From ccece4ef5765e85f9a3d7ba0c795a1d0ce3dd7fd Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rich@annexia.org>
Date: Tue, 15 Apr 2008 13:40:51 +0000
Subject: [PATCH] string_of_bitstring.

---
 bitmatch.ml  | 27 ++++++++++++++++++++++++++-
 bitmatch.mli | 14 +++++++++++++-
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/bitmatch.ml b/bitmatch.ml
index c1d6cd1..861cd5a 100644
--- a/bitmatch.ml
+++ b/bitmatch.ml
@@ -15,7 +15,7 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  *
- * $Id: bitmatch.ml,v 1.8 2008-04-02 11:06:07 rjones Exp $
+ * $Id: bitmatch.ml,v 1.9 2008-04-15 13:40:51 rjones Exp $
  *)
 
 open Printf
@@ -601,6 +601,31 @@ let construct_int64_be_unsigned buf v flen exn =
   I64.map_bytes_be (Buffer._add_bits buf) (Buffer.add_byte buf) v flen
 
 (*----------------------------------------------------------------------*)
+(* Extract a string from a bitstring. *)
+
+let string_of_bitstring (data, off, len) =
+  if off land 7 = 0 && len land 7 = 0 then
+    (* Easy case: everything is byte-aligned. *)
+    String.sub data (off lsr 3) (len lsr 3)
+  else (
+    (* Bit-twiddling case. *)
+    let strlen = (len + 7) lsr 3 in
+    let str = String.make strlen '\000' in
+    let rec loop data off len i =
+      if len >= 8 then (
+	let c, off, len = extract_char_unsigned data off len 8 in
+	str.[i] <- Char.chr c;
+	loop data off len (i+1)
+      ) else if len > 0 then (
+	let c, off, len = extract_char_unsigned data off len len in
+	str.[i] <- Char.chr c
+      )
+    in
+    loop data off len 0;
+    str
+  )
+
+(*----------------------------------------------------------------------*)
 (* Display functions. *)
 
 let isprint c =
diff --git a/bitmatch.mli b/bitmatch.mli
index 9a38808..b700f2b 100644
--- a/bitmatch.mli
+++ b/bitmatch.mli
@@ -15,7 +15,7 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  *
- * $Id: bitmatch.mli,v 1.12 2008-04-02 13:59:37 rjones Exp $
+ * $Id: bitmatch.mli,v 1.13 2008-04-15 13:40:51 rjones Exp $
  *)
 
 (**
@@ -577,6 +577,18 @@ val bitstring_length : bitstring -> int
 (** [bitstring_length bitstring] returns the length of
     the bitstring in bits. *)
 
+val string_of_bitstring : bitstring -> string
+(** [string_of_bitstring bitstring] converts a bitstring to a string
+    (eg. to allow comparison).
+
+    This function is inefficient.  In the best case when the bitstring
+    is nicely byte-aligned we do a [String.sub] operation.  If the
+    bitstring isn't aligned then this involves a lot of bit twiddling
+    and is particularly inefficient.
+
+    XXX This function wouldn't be needed so much if the [bitmatch]
+    operator allowed us to pattern-match on strings. *)
+
 (** {3 Bitstring buffer} *)
 
 module Buffer : sig
-- 
1.8.3.1