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Date: Fri, 17 May 2013 17:21:49 +0100
From: "Richard W.M. Jones" <rich@annexia.org>
To: caml-list@inria.fr
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 <5192DFA0.2080706@riken.jp>
 <20130517161559.GE32353@annexia.org>
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User-Agent: Mutt/1.5.20 (2009-06-14)
Subject: Re: [Caml-list] ANNOUNCE: ocaml-bitstring 2.0.4

On Fri, May 17, 2013 at 05:15:59PM +0100, Richard W.M. Jones wrote:
> On Wed, May 15, 2013 at 10:06:40AM +0900, Francois Berenger wrote:
> > I mean, the bitstring can be huge, like millions of bits?
> 
> They can be as long as strings in OCaml.  Hence there's a limit of 16
> MBytes on 32 bit architectures.  It's (generally) advisable to use a
> 64 bit machine, and bitstrings are effectively unlimited on 64 bit.
> 
> Sub-bitstrings are implemented efficiently, without copying.

To be plainer, this is the definition of a bitstring:

  (* 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.
   *)
  type bitstring = string * int * int

and this is how a subbitstring is made:

  let subbitstring (data, off, len) off' len' =
    let off = off + off' in
    if off' < 0 || len' < 0 || off' > len - len' then
      invalid_arg "subbitstring";
    (data, off, len')

Rich.

-- 
Richard Jones
Red Hat