DEFINITION MODULE Queues;

(*
 This module provides procedures for queue'ing (lifo, fifo, and combinations)
 objects up to 512 bytes in size.
 A queue may be made of non-homogeneous objects, but care muse be taken to pop
 data into the same format as there object which was push'ed.
 If an item longer than 512 bytes is pushed, then only the first 512 bytes is
 stored on the queue. And conversely, only as much data is pop'ed as can be
 stored in the object which will contain the pop'ed item.
*)
(* J. Andrea, Oct.11/91 *)
(* This code may be freely used and distributed, it may not be sold. *)

FROM SYSTEM IMPORT BYTE;

EXPORT QUALIFIED Queue, Directions, InitQueue, Pop, Push, DelQueue, QueueInfo;

TYPE
   Queue; (* hidden *)

   Directions = (Top,Bottom);

PROCEDURE InitQueue( VAR a_queue :Queue );
(* Create a queue, this must be called before the first Push *)

PROCEDURE Push( a_queue :Queue; which_end :Directions; data :ARRAY OF BYTE );
(* Push an item onto the Queue, any item up to 512 bytes in size *)

PROCEDURE Pop( a_queue :Queue; which_end :Directions;
               VAR data :ARRAY OF BYTE; VAR nothing_to_pop :BOOLEAN );
(* Pop the next item from the queue,
   return 'empty' as TRUE is there is nothing to get popped *)

PROCEDURE DelQueue( VAR a_queue :Queue );
(* Delete the queue completely *)

PROCEDURE QueueInfo( a_queue :Queue; which_end :Directions;
                     VAR n_items, size :CARDINAL );
(* Get some information about the queue
   'n_items' is the number of items currently on the queue,
   'size'    is the size of the item at the specified end of the queue *)

END Queues.