(* 
 * An oogg file is an abstract type that can be accessed as a stream at a 
 * particular byte location.  Internally, we use a file descriptor.
 *)

type file = Unix.file_descr;;
type stream = IO.input;;
type outStream = unit IO.output;;

(*
 * opening an ooggFile
 *)

let file_open ?(writable=false) name = 
  if writable then
    Unix.openfile name [Unix.O_RDWR ; Unix.O_CREAT ; Unix.O_TRUNC] 0o644
  else
    Unix.openfile name [Unix.O_RDONLY] 0o644;;

(*
 * align a stream on a page boundary.  We need to look for 'OggS'.
 * return characters consumed
 *)
let rec align stream =
  let rec _check_value value = 
    match value with
    | "OggS" -> 4
    | l      -> 4 + if (String.contains l 'O') 
                    then
                      begin
                        let p = String.index l 'O' in
                        let next_read = IO.really_nread stream p in
                        let result = String.create 4 in
                        String.blit l p result 0 (4 - p); 
                        String.blit next_read 0 result (4 - p) p;
                        _check_value result + p
                      end
                    else align stream in 
  let first_read = IO.really_nread stream 4 in
  _check_value first_read;;

(*
 * retrieving a stream from an ooggFile.  Always align to an ogg page.
 *)
let to_stream file position =
  ignore (Unix.lseek file position Unix.SEEK_SET);
  let input = IO.input_channel (Unix.in_channel_of_descr file) in
  let offset = (align input) - 4 in
  ignore (Unix.lseek file (position + offset) Unix.SEEK_SET);
  IO.input_channel (Unix.in_channel_of_descr file);;

let to_output_stream file position =
  ignore (Unix.lseek file position Unix.SEEK_SET);
  IO.output_channel (Unix.out_channel_of_descr file);;