Re: bitstreams

[srini <r.srinivasan@worldnet.att.net>]

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I do not know much about mp3 etc but I have developed a bit stream
package which allows you to perform reads in terms of bits. I use these
in my own compression/decompression test software. The spec is attached.
I would be happy to send you the body as well if you are interested.


Daniel Nilsson wrote:
> 
> Hi.
> In my mp3 decoder I will have to search for bit-patterns in a buffer of
> bytes, how is this best done, and How can I "shift" in a sequence of bits
> into a byte, or word efficiently? (I want ex. take 5 bits from a buffer and
> put them rightadjusted in a unsigned_32)
> 
> /Daniel Nilsson

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with Interfaces ;
with ada.streams ;
with ada.streams.stream_io ;
----------------------------------------------------------------------
-- Abstract : Bits_Io
--      This package converts a normal I/O stream into a bit level I/O
----------------------------------------------------------------------
package Bits_Io is

   type File_Type is limited private;
   type File_Mode is (In_File, Out_File, Append_File);

   type Vector_type is array (natural range <>) of boolean ;
   pragma pack( Vector_Type ) ;

   -- Abstract : Open
   --    A file is opened using normal File Management facilities first and then
   -- a Bit Stream File is created using the following procedure. It is the same
   -- procedure whether performing input or output. It is assumed that the file
   -- is used only for one of these purposes at any time.
   procedure Open
     (File : in out File_Type;
      Mode : in File_Mode ;
      Stream : access Ada.Streams.Root_Stream_Type'Class ) ;

   -- Abstract : Read/Write
   --    The following two procedures allow an Unsigned_8 variable to read or
   -- write. Similar procedures for Unsigned_16 and Unsigned_32 are also provided
   procedure Read
     (File : in out File_Type ;
      Item : out Interfaces.Unsigned_8 ;
      Last : out Ada.Streams.Stream_Element_Offset) ;
                                       -- Returns the number of bits actually read

   procedure Write
     (File : in out File_Type ;
      Item : in Interfaces.Unsigned_8 ;
      Last : in integer );             -- How many bits should really be written?

   procedure Read
     (File : in out File_Type ;
      Item : out Interfaces.Unsigned_16 ;
      Last : out Ada.Streams.Stream_Element_Offset) ;

   procedure Write
     (File : in out File_Type ;
      Item : in Interfaces.Unsigned_16 ;
      Last : in integer );

   procedure Read
     (File : in out File_Type ;
      Item : out Interfaces.Unsigned_32 ;
      Last : out Ada.Streams.Stream_Element_Offset) ;

   procedure Write
     (File : in out File_Type ;
      Item : in Interfaces.Unsigned_32 ;
      Last : in integer );

   -- Abstract : Read/Write
   --    These are the real I/O procedures where all the work is done. The above
   -- routines simply call these internally.
   procedure Read
     (File : in out File_Type ;
      Item : out Vector_Type ;
      Last : out Ada.Streams.Stream_Element_Offset) ;

   procedure Write
     (File : in out File_Type ;
      Item : in Vector_Type );

   -- Abstract : Flush
   --    Used for writing operations, sends any residual bits to the output stream
   -- packed in a byte.
   procedure Flush (File : in out File_Type ) ;

private

    type Root_Stream_Access_Type is access all Ada.Streams.Root_Stream_Type'Class ;

    type File_Type is
       record
          Opened : Boolean := False ;
          Ac : Root_Stream_Access_Type ;
          Mode : File_Mode ;
          Residue : Interfaces.Unsigned_8 ;
          Bits_Used : Integer ;
       end record ;

end Bits_Io ;

-- $Author: Administrator $
-- $Revision: 1.1.1.1 $
-- $Log: bits_io.ads,v $
-- Revision 1.1.1.1  2000/08/04 06:07:07  Administrator
-- Initial Checkin
--
-- Revision 1.1  1998/09/12 04:43:32  srinivr
-- Bit level Stream Input/Output routines. Initial Check in with
-- some known problems.
--
-- Revision 1.1  1998/01/27 03:39:33  srinivr
-- Added annotations
--