source: trunk/athena/etc/larvnetd/timer.c @ 11936

/* Copyright 1998 by the Massachusetts Institute of Technology.
 *
 * Permission to use, copy, modify, and distribute this
 * software and its documentation for any purpose and without
 * fee is hereby granted, provided that the above copyright
 * notice appear in all copies and that both that copyright
 * notice and this permission notice appear in supporting
 * documentation, and that the name of M.I.T. not be used in
 * advertising or publicity pertaining to distribution of the
 * software without specific, written prior permission.
 * M.I.T. makes no representations about the suitability of
 * this software for any purpose.  It is provided "as is"
 * without express or implied warranty.
 */

/* This file implements a mini-library of functions for setting up
 * and processing timers.
 */

static const char rcsid[] = "$Id: timer.c,v 1.2 1998-09-15 15:03:37 ghudson Exp $";

#include <sys/types.h>
#include <sys/time.h>
#include <stdlib.h>
#include <time.h>
#include <syslog.h>
#include "larvnetd.h"
#include "timer.h"

/* DELTA is just an offset to keep the size a bit less than a power of
 * two.  It's measured in pointers, so it's 32 bytes on most systems.
 */
#define DELTA 8
#define INITIAL_HEAP_SIZE (1024 - DELTA)

/* We have three operations which we need to be able to perform
 * quickly: adding a timer, deleting a timer given a pointer to
 * it, and determining which timer will be the next to go off.  A
 * heap is an ideal data structure for these purposes, so we use
 * one.  The heap is an array of pointers to timers, and each timer
 * knows the position of its pointer in the heap.
 *
 * Okay, what is the heap, exactly?  It's a data structure,
 * represented as an array, with the invariant condition that
 * the timeout of heap[i] is less than or equal to the timeout of
 * heap[i * 2 + 1] and heap[i * 2 + 2] (assuming i * 2 + 1 and
 * i * 2 + 2 are valid * indices).  An obvious consequence of this
 * is that heap[0] has the lowest timer value, so finding the first
 * timer to go off is easy.  We say that an index i has "children"
 * i * 2 + 1 and i * 2 + 1, and the "parent" (i - 1) / 2.
 *
 * To add a timer to the heap, we start by adding it to the end, and
 * then keep swapping it with its parent until it has a parent with
 * a timer value less than its value.  With a little bit of thought,
 * you can see that this preserves the heap property on all indices
 * of the array.
 *
 * To delete a timer at position i from the heap, we discard it and
 * fill in its position with the last timer in the heap.  In order
 * to restore the heap, we have to consider two cases: the timer
 * value at i is less than that of its parent, or the timer value at
 * i is greater than that of one of its children.  In the first case,
 * we propagate the timer at i up the tree, swapping it with its
 * parent, until the heap is restored; in the second case, we
 * propagate the timer down the tree, swapping it with its least
 * child, until the heap is restored.
 */

/* In order to ensure that the back pointers from timers are consistent
 * with the heap pointers, all heap assignments should be done with the
 * HEAP_ASSIGN() macro, which sets the back pointer and updates the
 * heap at the same time.
 */
#define PARENT(i) (((i) - 1) / 2)
#define CHILD1(i) ((i) * 2 + 1)
#define CHILD2(i) ((i) * 2 + 2)
#define TIME(i) (heap[i]->abstime)
#define HEAP_ASSIGN(pos, tmr) ((heap[pos] = (tmr))->heap_pos = (pos))

static Timer **heap;
static int num_timers = 0;
static int heap_size = 0;

static void timer_botch(void *);
static Timer *add_timer(Timer *);

Timer *timer_set_rel(int reltime, Timer_proc proc, void *arg)
{
  return timer_set_abs(time(NULL) + reltime, proc, arg);
}

Timer *timer_set_abs(time_t abstime, Timer_proc proc, void *arg)
{
  Timer *timer;

  timer = (Timer *) emalloc(sizeof(Timer));
  timer->abstime = abstime;
  timer->func = proc;
  timer->arg = arg;
  return add_timer(timer);
}

void *timer_reset(Timer *timer)
{
  int pos, min;
  void *arg;

  /* Free the timer, saving its heap position and argument. */
  pos = timer->heap_pos;
  arg = timer->arg;
  free(timer);

  if (pos != num_timers - 1)
    {
      /* Replace the timer with the last timer in the heap and
       * restore the heap, propagating the timer either up or
       * down, depending on which way it violates the heap
       * property to insert the last timer in place of the
       * deleted timer.
       */
      if (pos > 0 && TIME(num_timers - 1) < TIME(PARENT(pos)))
        {
          do
            {
              HEAP_ASSIGN(pos, heap[PARENT(pos)]);
              pos = PARENT(pos);
            }
          while (pos > 0 && TIME(num_timers - 1) < TIME(PARENT(pos)));
          HEAP_ASSIGN(pos, heap[num_timers - 1]);
        }
      else
        {
          while (CHILD2(pos) < num_timers)
            {
              min = num_timers - 1;
              if (TIME(CHILD1(pos)) < TIME(min))
                min = CHILD1(pos);
              if (TIME(CHILD2(pos)) < TIME(min))
                min = CHILD2(pos);
              HEAP_ASSIGN(pos, heap[min]);
              pos = min;
            }
          if (pos != num_timers - 1)
            HEAP_ASSIGN(pos, heap[num_timers - 1]);
        }
    }
  num_timers--;
  return arg;
}

static Timer *add_timer(Timer *new)
{
  int pos;

  /* Create or resize the heap as necessary. */
  if (heap_size == 0)
    {
      heap_size = INITIAL_HEAP_SIZE;
      heap = (Timer **) emalloc(heap_size * sizeof(Timer *));
    }
  else if (num_timers >= heap_size)
    {
      heap_size = heap_size * 2 + DELTA;
      heap = (Timer **) erealloc(heap, heap_size * sizeof(Timer *));
    }

  /* Insert the Timer *into the heap. */
  pos = num_timers;
  while (pos > 0 && new->abstime < TIME(PARENT(pos)))
    {
      HEAP_ASSIGN(pos, heap[PARENT(pos)]);
      pos = PARENT(pos);
    }
  HEAP_ASSIGN(pos, new);
  num_timers++;

  return new;
}

void timer_process(void)
{
  Timer *t;
  Timer_proc func;
  void *arg;

  if (num_timers == 0 || heap[0]->abstime > time(NULL))
    return;

  /* Remove the first timer from the heap, remembering its
   * function and argument.
   */
  t = heap[0];
  func = t->func;
  arg = t->arg;
  t->func = timer_botch;
  t->arg = NULL;
  timer_reset(t);

  /* Run the function. */
  func(arg);
}

struct timeval *timer_timeout(struct timeval *tvbuf)
{
  if (num_timers > 0)
    {
      tvbuf->tv_sec = heap[0]->abstime - time(NULL);
      if (tvbuf->tv_sec < 0)
        tvbuf->tv_sec = 0;
      tvbuf->tv_usec = 0;
      return tvbuf;
    }
  else
    return NULL;
}

static void timer_botch(void *arg)
{
  syslog(LOG_ALERT, "timer botch");
  abort();
}