#include "timerwheel.h"

struct _TWEntry
{
  TWEntry *next, **pprev;
  guint64 target;
  TWCallbackFunc callback;
  gpointer user_data;
  GDestroyNotify user_data_free;
};

void
tw_entry_prepend(TWEntry **pfirst, TWEntry *new)
{
  new->next = *pfirst;
  new->pprev = pfirst;
  if (*pfirst)
    (*pfirst)->pprev = &new->next;
  *pfirst = new;
}

void
tw_entry_unlink(TWEntry *entry)
{
  if (entry->next)
    entry->next->pprev = entry->pprev;
  *(entry->pprev) = entry->next;
}

static void
tw_entry_free(TWEntry *entry)
{
  if (entry->user_data && entry->user_data_free)
    entry->user_data_free(entry->user_data);
  g_free(entry);
}

#if ENABLE_DEBUG
static void
tw_entry_list_validate(TWEntry **head)
{
  TWEntry *last, *first, *p;

  last = NULL;
  first = *head;
  g_assert(first->pprev == head);
  for (p = first; p; p = p->next)
    {
      if (last)
        g_assert(p->pprev == &last->next);
      last = p;
    }
}

#else

static inline void
tw_entry_list_validate(TWEntry **head)
{
}

#endif

typedef struct _TWLevel
{
  /* mask of the bits that index the current level */
  guint64 mask;
  /* */
  guint64 slot_mask;
  guint16 num;
  guint8  shift;
  TWEntry *slots[0];
} TWLevel;

TWLevel *
tw_level_new(gint bits, gint shift)
{
  TWLevel *self;
  gint num;

  num = (1 << bits);
  self = g_malloc0(sizeof(TWLevel) + num * sizeof(self->slots[0]));
  self->shift = shift;
  self->mask = (num - 1) << shift;
  self->slot_mask = (1 << shift) - 1;
  self->num = num;
  return self;
}

void
tw_level_free(TWLevel *self)
{
  gint i;

  for (i = 0; i < self->num; i++)
    {
      TWEntry *entry;
      TWEntry *next;

      for (entry = self->slots[i]; entry; entry = next)
        {
          next = entry->next;
          tw_entry_free(entry);
        }
    }
  g_free(self);
}

struct _TimerWheel
{
  TWLevel *levels[4];
  TWEntry *future;
  guint64 now;
  guint64 base;
  gint num_timers;
};

void
timer_wheel_add_timer_entry(TimerWheel *self, TWEntry *entry)
{
  guint64 level_base;
  gint slot;
  gint level_ndx;

  for (level_ndx = 0; level_ndx < G_N_ELEMENTS(self->levels); level_ndx++)
    {
      TWLevel *level = self->levels[level_ndx];

      /* level_base contains the time of associated with level->slots[0] */
      level_base = self->base & ~level->slot_mask & ~level->mask;

      if (entry->target > level_base + (level->num << level->shift))
        {
          if (entry->target < level_base + (level->num << level->shift) * 2 &&
              (self->now & level->mask) > (entry->target & level->mask))
            {
              /* target is in the next slot at the next level and
                 we've already crossed the slot in the current
                 level */
            }
          else
            {
              /* target over the current array */
              continue;
            }
        }

      slot = (entry->target & level->mask) >> level->shift;
      tw_entry_prepend(&level->slots[slot], entry);
      tw_entry_list_validate(&level->slots[slot]);
      break;
    }
  if (level_ndx >= G_N_ELEMENTS(self->levels))
    {
      tw_entry_prepend(&self->future, entry);
      tw_entry_list_validate(&self->future);
    }

}

TWEntry *
timer_wheel_add_timer(TimerWheel *self, gint timeout, TWCallbackFunc cb, gpointer user_data, GDestroyNotify user_data_free)
{
  TWEntry *entry;

  entry = g_new0(TWEntry, 1);
  entry->target = self->now + timeout;
  entry->callback = cb;
  entry->user_data = user_data;
  entry->user_data_free = user_data_free;

  timer_wheel_add_timer_entry(self, entry);
  self->num_timers++;
  return entry;
}

void
timer_wheel_del_timer(TimerWheel *self, TWEntry *entry)
{
  tw_entry_unlink(entry);
  tw_entry_free(entry);
  self->num_timers--;
}

void
timer_wheel_mod_timer(TimerWheel *self, TWEntry *entry, gint new_timeout)
{
  tw_entry_unlink(entry);
  entry->target = self->now + new_timeout;
  timer_wheel_add_timer_entry(self, entry);
}

guint64
timer_wheel_get_timer_expiration(TimerWheel *self, TWEntry *entry)
{
  return entry->target;
}

static void
timer_wheel_cascade(TimerWheel *self)
{
  gint level_ndx;
  TWLevel *source_level, *target_level;
  TWEntry *next, **source_head, **target_head, *entry;
  gint source_slot, target_slot;
  guint64 target_level_base;

  for (level_ndx = 1; level_ndx < G_N_ELEMENTS(self->levels); level_ndx++)
    {
      source_level = self->levels[level_ndx];
      target_level = self->levels[level_ndx - 1];

      source_slot = ((self->now & source_level->mask) >> source_level->shift);
      if (source_slot == source_level->num - 1)
        source_slot = 0;
      else
        source_slot++;

      source_head = &source_level->slots[source_slot];
      for (entry = *source_head; entry; entry = next)
        {
          next = entry->next;

          target_slot = (entry->target & target_level->mask) >> target_level->shift;
          target_head = &target_level->slots[target_slot];

          /* we don't need to unlink entry from the list as that list is completely emptied anyway */
          tw_entry_prepend(target_head, entry);
          tw_entry_list_validate(target_head);
        }
      *source_head = NULL;

      if (source_slot < source_level->num - 1)
        break;
    }

  if (level_ndx == G_N_ELEMENTS(self->levels))
    {
      source_head = &self->future;
      target_level = self->levels[level_ndx - 1];
      for (entry = *source_head; entry; entry = next)
        {
          next = entry->next;

          target_level_base = self->base & ~target_level->slot_mask & ~target_level->mask;
          if (entry->target < target_level_base + 2 * (target_level->num << target_level->shift))
            {
              target_slot = (entry->target & target_level->mask) >> target_level->shift;
              target_head = &target_level->slots[target_slot];

              /* unlink current entry */
              tw_entry_unlink(entry);
              tw_entry_list_validate(source_head);
              tw_entry_prepend(target_head, entry);
              tw_entry_list_validate(target_head);
            }
        }
    }

  self->base += self->levels[0]->num;
}


/*
 * Main time adjustment function
 */
void
timer_wheel_set_time(TimerWheel *self, guint64 new_now)
{
  /* time is not allowed to go backwards */
  if (self->now >= new_now)
    return;

  if (self->num_timers == 0)
    {
      /* if there are no timers registered with the current base, we
         can simply switch to a new one */

      self->now = new_now;
      self->base = new_now & ~self->levels[0]->mask;
      return;
    }

  for (; self->now < new_now; self->now++)
    {
      TWEntry **head, *next, *entry;
      gint slot;
      TWLevel *level = self->levels[0];

      slot = (self->now & level->mask) >> level->shift;

      head = &self->levels[0]->slots[slot];
      for (entry = *head; entry; entry = next)
        {
          next = entry->next;

          entry->callback(self->now, entry->user_data);
          tw_entry_free(entry);
          self->num_timers--;
        }
      *head = NULL;

      if (self->num_timers == 0)
        {
          self->now = new_now;
          self->base = new_now & ~self->levels[0]->mask;
          break;
        }

      if (slot == level->num - 1)
        timer_wheel_cascade(self);
    }
}

guint64
timer_wheel_get_time(TimerWheel *self)
{
  return self->now;
}

void
timer_wheel_expire_all(TimerWheel *self)
{
  guint64 now;

  now = self->now;
  timer_wheel_set_time(self, (guint64) -1);
  self->now = now;
}

TimerWheel *
timer_wheel_new(void)
{
  TimerWheel *self;
  gint bits[] = { 10, 6, 6, 6, 0 };
  gint shift = 0;
  gint i;

  self = g_new0(TimerWheel, 1);
  for (i = 0; i < G_N_ELEMENTS(self->levels); i++)
    {
      self->levels[i] = tw_level_new(bits[i], shift);
      shift += bits[i];
    }
  return self;
}

void
timer_wheel_free(TimerWheel *self)
{
  gint i;
  for (i = 0; i < G_N_ELEMENTS(self->levels); i++)
    tw_level_free(self->levels[i]);
  g_free(self);
}
