extract_char_unsigned

[ocaml-bitstring.git] / bitmatch.ml

diff --git a/bitmatch.ml b/bitmatch.ml
index 3135c89..cde163c 100644 (file)
--- a/bitmatch.ml
+++ b/bitmatch.ml
@@ -1,7 +1,9 @@
 (* Bitmatch library.
- * $Id: bitmatch.ml,v 1.1 2008-03-31 22:52:17 rjones Exp $
+ * $Id: bitmatch.ml,v 1.2 2008-04-01 08:56:43 rjones Exp $
  *)
 
+open Printf
+
 (* A bitstring is simply the data itself (as a string), and the
  * bitoffset and the bitlength within the string.  Note offset/length
  * are counted in bits, not bytes.
@@ -31,18 +33,48 @@ let bitstring_of_file fname =
   close_in chan;
   bs
 
-(* Extraction functions (internal: called from the generated macros,
- * and the parameters should have been checked for sanity already).
+(*----------------------------------------------------------------------*)
+(* Extraction functions.
+ *
+ * NB: internal functions, called from the generated macros, and
+ * the parameters should have been checked for sanity already).
  *)
+
+(* Bitstrings. *)
 let extract_bitstring data off len flen =
   (data, off, flen), off+flen, len-flen
 
 let extract_remainder data off len =
   (data, off, len), off+len, 0
 
-(* Extract and convert to numeric. *)
+(* Extract and convert to numeric.  A single bit is returned as
+ * a boolean.  There are no endianness or signedness considerations.
+ *)
 let extract_bit data off len _ =       (* final param is always 1 *)
   let byteoff = off lsr 3 in
   let bitmask = 1 lsl (7 - (off land 7)) in
   let b = Char.code data.[byteoff] land bitmask <> 0 in
   b, off+1, len-1
+
+(* Extract [2..8] bits.  Because the result fits into a single
+ * byte we don't have to worry about endianness, only signedness.
+ *)
+let extract_char_unsigned data off len flen =
+  let byteoff = off lsr 3 in
+
+  (* Extract the 16 bits at byteoff and byteoff+1 (note that the
+   * second byte might not exist in the original string).
+   *)
+  let word =
+    (Char.code data.[byteoff] lsl 8) +
+      (if String.length data > byteoff+1 then Char.code data.[byteoff+1]
+       else 0) in
+
+  (* Mask off the top bits. *)
+  let bitmask = (1 lsl (16 - (off land 7))) - 1 in
+  let word = word land bitmask in
+  (* Shift right to get rid of the bottom bits. *)
+  let shift = 16 - ((off land 7) + flen) in
+  let word = word lsr shift in
+
+  word, off+flen, len-flen