object
                      , PDF.Output
+                     , PDF.Update
   other-modules:       Data.ByteString.Char8.Util
                      , PDF.Body
                      , PDF.Parser

examples/getObj.hs

@@ -3,7 +3,7 @@
 import Codec.Compression.Zlib (decompress)
 import Data.ByteString.Lazy.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as BS (readFile)
-import qualified Data.ByteString.Lazy.Char8 as Lazy (concat, fromStrict, putStr, toStrict)
+import qualified Data.ByteString.Lazy.Char8 as Lazy (fromStrict, putStr, toStrict)
 import Data.Map ((!))
 import qualified Data.Map as Map (lookup)
 import PDF (Document(..), parseDocument)
@@ -11,6 +11,7 @@ import qualified PDF.EOL as EOL (Style)
 import PDF.Object (Content(..), DirectObject(..), Object(..), Name(..))
 import PDF.Output (ObjectId(..))
 import qualified PDF.Output as Output (render)
+import PDF.Update (unify)
 import System.Environment (getArgs)
 import System.IO (hPutStrLn, stderr)
 
@@ -26,12 +27,14 @@ display eolStyle s@(Stream {header, streamContent}) = Output.render eolStyle $
 
 extractObject :: ObjectId -> Document -> ByteString
 extractObject objectId (Document {eolStyle, updates}) =
-  Lazy.concat $ display eolStyle . (!objectId) . objects  <$> updates
+  display eolStyle . (!objectId) $ objects  content
+  where
+    content = unify updates
 
 main :: IO ()
 main = do
   [inputFile, objectId] <- getArgs
   result <- parseDocument <$> BS.readFile inputFile
   case result of
-    Left parseError -> hPutStrLn stderr $ show parseError
+    Left parseError -> hPutStrLn stderr parseError
     Right doc -> Lazy.putStr $ extractObject (ObjectId (read objectId)) doc

src/PDF/Update.hs

@@ -0,0 +1,47 @@
+{-# LANGUAGE NamedFieldPuns #-}
+module PDF.Update (
+    unify
+  ) where
+
+import Data.Map (member)
+import qualified Data.Map as Map (empty, union)
+import PDF.Object (
+      Content(..), IndexedObjects, IndirectObjCoordinates(..), Occurrence(..)
+    , Structure(..)
+  )
+
+emptyContent :: Content
+emptyContent = Content {
+      docStructure = Structure {xRef = Map.empty, trailer = Map.empty}
+    , objects = Map.empty
+    , occurrences = []
+  }
+
+unify :: [Content] -> Content
+unify = foldl complete emptyContent
+  where
+    complete tmpContent older =
+      let mergedObjects = Map.union (objects tmpContent) (objects older) in
+      Content {
+          docStructure =
+            unifyDocStructure (docStructure tmpContent) (docStructure older)
+        , objects = mergedObjects
+        , occurrences = 
+            unifyOccurrences mergedObjects (occurrences tmpContent) (occurrences older)
+      }
+
+unifyDocStructure :: Structure -> Structure -> Structure
+unifyDocStructure update original = Structure {
+      xRef = Map.union (xRef update) (xRef original)
+    , trailer = Map.union (trailer update) (trailer original)
+  }
+
+unifyOccurrences :: IndexedObjects -> [Occurrence] -> [Occurrence] -> [Occurrence]
+unifyOccurrences objects update = foldr addOlder update
+  where
+    addOlder occurrence@(Comment _) existing = occurrence : existing
+    addOlder occurrence@(Indirect indirect) existing =
+      if objectId indirect `member` objects
+      then occurrence : existing
+      else existing
+