//                    queue.cpp      S. Murrell    11-11-99

// This is an INTERLOCKED QUEUE of messages, suitable for communicating
// between threads. It needs the threads library and the -pthread option
// to CC when linking. It also needs the MESSAGE class, because those are
// the things it makes a queue of.

#include <string>
#include "threads.h"
#include <iostream.h>
#include "message.h"

class Queue
{ public:
    Queue(void): front(NULL), back(NULL) { s=create_semaphore(); }
    void add(Message *m);
    Message *remove(void);
    string look(void);
    int isempty(void);
  protected:
    Message *front, *back;
    semaphore s; };

void Queue::add(Message *m)
{ grab(s);
  if (back==NULL)
  { front=m; back=m; }
  else
  { back->next=m;
    back=m; }
  release(s); }

Message *Queue::remove(void)
{ grab(s);
  if (front==NULL)
  { release(s);
    return NULL; }
  Message *m=front;
  front=front->next;
  if (front==NULL) back=NULL;
  release(s);
  return m; }

string Queue::look(void)
{ grab(s);
  if (front==NULL)
  { release(s);
    return ""; }
  string r=front->msg;
  release(s);
  return r; }

int Queue::isempty(void)
{ return (front==NULL); }