File: pdfcodec.ml

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(* \chaptertitle{PDFCodec}{PDF compression and decompression} *)
open Utility
open Pdfio
open Pdf

(* \section{Preliminaries} *)

(* Get the next non-whitespace character in a stream. *)
let rec get_streamchar skipped s =
  match s.input_byte () with
  | x when x = Pdfio.no_more -> dpr "I"; raise End_of_file
  | x ->
    let chr = char_of_int x in
      if is_whitespace chr then
        begin
          incr skipped;
          get_streamchar skipped s
        end
      else chr

(* Same, but return an option type instead of raising an exception at end of
input. *)
let get_streamchar_option skipped s =
  try Some (get_streamchar skipped s) with
    End_of_file -> dpr "2O"; None

(* \intf Raised if there was bad data. *)
exception Couldn'tDecodeStream of string

(* \intf Raised if the codec was not supported. *)
exception DecodeNotSupported of string

(* \section{ASCIIHex} *)

(* We build a list of decoded characters from the input stream, and then
convert this to the output stream. *)
let encode_ASCIIHex stream =
  let size = stream_size stream in
    let stream' = mkstream (size * 2 + 1) in
      sset stream' (size * 2) (int_of_char '>'); (*r ['>'] is end-of-data *)
      for p = 0 to size - 1 do
        let chars = explode (Printf.sprintf "%02X" (sget stream p)) in
          sset stream' (p * 2) (int_of_char (hd chars));
          sset stream' (p * 2 + 1) (int_of_char (hd (tl chars)))
      done;
      stream'

(* Decode ASCIIHex *)

(* Calulate a character from two hex digits a and b *)
let char_of_hex a b =
  char_of_int (int_of_string ("0x" ^ string_of_char a ^ string_of_char b))

let decode_ASCIIHex i =
  let output = ref []
  and enddata = ref false in
    try
      while not !enddata do
        let b = get_streamchar (ref 0) i in
          let b' = get_streamchar (ref 0) i in
            match b, b' with
            | '>', _ -> set enddata
            | ('0'..'9' | 'a'..'f' | 'A'..'F') as c, '>' ->
                output =| char_of_hex c '0';
                set enddata
            | ('0'..'9' | 'a'..'f' | 'A'..'F' as c),
              ('0'..'9' | 'a'..'f' | 'A'..'F' as c') ->
                output =| char_of_hex c c'
            | _ -> raise Not_found (*r Bad data. *)
      done;
      bytestream_of_charlist (rev !output)
    with
      | End_of_file ->
          (* We ran out of data. This is a normal exit. *)
          dpr "J";
          bytestream_of_charlist (rev !output)
      | Not_found ->
          raise (Couldn'tDecodeStream "ASCIIHex")

(* \section{ASCII85}*)

(* Decode five characters. *)
let decode_5bytes (c1, c2, c3, c4, c5) =
  let d x p =
    i32mul (i32ofi (int_of_char x - 33)) (i32ofi (pow p 85))
  in
    let total =
      fold_left i32add 0l [d c1 4; d c2 3; d c3 2; d c4 1; d c5 0]
    in
      let extract t =
        char_of_int (i32toi (lsr32 (lsl32 total (24 - t)) 24))
      in
        extract 24, extract 16, extract 8, extract 0

(* Main function *)        
let decode_ASCII85 i =
  let output = ref []
  and enddata = ref false
  and skipped = ref 0 in
    try
      while not !enddata do
        let c1 = get_streamchar_option skipped i in
        (*r Ignore any whitespace skipped before getting to the first char of
         interest. This prevents us sliding too much back and picking up z
         characters twice. *)
        skipped := 0;
        let c2 = get_streamchar_option skipped i in
        let c3 = get_streamchar_option skipped i in
        let c4 = get_streamchar_option skipped i in
        let c5 = get_streamchar_option skipped i in
          let ischar c = c >= '!' && c <= 'u' in
            match c1, c2, c3, c4, c5 with
            | Some 'z', _, _, _, _ ->
                i.seek_in (possub (i.pos_in ()) (posadd (posofi 4) (posofi !skipped)));
                output := '\000'::'\000'::'\000'::'\000'::!output
            | Some c1, Some c2, Some c3, Some c4, Some c5 when
                ischar c1 && ischar c2 && ischar c3 && ischar c4 && ischar c5 ->
                  let b1, b2, b3, b4 = decode_5bytes (c1, c2, c3, c4, c5) in
                    output := b4::b3::b2::b1::!output
            | Some '~', Some '>', _, _, _ ->
                set enddata
            | Some c1, Some c2, Some '~', Some '>', _
                when ischar c1 && ischar c2 ->
                  let b1, b2, b3, b4 = decode_5bytes (c1, c2, '~', '>', '!') in
                    set enddata; output := b1::!output
            | Some c1, Some c2, Some c3, Some '~', Some '>'
                when ischar c1 && ischar c2 && ischar c3 ->
                  let b1, b2, _, _ = decode_5bytes (c1, c2, c3, '~', '>') in
                    set enddata; output := b2::b1::!output
            | Some c1, Some c2, Some c3, Some c4, Some '~'
                when ischar c1 && ischar c2 && ischar c3 && ischar c4 ->
                  let b1, b2, b3, _ = decode_5bytes (c1, c2, c3, c4, '~') in
                    set enddata; output := b3::b2::b1::!output
            | _ -> raise End_of_file
      done;
      bytestream_of_charlist (rev !output)
    with
      End_of_file -> raise (Couldn'tDecodeStream "ASCII85")

(* Encode a single symbol set. *)
let encode_4bytes = function
  | [b1; b2; b3; b4] ->
      let ( * ) = Int64.mul
      and ( - ) = Int64.sub
      and ( / ) = Int64.div 
      and rem = Int64.rem in
        let numbers =
          [i64ofi (int_of_char b1) * i64ofi (pow 3 256);
           i64ofi (int_of_char b2) * i64ofi (pow 2 256);
           i64ofi (int_of_char b3) * i64ofi (pow 1 256);
           i64ofi (int_of_char b4) * i64ofi (pow 0 256)]
        in
          let t = fold_left Int64.add Int64.zero numbers
          and one85 = i64ofi (pow 1 85) and two85 = i64ofi (pow 2 85)
          and three85 = i64ofi (pow 3 85) and zero85 = i64ofi (pow 0 85)
          and four85 = i64ofi (pow 4 85) in
            let t, c5 = t - rem t one85, rem t one85 / zero85 in
              let t, c4 = t - rem t two85, rem t two85 / one85 in
                let t, c3 = t - rem t three85, rem t three85 / two85 in
                  let t, c2 = t - rem t four85, rem t four85 / three85 in
                    i64toi (t / four85), i64toi c2, i64toi c3, i64toi c4, i64toi c5
  | _ -> raise (Assert_failure ("encode_4bytes", 0, 0))

(* Encode a stream. *)
let encode_ASCII85 stream =
  let output = ref []
  and enddata = ref false
  and istream = input_of_bytestream stream in
    while not !enddata do
      let b1 = istream.input_char () in
      let b2 = istream.input_char () in
      let b3 = istream.input_char () in
      let b4 = istream.input_char () in
        match b1, b2, b3, b4 with
        | Some b1, Some b2, Some b3, Some b4 ->
            output := [b1; b2; b3; b4]::!output
        | Some b1, Some b2, Some b3, None ->
            set enddata; output := [b1; b2; b3]::!output
        | Some b1, Some b2, None, None ->
            set enddata; output := [b1; b2]::!output
        | Some b1, None, None, None ->
            set enddata; output := [b1]::!output
        | None, _, _, _ -> set enddata
        | _ -> assert false
    done;
    let fix k = char_of_int (k + 33) in
      let charlists' =
        rev_map
          (fun l ->
             let len = length l in
               if len < 4
               then
                 let l' = l @ (many '\000' (4 - len)) in
                   let c1, c2, c3, c4, c5 = encode_4bytes l' in
                     take [fix c1; fix c2; fix c3; fix c4; fix c5] (len + 1)
               else
                   let c1, c2, c3, c4, c5 = encode_4bytes l in
                     if c1 + c2 + c3 + c4 + c5 = 0
                       then ['z']
                       else [fix c1; fix c2; fix c3; fix c4; fix c5])
          !output
        in
          bytestream_of_charlist (flatten charlists' @ ['~'; '>'])

(* \section{Flate} *)

(* Make a bytestream from a list of strings by taking the contents, in order
from the items, in order. *)
let bytestream_of_strings strings =
  let total_length =
    fold_left ( + ) 0 (map String.length strings)
  in
    let s = mkstream total_length
    and pos = ref 0 in
      iter
        (fun str ->
           for x = 0 to String.length str - 1 do
             sset s !pos (int_of_char str.[x]); incr pos
           done)
        strings;
      s


(*IF-OCAML*)
let flate_process f data =
  let strings = ref []
  and pos = ref 0
  and inlength = stream_size data in
    let input =
      (fun buf ->
         let s = String.length buf in
           let towrite = min (inlength - !pos) s in
             for x = 0 to towrite - 1 do
               buf.[x] <- char_of_int (sget data !pos); incr pos
             done;
             towrite)
    and output =
      (fun buf length ->
         if length > 0 then strings =| String.sub buf 0 length)
    in
      f input output;
      bytestream_of_strings (rev !strings)

let decode_flate_input i =
  let strings = ref [] in
    let input =
      (fun buf ->
         let s = String.length buf in
           if s > 0 then
             begin
               match i.input_byte () with
               | x when x = Pdfio.no_more -> raise End_of_file
               | x ->
                   buf.[0] <- char_of_int x; 1
             end
           else 0)
    and output =
      (fun buf length ->
         if length > 0 then strings =| String.sub buf 0 length)
    in
      Zlib.uncompress input output;
      bytestream_of_strings (rev !strings)

let encode_flate stream =
  flate_process Zlib.compress stream

let decode_flate stream =
  try flate_process Zlib.uncompress stream with
    Zlib.Error (a, b) -> raise (Couldn'tDecodeStream "Flate")
(*ENDIF-OCAML*)

(*i*)(*F#
(* When decoding from an input (for an inline image), we proceed one byte at a
time so as to leave the input pointer at the correct place. *)
let io_of_pdfio i =
  let read () =
    match i.input_byte () with
    | x when x = Pdfio.no_more -> raise Io.No_more_input
    | x -> char_of_int x
  and input s i j =
    0
  and close () =
    ()
  in
    Io.create_in read input close

(* Encode a bigarray using the ZSharpLib library, returning another bigarray. *)
let encode_flate s =
  let arr = Array.make (stream_size s) (Byte.of_int 0) in
    for x = 0 to Array.length arr - 1 do
      arr.(x) <- Byte.of_int (sget s x)
    done;
    let memstream = new System.IO.MemoryStream() in
      let deflater =
        new ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream(memstream)
      in
        deflater.IsStreamOwner <- false;
        deflater.Write(arr, 0, stream_size s);
        deflater.Flush();
        deflater.Finish();
        ignore (memstream.Seek(0L, System.IO.SeekOrigin.Begin));
        let bytes = ref []
        and finished = ref false in
          while not !finished do
            let x = memstream.ReadByte() in
              if x = ~-1 then set finished else bytes =| x
          done;
          bytestream_of_list (rev !bytes)

(* Encode a bigarray using the ZSharpLib library, returning another bigarray. *)
let decode_flate s =
  let arr = Array.make (stream_size s) (Byte.of_int 0) in
    for x = 0 to Array.length arr - 1 do
      arr.(x) <- Byte.of_int (sget s x)
    done;
    let memstream = new System.IO.MemoryStream(arr) in
      let inflater =
        new ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream(memstream)
      in
        let bytes = ref []
        and finished = ref false in
          while not !finished do
            let x = inflater.ReadByte() in
              if x = ~-1 then set finished else bytes =| x
          done;
          bytestream_of_list (rev !bytes)

(* The same, but we don't know where the data ends. Need to deflate until the
end of the stream, and leave the input pointer pointing to the byte following
the compressed data. *)
let decode_flate_input i =
  let startpos = i.pos_in () in
    try
      stream_of_int_array (Io.read_all (Unzip.inflate true (io_of_pdfio i)))
    with
      _ ->
        dpr "2P";
        i.seek_in startpos;
        stream_of_int_array (Io.read_all (Unzip.inflate false (io_of_pdfio i)))

F#*)(*i*)

(* \section{LZW} *)

(* Decode LZW. *)
let decode_lzw early i =
  let prefix_code = Array.make 4096 0
  and append_character = Array.make 4096 0
  and bit_count = ref 0
  and bit_buffer = ref 0l
  and endflush = ref 4
  and code_length = ref 9
  and next_code = ref 258
  and new_code = ref 0
  and old_code = ref 256
  and character = ref 0 in
    let rec decode_string code str =
      if code > 255 then
        decode_string prefix_code.(code) (append_character.(code)::str)
      else
        code::str
    and input_code stream =
      while !bit_count <= 24 do
        let streambyte =
          match stream.input_byte () with
          | b when b = Pdfio.no_more ->
              if !endflush = 0 then raise End_of_file else (decr endflush; 0)
          | b -> b
        in
          bit_buffer := lor32 !bit_buffer (lsl32 (i32ofi streambyte) (24 - !bit_count));
          bit_count += 8
      done;
      let result = Int32.to_int (lsr32 !bit_buffer (32 - !code_length)) in
        bit_buffer := lsl32 !bit_buffer !code_length;
        bit_count -= !code_length;
        result
    and strip_cleartable_codes stream =
      while !old_code = 256 do
        old_code := input_code stream
      done
    and reset_table () =
      next_code := 258;
      code_length := 9;
      old_code := 256
    in
      (* FIXME: How could it ever be 257? It's a byte... *)
      match peek_byte i with 257 -> mkstream 0 | _ ->
        bit_count := 0; bit_buffer := 0l;
        endflush := 4; reset_table ();
        let outstream_data = {pos = 0; data = mkstream 16034} in
          let outstream = output_of_stream outstream_data
          and finished = ref false in
            strip_cleartable_codes i;
            match !old_code with
            | 257 -> mkstream 0
            | _ ->
                character := !old_code;
                outstream.output_byte !old_code;
                while not !finished do
                  new_code := input_code i;
                  match !new_code with
                  | 257 -> set finished
                  | 256 ->
                     reset_table ();
                     set_array prefix_code 0;
                     set_array append_character 0;
                     strip_cleartable_codes i;
                     character := !old_code;
                     outstream.output_byte !old_code
                  | _ ->
                    let chars =
                      if !new_code >= !next_code
                        then (decode_string !old_code []) @ [!character]
                        else decode_string !new_code []
                    in
                      character := hd chars;
                      iter outstream.output_byte chars;
                      prefix_code.(!next_code) <- !old_code;
                      append_character.(!next_code) <- !character;
                      incr next_code;
                      old_code := !new_code;
                      match !next_code + early with
                      | 512 | 1024 | 2048 -> incr code_length
                      | _ -> ()
                done;
                let out = mkstream outstream_data.pos in
                  for x = 0 to stream_size out - 1 do
                    sset out x (sget outstream_data.data x);
                  done;
                  out

(* \section{CCITT} *)          
(* Decode a CCITT-encoded stream. Parameter names:
  \begin{itemize}
     \item [eol] -- /EndOfLine
     \item [eba] -- /EncodedByteAlign
     \item [eob] -- /EndOfBlock
     \item [bone] -- /BlackIs1
     \item [dra] -- /DamagedRowsBeforeError
     \item [c] -- /Columns
     \item [r] -- /Rows
  \end{itemize}
*)

let rec read_white_code i =
  let a = getbitint i in
  let b = getbitint i in
  let c = getbitint i in
  let d = getbitint i in
    match a, b, c, d with
    | 0, 1, 1, 1 -> 2
    | 1, 0, 0, 0 -> 3
    | 1, 0, 1, 1 -> 4
    | 1, 1, 0, 0 -> 5
    | 1, 1, 1, 0 -> 6
    | 1, 1, 1, 1 -> 7
    | _ ->
  let e = getbitint i in
    match a, b, c, d, e with
    | 1, 0, 0, 1, 1 -> 8
    | 1, 0, 1, 0, 0 -> 9
    | 0, 0, 1, 1, 1 -> 10
    | 0, 1, 0, 0, 0 -> 11
    | 1, 1, 0, 1, 1 -> 64 + read_white_code i
    | 1, 0, 0, 1, 0 -> 128 + read_white_code i
    | _ ->
  let f = getbitint i in
    match a, b, c, d, e, f with
    | 0, 0, 0, 1, 1, 1 -> 1
    | 0, 0, 1, 0, 0, 0 -> 12
    | 0, 0, 0, 0, 1, 1 -> 13
    | 1, 1, 0, 1, 0, 0 -> 14
    | 1, 1, 0, 1, 0, 1 -> 15
    | 1, 0, 1, 0, 1, 0 -> 16
    | 1, 0, 1, 0, 1, 1 -> 17
    | 0, 1, 0, 1, 1, 1 -> 192 + read_white_code i
    | 0, 1, 1, 0, 0, 0 -> 1664 + read_white_code i
    | _ ->
  let g = getbitint i in
    match a, b, c, d, e, f, g with
    | 0, 1, 0, 0, 1, 1, 1 -> 18
    | 0, 0, 0, 1, 1, 0, 0 -> 19
    | 0, 0, 0, 1, 0, 0, 0 -> 20
    | 0, 0, 1, 0, 1, 1, 1 -> 21
    | 0, 0, 0, 0, 0, 1, 1 -> 22
    | 0, 0, 0, 0, 1, 0, 0 -> 23
    | 0, 1, 0, 1, 0, 0, 0 -> 24
    | 0, 1, 0, 1, 0, 1, 1 -> 25
    | 0, 0, 1, 0, 0, 1, 1 -> 26
    | 0, 1, 0, 0, 1, 0, 0 -> 27
    | 0, 0, 1, 1, 0, 0, 0 -> 28
    | 0, 1, 1, 0, 1, 1, 1 -> 256 + read_white_code i
    | _ ->
  let h = getbitint i in
    match a, b, c, d, e, f, g, h with
    | 0, 0, 1, 1, 0, 1, 0, 1 -> 0
    | 0, 0, 0, 0, 0, 0, 1, 0 -> 29
    | 0, 0, 0, 0, 0, 0, 1, 1 -> 30
    | 0, 0, 0, 1, 1, 0, 1, 0 -> 31
    | 0, 0, 0, 1, 1, 0, 1, 1 -> 32
    | 0, 0, 0, 1, 0, 0, 1, 0 -> 33
    | 0, 0, 0, 1, 0, 0, 1, 1 -> 34
    | 0, 0, 0, 1, 0, 1, 0, 0 -> 35
    | 0, 0, 0, 1, 0, 1, 0, 1 -> 36
    | 0, 0, 0, 1, 0, 1, 1, 0 -> 37
    | 0, 0, 0, 1, 0, 1, 1, 1 -> 38
    | 0, 0, 1, 0, 1, 0, 0, 0 -> 39
    | 0, 0, 1, 0, 1, 0, 0, 1 -> 40
    | 0, 0, 1, 0, 1, 0, 1, 0 -> 41
    | 0, 0, 1, 0, 1, 0, 1, 1 -> 42
    | 0, 0, 1, 0, 1, 1, 0, 0 -> 43
    | 0, 0, 1, 0, 1, 1, 0, 1 -> 44
    | 0, 0, 0, 0, 0, 1, 0, 0 -> 45
    | 0, 0, 0, 0, 0, 1, 0, 1 -> 46
    | 0, 0, 0, 0, 1, 0, 1, 0 -> 47
    | 0, 0, 0, 0, 1, 0, 1, 1 -> 48
    | 0, 1, 0, 1, 0, 0, 1, 0 -> 49
    | 0, 1, 0, 1, 0, 0, 1, 1 -> 50
    | 0, 1, 0, 1, 0, 1, 0, 0 -> 51
    | 0, 1, 0, 1, 0, 1, 0, 1 -> 52
    | 0, 0, 1, 0, 0, 1, 0, 0 -> 53
    | 0, 0, 1, 0, 0, 1, 0, 1 -> 54
    | 0, 1, 0, 1, 1, 0, 0, 0 -> 55
    | 0, 1, 0, 1, 1, 0, 0, 1 -> 56
    | 0, 1, 0, 1, 1, 0, 1, 0 -> 57
    | 0, 1, 0, 1, 1, 0, 1, 1 -> 58
    | 0, 1, 0, 0, 1, 0, 1, 0 -> 59
    | 0, 1, 0, 0, 1, 0, 1, 1 -> 60
    | 0, 0, 1, 1, 0, 0, 1, 0 -> 61
    | 0, 0, 1, 1, 0, 0, 1, 1 -> 62
    | 0, 0, 1, 1, 0, 1, 0, 0 -> 63
    | 0, 0, 1, 1, 0, 1, 1, 0 -> 320 + read_white_code i
    | 0, 0, 1, 1, 0, 1, 1, 1 -> 384 + read_white_code i
    | 0, 1, 1, 0, 0, 1, 0, 0 -> 448 + read_white_code i
    | 0, 1, 1, 0, 0, 1, 0, 1 -> 512 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 0, 0 -> 576 + read_white_code i
    | 0, 1, 1, 0, 0, 1, 1, 1 -> 640 + read_white_code i
    | _ ->
  let j = getbitint i in
    match a, b, c, d, e, f, g, h, j with
    | 0, 1, 1, 0, 0, 1, 1, 0, 0 -> 704 + read_white_code i
    | 0, 1, 1, 0, 0, 1, 1, 0, 1 -> 768 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 0, 1, 0 -> 832 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 0, 1, 1 -> 896 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 1, 0, 0 -> 960 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 1, 0, 1 -> 1024 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 1, 1, 0 -> 1088 + read_white_code i
    | 0, 1, 1, 0, 1, 0, 1, 1, 1 -> 1152 + read_white_code i
    | 0, 1, 1, 0, 1, 1, 0, 0, 0 -> 1216 + read_white_code i
    | 0, 1, 1, 0, 1, 1, 0, 0, 1 -> 1280 + read_white_code i
    | 0, 1, 1, 0, 1, 1, 0, 1, 0 -> 1344 + read_white_code i
    | 0, 1, 1, 0, 1, 1, 0, 1, 1 -> 1408 + read_white_code i
    | 0, 1, 0, 0, 1, 1, 0, 0, 0 -> 1472 + read_white_code i
    | 0, 1, 0, 0, 1, 1, 0, 0, 1 -> 1536 + read_white_code i
    | 0, 1, 0, 0, 1, 1, 0, 1, 0 -> 1600 + read_white_code i
    | 0, 1, 0, 0, 1, 1, 0, 1, 1 -> 1728 + read_white_code i
    | _ ->
  let k = getbitint i in
  let l = getbitint i in
    match a, b, c, d, e, f, g, h, j, k, l with
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 -> 1792 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0 -> 1856 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 1 -> 1920 + read_white_code i
    | _ ->
  let m = getbitint i in
    match a, b, c, d, e, f, g, h, j, k, l, m with
    | 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 -> ~-1
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0 -> 1984 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1 -> 2048 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0 -> 2112 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1 -> 2176 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0 -> 2240 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 1 -> 2304 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0 -> 2368 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1 -> 2432 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0 -> 2496 + read_white_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1 -> 2560 + read_white_code i
    | _ -> raise (Failure "bad white code")

let rec read_black_code i =
  let a = getbitint i in
  let b = getbitint i in
    match a, b with
    | 1, 1 -> 2
    | 1, 0 -> 3
    | _ ->
  let c = getbitint i in
    match a, b, c with
    | 0, 1, 0 -> 1
    | 0, 1, 1 -> 4
    | _ ->
  let d = getbitint i in
    match a, b, c, d with
    | 0, 0, 1, 1 -> 5
    | 0, 0, 1, 0 -> 6
    | _ ->
  let e = getbitint i in
    match a, b, c, d, e with
    | 0, 0, 0, 1, 1 -> 7
    | _ ->
  let f = getbitint i in
    match a, b, c, d, e, f with
    | 0, 0, 0, 1, 0, 1 -> 8
    | 0, 0, 0, 1, 0, 0 -> 9
    | _ ->
  let g = getbitint i in
    match a, b, c, d, e, f, g with
    | 0, 0, 0, 0, 1, 0, 0 -> 10
    | 0, 0, 0, 0, 1, 0, 1 -> 11
    | 0, 0, 0, 0, 1, 1, 1 -> 12
    | _ ->
  let h = getbitint i in
    match a, b, c, d, e, f, g, h with
    | 0, 0, 0, 0, 0, 1, 0, 0 -> 13
    | 0, 0, 0, 0, 0, 1, 1, 1 -> 14
    | _ ->
  let j = getbitint i in
    match a, b, c, d, e, f, g, h, j with
    | 0, 0, 0, 0, 1, 1, 0, 0, 0 -> 15
    | _ ->
  let k = getbitint i in
    match a, b, c, d, e, f, g, h, j, k with
    | 0, 0, 0, 0, 1, 1, 0, 1, 1, 1 -> 0
    | 0, 0, 0, 0, 0, 1, 0, 1, 1, 1 -> 16
    | 0, 0, 0, 0, 0, 1, 1, 0, 0, 0 -> 17
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 -> 18
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 -> 64 + read_black_code i
    | _ ->
  let l = getbitint i in
    match a, b, c, d, e, f, g, h, j, k, l with
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1 -> 19
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0 -> 20
    | 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 0 -> 21
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1 -> 22
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0 -> 23
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 1 -> 24
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0 -> 25
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 -> 1792 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0 -> 1856 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 1 -> 1920 + read_black_code i
    | _ ->
  let m = getbitint i in
    match a, b, c, d, e, f, g, h, j, k, l, m with
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0 -> 26
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1 -> 27
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0 -> 28
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1 -> 29
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0 -> 30
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1 -> 31
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0 -> 32
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1 -> 33
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1, 0 -> 34
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1, 1 -> 35
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0 -> 36
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 1 -> 37
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1, 0 -> 38
    | 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1 -> 39
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 0 -> 40
    | 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 1 -> 41
    | 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 1, 0 -> 42
    | 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 1, 1 -> 43
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0 -> 44
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1 -> 45
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0 -> 46
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 1, 1 -> 47
    | 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0 -> 48
    | 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1 -> 49
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0 -> 50
    | 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1 -> 51
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0 -> 52
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1 -> 53
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0 -> 54
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 1 -> 55
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0 -> 56
    | 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 0, 0 -> 57
    | 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 0, 1 -> 58
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 1 -> 59
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 0 -> 60
    | 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0 -> 61
    | 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 0 -> 62
    | 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1 -> 63
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0, 0 -> 128 + read_black_code i
    | 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0, 1 -> 192 + read_black_code i
    | 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1 -> 256 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1 -> 320 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0 -> 384 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1 -> 448 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0 -> 1984 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1 -> 2048 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0 -> 2112 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1 -> 2176 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0 -> 2240 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 1 -> 2304 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0 -> 2368 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1 -> 2432 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0 -> 2496 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1 -> 2560 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 -> ~-1
    | _ ->
  let n = getbitint i in
    match a, b, c, d, e, f, g, h, j, k, l, m, n with
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 0 -> 512 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 1 -> 576 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0 -> 640 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1 -> 704 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0 -> 768 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1 -> 832 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 1, 0 -> 896 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 1, 1 -> 960 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 0 -> 1024 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1 -> 1088 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 0 -> 1152 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 1 -> 1216 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0 -> 1280 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1 -> 1344 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0 -> 1408 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1 -> 1472 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0 -> 1536 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1 -> 1600 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0 -> 1664 + read_black_code i
    | 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1 -> 1728 + read_black_code i
    | _ -> raise (Failure "bad black code")

(* Group 4 Fax decoder. *)
type modes =
  | Pass
  | Horizontal
  | Vertical of int
  | Uncompressed
  | EOFB

let read_mode i =
  let a = getbitint i in
    match a with
    | 1 -> Vertical 0
    | _ ->
  let b = getbitint i in
  let c = getbitint i in
    match a, b, c with
    | 0, 1, 1 -> Vertical ~-1
    | 0, 1, 0 -> Vertical 1
    | 0, 0, 1 -> Horizontal
    | _ ->
  let d = getbitint i in
    match a, b, c, d with
    | 0, 0, 0, 1 -> Pass
    | _ ->
  let e = getbitint i in
  let f = getbitint i in
    match a, b, c, d, e, f with
    | 0, 0, 0, 0, 1, 1 -> Vertical ~-2
    | 0, 0, 0, 0, 1, 0 -> Vertical 2
    | _ ->
  let g = getbitint i in
    match a, b, c, d, e, f, g with
    | 0, 0, 0, 0, 0, 1, 1 -> Vertical ~-3
    | 0, 0, 0, 0, 0, 1, 0 -> Vertical 3
    | _ ->
  let h = getbitint i in
  let j = getbitint i in
  let k = getbitint i in
  let l = getbitint i in
  let m = getbitint i in
    match a, b, c, d, e, f, g, h, j, k, l, m with
    | 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1 -> Uncompressed
    | 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 ->
      let a = getbitint i in
      let b = getbitint i in
      let c = getbitint i in
      let d = getbitint i in
      let e = getbitint i in
      let f = getbitint i in
      let g = getbitint i in
      let h = getbitint i in
      let j = getbitint i in
      let k = getbitint i in
      let l = getbitint i in
      let m = getbitint i in
        begin match a, b, c, d, e, f, g, h, j, k, l, m with
        | 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 -> EOFB
        | _ -> raise (Failure "Not a valid code on EOFB")
        end
  | _ -> raise (Failure "Not a valid code")

let decode_CCITTFax k eol eba c r eob bone dra input =
  if k > 0 then raise (DecodeNotSupported "CCITTFax k > 0") else
    let whiteval, blackval = if bone then 0, 1 else 1, 0
    and output = make_write_bitstream () in
      let b = bitstream_of_input input
      and column = ref 0
      and row = ref 0
      and refline = ref (Array.make c whiteval)
      and currline = ref (Array.make c 0)
      and white = ref true
      and output_line line =
        Array.iter (putbit output) line;
        align_write output
      in
        let output_span l v =
          if l < 0 then raise (Failure "Bad CCITT stream") else
            begin
              for x = !column to !column + l - 1 do
                let r = !currline in r.(x) <- v
              done;
              column += l
            end
        and find_b1 () =
          let pos = ref !column
          and curr, opp = if !white then whiteval, blackval else blackval, whiteval in
            (* Altered to get rid of exception - test *)
            let find v =
              while
                let r = !refline in
                  if !pos >= 0 && !pos < Array.length r then
                    r.(!pos) <> v
                  else
                    false
              do
                incr pos
              done; !pos
            in
              try
                (* Careful to skip imaginary black at beginning *)
                ignore (if !column = 0 && !white then 0 else find curr);
                find opp
              with
                _ -> dpr "2Q"; c
        and find_b2 () =
          let pos = ref !column
          and curr, opp = if !white then whiteval, blackval else blackval, whiteval in
            (* Altered to get rid of exception - test *)
            let find v =
              while
                let r = !refline in
                  if !pos >=0 && !pos < Array.length r then
                    r.(!pos) <> v
                  else
                    false
              do
                incr pos
              done; !pos
            in
              try
                (* Careful to skip imaginary black at beginning *)
                ignore (if !column = 0 && !white then 0 else find curr);
                ignore (find opp);
                find curr
              with
                _ -> dpr "2R"; c
        in
          try
            while true do
              if !column >= c then
                begin
                  output_line !currline;
                  refline := !currline;
                  column := 0;
                  set white;
                  if eba then align b;
                  incr row;
                  if !row >= r && r > 0 then raise End_of_file
                end
              else
               begin
                  if k < 0 then
                    (* Group 4 *)
                    match read_mode b with
                    | Pass ->
                        output_span (find_b2 () - !column) (if !white then whiteval else blackval)
                    | Horizontal ->
                        if !white then
                          begin
                            output_span (read_white_code b) whiteval;
                            output_span (read_black_code b) blackval;
                          end
                        else
                          begin
                            output_span (read_black_code b) blackval;
                            output_span (read_white_code b) whiteval;
                          end
                    | Vertical n ->
                        output_span (find_b1 () - !column - n) (if !white then whiteval else blackval);
                        flip white
                    | EOFB -> raise End_of_file
                    | Uncompressed -> raise (DecodeNotSupported "CCITT Uncompressed")
                  else if k = 0 then
                    (* Group 3 *)
                    begin match (if !white then read_white_code else read_black_code) b with
                    | -1 ->
                       (* Pad it out *)
                       if !column > 0 then output_span (c - !column) whiteval
                    | l ->
                      begin
                        output_span l (if !white then whiteval else blackval);
                        flip white
                      end
                    end
                  else
                    raise (DecodeNotSupported "CCITT k")
                end
            done;
            mkstream 0
          with
            | End_of_file -> dpr "K"; bytestream_of_write_bitstream output
            | _ -> raise (Failure "Bad CCITT Stream") 

(* \section{PNG and TIFF Predictors} *)

(* Get the value at index [i] from an int array [a], giving zero if the index is
too low. Fails in the usual manner if the index is too high. *)
let get0 a i =
  if i < 0 then 0 else a.(i)

(* TIFF prediction. 8bpp only for now. *)
let decode_tiff_predictor colors bpc columns stream =
  match bpc with
  | 8 ->
      let scanline_width = (colors * bpc * columns + 7) / 8 in
        for line = 0 to stream_size stream / scanline_width - 1 do
          let linestart = line * scanline_width in
            for p = 1 to scanline_width - 1 do
              sset stream (linestart + p)
                ((sget stream (linestart + p - 1) + sget stream (linestart + p)) mod 256)
            done
        done;
        stream
  | _ ->
     raise (DecodeNotSupported "Tiff predictor not 8bpp")

(* Given two scanlines, the previous and current, and the predictor function
[p], calculate the output scanline as a list of bytes. *)
let decode_scanline_pair prior_encoded prior_decoded current pred bpc cols =
  let output = Array.copy current in
    begin match pred with
    | 0 -> (* None *)
        ()
    | 1 -> (* Sub *)
        for x = 0 to Array.length output - 1 do
          output.(x) <- (get0 current x + get0 output (x - cols)) mod 256
        done
    | 2 -> (* Up *)
        for x = 0 to Array.length output - 1 do
          output.(x) <- (get0 current x + get0 prior_decoded x) mod 256
        done
    | 3 -> (* Average -- No test case yet found. *)
        for x = 0 to Array.length output - 1 do
          output.(x) <-
            (get0 current x +
              (get0 output (x - cols) + get0 prior_decoded x) / 2) mod 256
        done
    | 4 -> (* Paeth *)
        let paeth a b c =
          let p = a + b - c in
            let pa = abs (p - a) and pb = abs (p - b) and pc = abs (p - c) in
              if pa <= pb && pa <= pc then a
              else if pb <= pc then b
              else c
          in
            for x = 0 to Array.length output - 1 do
              output.(x) <-
                let curr = get0 current x
                and currback = get0 output (x - cols)
                and decoded = get0 prior_decoded x
                and decodedback = get0 prior_decoded (x - cols) in
                  (curr + paeth currback decoded decodedback) mod 256
            done
    | _ -> raise (DecodeNotSupported "unknown PNG predictor")
    end;
    output

(* Main function. Given predictor, number of channels, bits-per-channel,
columns and the stream data, perform the decoding. *)
let decode_predictor pred colors bpc columns stream =
  if pred = 2 then decode_tiff_predictor colors bpc columns stream else
    let i = input_of_bytestream stream
    and scanline_width = (colors * bpc * columns + 7) / 8 in
      let blank () = ref (Array.make scanline_width 0) in
        let prev, curr, prior_decoded = blank (), blank (), blank ()
        and outputlines = ref []
        and finished = ref false
        and pred = ref 0
        and got_predictor = ref false in
          while not !finished do
            clear got_predictor;
            begin match i.input_byte () with
            | x when x = Pdfio.no_more -> set finished
            | x -> pred := x
            end;
            if !finished then () else
              begin
                set got_predictor;
                prev := !curr;
                for x = 0 to scanline_width - 1 do
                  match i.input_byte () with
                  | x when x = Pdfio.no_more -> set finished
                  | b -> (!curr).(x) <- b
                done
              end;
            (* We allow an unfinished final line only if we managed to get a
            predictor byte *)
            if !got_predictor then
              begin
                prior_decoded :=
                decode_scanline_pair
                  !prev !prior_decoded !curr !pred bpc ((bpc * colors + 7) / 8);
                outputlines =| !prior_decoded
              end
          done;
          bytestream_of_arraylist (rev !outputlines)

(* \section{Run Length Encoding} *)
let encode_runlength stream =
  let i = input_of_bytestream stream in
    let data_in = ref [] in
      begin try
        while true do
          data_in =|
            begin match i.input_byte () with
            | x when x = Pdfio.no_more -> raise End_of_file
            | x -> x
            end
        done
      with
        End_of_file -> dpr "M"; data_in := rev !data_in
      end;
      let rec runs_of_data prev = function
        | [] -> rev prev
        | h::t ->
            let same, rest = cleavewhile (eq h) (h::t) in
              runs_of_data ((length same, hd same)::prev) rest
      in
        let runs = ref (runs_of_data [] !data_in)
        and outbytes = ref []
        and chunksize = ref 0
        and chunkdata = ref [] in
          let writechunk () =
            if !chunksize > 0 then
              begin
                outbytes =| !chunksize - 1; 
                iter (( =| ) outbytes) (rev !chunkdata);
                chunkdata := [];
                chunksize := 0;
              end
          in
              while !runs <> [] do
                begin match hd !runs with
                | (l, x) when l < 1 ->
                    assert false
                | (l, x) when l < 3 ->
                    if l + !chunksize > 128 then writechunk ();
                    chunkdata =@ many x l;
                    chunksize += l 
                | (l, x) ->
                    writechunk ();
                    let l = ref l in
                      while !l > 0 do
                        outbytes =| 257 - min !l 128;
                        outbytes =| x;
                        l -= 128
                      done
                end;
                runs := tl !runs
              done;
              writechunk ();
              outbytes =| 128; (*r End-of-data *)
              bytestream_of_list (rev !outbytes)

let decode_runlength i =
  let s =  {pos = 0; data = mkstream 4096} in
    let o = output_of_stream s in
      let eod = ref false in
        try
          while not !eod do
            let l =
              match i.input_byte () with
              | x when x = Pdfio.no_more -> raise End_of_file
              | x -> x
            in
              if l < 128 then
                for x = 1 to l + 1 do
                  o.output_byte
                    begin match i.input_byte () with
                      | x when x = Pdfio.no_more -> raise End_of_file
                      | x -> x
                    end
                done
              else if l > 128 then
                let towrite =
                  begin match i.input_byte () with
                  | x when x = Pdfio.no_more -> raise End_of_file
                  | x -> x
                  end;
                in
                  for x = 1 to 257 - l do
                    o.output_byte towrite
                  done
              else
                set eod
          done;
          let osize = postoi (o.out_channel_length ()) in
            let output = mkstream osize in
              for x = 0 to osize - 1 do
                sset output x (sget s.data x)
              done;
              output
        with
          End_of_file -> raise (Couldn'tDecodeStream "RunLength")

(* \section{Decoding PDF streams} *)
type source =
  | StreamSource of bytestream
  | InputSource of input

let decoder pdf dict source name =
  let input_of_source = function
    | InputSource i -> i
    | StreamSource s -> input_of_bytestream s
  in
    let i = input_of_source source in
      match name with
      | "/ASCIIHexDecode" | "/AHx" -> decode_ASCIIHex i
      | "/ASCII85Decode" | "/A85" -> decode_ASCII85 i
      | "/FlateDecode" | "/Fl" ->
          begin match source with
          | StreamSource s -> decode_flate s
          | InputSource i -> decode_flate_input i
          end
      | "/RunLengthDecode" | "/RL" -> decode_runlength i
      | "/LZWDecode" | "/LZW" ->
          let early =
            match lookup_direct_orelse pdf "/DecodeParms" "/DP" dict with
            | None -> 1
            | Some d ->
                match lookup_direct pdf "/EarlyChange" d with
                | Some (Integer n) -> n
                | None -> 1
                | _ -> raise (PDFError "malformed /EarlyChange")
          in
            decode_lzw early i
      | "/CCITTFaxDecode" | "/CCF" ->
          begin match lookup_direct_orelse pdf "/DecodeParms" "/DP" dict with
          | None -> decode_CCITTFax 0 false false 1728 0 true false 0 i
          | Some (Pdf.Dictionary _ as dparms)
          | Some (Array (dparms::_)) ->
              let dparms = direct pdf dparms in
              let k =
                match lookup_direct pdf "/K" dparms with
                | Some (Integer i) -> i
                | _ -> 0
              and eol =
                match lookup_direct pdf "/EndOfLine" dparms with
                | Some (Boolean b) -> b
                | _ -> false
              and eba =
                match lookup_direct pdf "/EncodedByteAlign" dparms with
                | Some (Boolean b) -> b
                | _ -> false
              and c =
                match lookup_direct pdf "/Columns" dparms with
                | Some (Integer i) -> i
                | _ -> 1728
              and r =
                match lookup_direct pdf "/Rows" dparms with
                | Some (Integer i) -> i
                | _ -> 0
              and eob =
                match lookup_direct pdf "/EndOfBlock" dparms with
                | Some (Boolean b) -> b
                | _ -> true
              and bone =
                match lookup_direct pdf "/BlackIs1" dparms with
                | Some (Boolean b) -> b
                | _ -> false
              and dra =
                match lookup_direct pdf "/DamagedRowsBeforeError" dparms with
                | Some (Integer i) -> i
                | _ -> 0
              in
                decode_CCITTFax k eol eba c r eob bone dra i
            | _ -> raise (Pdf.PDFError "bad Decodeparms")
            end
      | name ->
          raise (DecodeNotSupported (Printf.sprintf "Unknown: %s" name))

(* Decode at most one stage. *)
let decode_one pdf dict source =
  match lookup_direct_orelse pdf "/Filter" "/F" dict with
  | None | Some (Array []) ->
      begin match source with
      | StreamSource s -> s
      | InputSource i -> raise (DecodeNotSupported "decode_one")
      end
  | Some (Name n) | Some (Array (Name n::_)) ->
      let decoded = decoder pdf dict source n in
        let decodeparms =
          match lookup_direct_orelse pdf "/DecodeParms" "/DP" dict with
          | Some (Dictionary d)
          | Some (Array (Dictionary d::_)) -> Dictionary d
          | _ -> Dictionary []
        in
          begin match lookup_direct pdf "/Predictor" decodeparms with
          | None | Some (Integer 1) -> decoded
          | Some (Integer pred) ->
              let colors =
                match lookup_direct pdf "/Colors" decodeparms with
                | Some (Integer n) -> n
                | None -> 1
                | _ -> raise (PDFError "malformed /Colors")
              and bits_per_component =
                match lookup_direct pdf "/BitsPerComponent" decodeparms with
                | Some (Integer n) -> n
                | None -> 8
                | _ -> raise (PDFError "malformed /BitsPerComponent")
              and columns =
                match lookup_direct pdf "/Columns" decodeparms with
                | Some (Integer n) -> n
                | None -> 1
                | _ -> raise (PDFError "malformed /Columns")
              in
                begin try
                  decode_predictor pred colors bits_per_component columns decoded
                with
                  _ -> raise (Couldn'tDecodeStream "Predictor")
                end
          | _ -> raise (PDFError "Malformed /Predictor")
          end
  | _ ->
    raise (PDFError "PDF.decode: Bad filter specification")

(* Remove a single decoder from a filter list. Also remove the first entry of a
 DecodeParms array *)
let remove_decoder d =
  let d' =
    match lookup "/Filter" d, lookup "/F" d with
    | None, None -> d
    | Some (Name _ | Array [_]), None -> lose (fun (n, _) -> n = "/Filter") d
    | None, Some (Name _ | Array [_]) -> lose (fun (n, _) -> n = "/F") d
    | Some (Array (_::t)), _ -> replace "/Filter" (Array t) d
    | _, Some (Array (_::t)) -> replace "/F" (Array t) d
    | _ -> raise (PDFError "PDF.remove_decoder: malformed /Filter")
  in
    match lookup "/DecodeParms" d', lookup "/DP" d' with
    | None, None -> d'
    | Some (Dictionary _ | Array []), _ -> remove "/DecodeParms" d'
    | _, Some (Dictionary _ | Array []) -> remove "/DP" d'
    | Some (Array (_::t)), _ -> replace "/DecodeParms" (Array t) d'
    | _, Some (Array (_::t)) -> replace "/DP" (Array t) d'
    | _ -> raise (PDFError "PDF.remove_decoder: malformed /DecodeParms")

(* \intf Decode at most one stage. *)
let rec decode_pdfstream_onestage pdf stream =
  getstream stream;
  match stream with
  | Stream ({contents = (Dictionary d as dict, Got s)} as stream_contents) ->
      begin match direct pdf (lookup_fail "no /Length" pdf "/Length" dict) with
      | Integer l -> () (*i if l <> stream_size s then raise (PDFError "Wrong /Length") i*)
      | _ -> raise (PDFError "No /Length")
      end;
      let stream' = decode_one pdf dict (StreamSource s) in
        let d' =
          replace "/Length" (Integer (stream_size stream')) (remove_decoder d)
        in
          stream_contents := Dictionary d', Got stream'
  | _ -> raise (PDFError "Pdf.decode_pdfstream: not a valid Stream")

(* \intf Decode until there's nothing left to do. *)
let rec decode_pdfstream pdf = function
  | Stream {contents = d, _} as stream ->
      getstream stream;
      begin match lookup_direct_orelse pdf "/Filter" "/F" d with
      | None -> ()
      | Some (Name _ | Array _) ->
            begin
              decode_pdfstream_onestage pdf stream;
              match stream with
              | Stream {contents = d', _} ->
                  if d = d' then () else
                  decode_pdfstream pdf stream
              | _ -> assert false
            end
      | _ -> raise (PDFError "Pdf.remove_decoder: malformed /Filter")
      end
  | Indirect i ->
      decode_pdfstream pdf (Pdf.direct pdf (Indirect i))
  | _ -> raise (PDFError "Pdf.decode_pdfstream: malformed Stream")

(* \intf Decode a stream until a decoding isn't supported. *)
let decode_pdfstream_until_unknown pdf s =
  try decode_pdfstream pdf s with
    DecodeNotSupported _ -> dpr "2T"; ()

(* Decode the first decoder from an input. Any further ones can be done in the
usual fashion. Fails if no decoder (you should have dealt with this already). *)
let decode_from_input i dict =
  match lookup_direct_orelse (Pdf.empty ()) "/F" "/Filter" dict with
  | Some (Name n) ->
      Some (decode_one (Pdf.empty ()) dict (InputSource i))
  | Some (Array (h::t)) ->
      let stream = decode_one (Pdf.empty ()) dict (InputSource i) in
        let rec decode_rest stream = function
          | [] -> stream
          | Name n::more ->
              let dict' = remove_dict_entry dict "/Filter" in
                let dict'' = remove_dict_entry dict' "/F" in
                  let stream' =
                    decode_one (Pdf.empty ()) dict'' (StreamSource stream)
                  in
                    decode_rest stream' more
          | _ -> raise (PDFError "Malformed filter array")
        in
          Some (decode_rest stream t)
  | _ -> raise (Couldn'tDecodeStream "No or bad filter")

(* \section {Encoding streams} *)

(* \intf Supported encodings. *)
type encoding =
  | ASCIIHex
  | ASCII85
  | RunLength
  | Flate

(* The name of an encoding. *)
let name_of_encoding = function
  | ASCIIHex -> "/ASCIIHexDecode"
  | ASCII85 -> "/ASCII85Decode"
  | RunLength -> "/RunLengthDecode"
  | Flate -> "/FlateDecode"

(* Add an encoding to the dictionary [d]. *)
let add_encoding length pdf encoding d =
  let filter' =
    match lookup_direct pdf "/Filter" d with
    | None ->
        Name (name_of_encoding encoding)
    | Some (Name n) ->
        Array (Name (name_of_encoding encoding)::[Name n])
    | Some (Array a) ->
        Array (Name (name_of_encoding encoding)::a)
    | _ -> raise (PDFError "Malformed /Filter")
  in
    replace_dict_entry (add_dict_entry d "/Filter" filter') "/Length" (Integer length)

(* Find the encoding function. *)
let encoder_of_encoding = function
  | ASCIIHex -> encode_ASCIIHex
  | ASCII85 -> encode_ASCII85
  | RunLength -> encode_runlength
  | Flate -> encode_flate

(* \intf Encode a PDF stream with an encoding. *)
let encode_pdfstream pdf encoding stream =
  getstream stream;
  match stream with
  | Stream ({contents = d, Got s} as stream) ->
      let data = encoder_of_encoding encoding s in
        let d' = add_encoding (stream_size data) pdf encoding d in
          stream := d', Got data
  | _ -> raise (PDFError "Pdf.encode_pdfstream: malformed Stream")