diff --git a/cpdfpng.ml b/cpdfpng.ml
index 7a02b75..7955b42 100644
--- a/cpdfpng.ml
+++ b/cpdfpng.ml
@@ -9,6 +9,72 @@ type t =
    height : int;
    idat : bytes}
 
+(* Writing *)
+let crc_table =
+  let f n =
+    let c = ref (i32ofi n) in
+      for _ = 0 to 7 do
+        c := lxor32 (lsr32 !c 1) (land32 0xedb88320l (i32succ (lnot32 (land32 !c 1l))))
+      done;
+      !c
+  in
+    Array.init 256 f
+
+let update_crc crc buf len =
+  let c = ref crc in
+    for n = 0 to len - 1 do
+      let e = i32ofi (int_of_char buf.[n]) in
+        c := lxor32 crc_table.(i32toi (land32 (lxor32 !c e) 0xffl)) (lsr32 !c 8)
+    done;
+    !c
+
+let png_crc buf len =
+  lnot32 (update_crc 0xffffffffl buf len)
+
+let bytes_of_word x =
+  i32toi (Int32.shift_right x 24),
+  i32toi (land32 0x000000FFl (Int32.shift_right x 16)),
+  i32toi (land32 0x000000FFl (Int32.shift_right x 8)),
+  i32toi (land32 0x000000FFl x)
+
+let write_crc o ctype cdata =
+  let crc = update_crc 0xffffffffl ctype 4 in
+  let crc = update_crc crc cdata (String.length cdata) in
+  let a, b, c, d = bytes_of_word crc in
+    o.output_byte a;
+    o.output_byte b;
+    o.output_byte c;
+    o.output_byte d
+
+let write_chunk o ctype data =
+  for x = 0 to 4 do o.output_byte (int_of_char ctype.[x]) done;
+  o.output_string (Bytes.unsafe_to_string data);
+  write_crc o ctype (Bytes.unsafe_to_string data)
+
+let write_word b p n =
+  let p, q, r, s = bytes_of_word n in
+  Bytes.set b p (char_of_int p);
+  Bytes.set b (p + 1) (char_of_int q);
+  Bytes.set b (p + 2) (char_of_int r);
+  Bytes.set b (p + 3) (char_of_int s)
+
+let write_png png o =
+  if bytes_size png.idat > 2147483647 then raise (Invalid_argument "write_png: too large") else
+  (* Signature *)
+  o.output_string "\137\080\078\071\013\010\026\010";
+  let ihdr = Bytes.create 13 in
+  write_word ihdr 0 (i32ofi png.width);
+  write_word ihdr 4 (i32ofi png.height);
+  Bytes.set ihdr 8 (char_of_int 8); (* bit depth *)
+  Bytes.set ihdr 9 (char_of_int 2); (* colour type *)
+  Bytes.set ihdr 10 (char_of_int 0); (* compression method *)
+  Bytes.set ihdr 11 (char_of_int 0); (* filter method *)
+  Bytes.set ihdr 12 (char_of_int 0); (* interlace method *)
+  write_chunk o "IHDR" ihdr;
+  write_chunk o "IDAT" (Bytes.unsafe_of_string (Pdfio.string_of_bytes png.idat));
+  write_chunk o "IEND" (Bytes.create 0)
+
+(* Reading *)
 let string_of_tag t =
   Printf.sprintf "%c%c%c%c"
     (char_of_int (i32toi (Int32.shift_right t 24)))
@@ -74,58 +140,3 @@ let read_png i =
       {width = i32toi width; height = i32toi height; idat = concat_bytes (rev !idat)}
   with
     e -> raise (Pdf.PDFError (Printf.sprintf "read_png: failed on %s" (Printexc.to_string e)))
-
-let (>>) = Int32.shift_right_logical
-let (&)  = Int32.logand
-let (^)  = Int32.logxor
-
-let crc_table =
-  let elem n =
-    let c = ref (Int32.of_int n) in
-    for _ = 0 to 7 do
-      c := (!c >> 1) ^ (0xedb88320l & (Int32.succ (Int32.lognot (!c & 1l))))
-    done; !c
-  in Array.init 256 elem
-
-let update_crc crc buf len =
-  let c = ref crc in
-  for n = 0 to len - 1 do
-    let e = Int32.of_int (int_of_char buf.[n]) in
-    c := crc_table.(Int32.to_int ((!c ^ e) & 0xffl)) ^ (!c >> 8)
-  done; !c
-
-let png_crc buf len =
-  Int32.lognot (update_crc 0xffffffffl buf len)
-
-let write_crc o ctype cdata =
-  o.output_byte 0;
-  o.output_byte 0;
-  o.output_byte 0;
-  o.output_byte 0
-
-let write_chunk o ctype data =
-  for x = 0 to 4 do o.output_byte (int_of_char ctype.[x]) done;
-  o.output_string (Bytes.unsafe_to_string data);
-  write_crc o ctype data
-
-let write_word b p n =
-  Bytes.set b p ' ';
-  Bytes.set b (p + 1) ' ';
-  Bytes.set b (p + 2) ' ';
-  Bytes.set b (p + 3) ' '
-
-let write_png png o =
-  if bytes_size png.idat > 2147483647 then raise (Invalid_argument "write_png: too large") else
-  (* Signature *)
-  o.output_string "\137\080\078\071\013\010\026\010";
-  let ihdr = Bytes.create 13 in
-  write_word ihdr 0 png.width;
-  write_word ihdr 4 png.height;
-  Bytes.set ihdr 8 (char_of_int 8); (* bit depth *)
-  Bytes.set ihdr 9 (char_of_int 2); (* colour type *)
-  Bytes.set ihdr 10 (char_of_int 0); (* compression method *)
-  Bytes.set ihdr 11 (char_of_int 0); (* filter method *)
-  Bytes.set ihdr 12 (char_of_int 0); (* interlace method *)
-  write_chunk o "IHDR" ihdr;
-  write_chunk o "IDAT" (Bytes.unsafe_of_string (Pdfio.string_of_bytes png.idat));
-  write_chunk o "IEND" (Bytes.create 0)