Avoid squeezing shared content streams

cpdf.ml

@@ -109,9 +109,13 @@ let really_squeeze pdf =
           pdf.Pdf.objects <- !pdfr.Pdf.objects;
           pdf.Pdf.trailerdict <- !pdfr.Pdf.trailerdict
 
-(* Squeeze the form xobject at objnum. FIXME: For old PDFs (< v1.2) any
+(* Squeeze the form xobject at objnum.
-resources from the page (or its ancestors in the page tree!) are also needed -
+
-we must merge them with the ones from the xobject itself. *)
+FIXME: For old PDFs (< v1.2) any resources from the page (or its ancestors in
+the page tree!) are also needed - we must merge them with the ones from the
+xobject itself. However, it it safe for now -- in the unlikely event that the
+resources actually need to be available, the parse will fail, the squeeze of
+this object will fail, and we bail out. *)
 let xobjects_done = ref []
 
 let squeeze_form_xobject pdf objnum =
@@ -142,9 +146,36 @@ let squeeze_form_xobject pdf objnum =
             end
       | _ -> ()
 
+(* For a list of indirects representing content streams, make sure that none of
+them are duplicated in the PDF. This indicates sharing, which parsing and
+rewriting the streams might destroy, thus making the file bigger. FIXME: The
+correct thing to do is to preserve the multiple content streams. *)
+let no_duplicates content_stream_numbers stream_numbers =
+  not
+    (mem false
+       (map
+         (fun n -> length (keep (eq n) content_stream_numbers) < 2)
+         stream_numbers))
+
+(* Give a list of content stream numbers, given a page reference number *)
+let content_streams_of_page pdf refnum =
+  match Pdf.direct pdf (Pdf.lookup_obj pdf refnum) with
+    Pdf.Dictionary dict ->
+      begin match lookup "/Contents" dict with
+        Some (Pdf.Indirect i) -> [i]
+      | Some (Pdf.Array x) ->
+          option_map (function Pdf.Indirect i -> Some i | _ -> None) x
+      | _ -> []
+      end
+  | _ -> []
+
 (* For each object in the PDF marked with /Type /Page, for each /Contents
 indirect reference or array of such, decode and recode that content stream. *)
 let squeeze_all_content_streams pdf =
+  let page_reference_numbers = Pdf.page_reference_numbers pdf in
+    let all_content_streams_in_doc =
+      flatten (map (content_streams_of_page pdf) page_reference_numbers)
+    in
       xobjects_done := [];
       Pdf.objiter
         (fun objnum _ ->
@@ -159,7 +190,6 @@ let squeeze_all_content_streams pdf =
                   | None -> Pdf.Dictionary []
                 in
                   begin try
-                let newstream =
                     let content_streams =
                       match lookup "/Contents" dict with
                         Some (Pdf.Indirect i) ->
@@ -170,6 +200,12 @@ let squeeze_all_content_streams pdf =
                       | Some (Pdf.Array x) -> x
                       | _ -> raise Not_found
                     in
+                      if
+                        no_duplicates
+                          all_content_streams_in_doc
+                          (map (function Pdf.Indirect i -> i | _ -> assert false) content_streams)
+                      then
+                        let newstream =
                           Pdfops.stream_of_ops
                             (Pdfops.parse_operators pdf resources content_streams)
                         in