/* GStreamer

 * Copyright (C) 2009 Edward Hervey <bilboed@bilboed.com>

 *               2011 Wim Taymans <wim.taymans@gmail.com>

 *

 * gstatomicqueue.c:

 *

 * This library is free software; you can redistribute it and/or

 * modify it under the terms of the GNU Library General Public

 * License as published by the Free Software Foundation; either

 * version 2 of the License, or (at your option) any later version.

 *

 * This library is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 * Library General Public License for more details.

 *

 * You should have received a copy of the GNU Library General Public

 * License along with this library; if not, write to the

 * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,

 * Boston, MA 02110-1301, USA.

 */

#include "gst_private.h"

#include <string.h>

#include <gst/gst.h>

#include "gstatomicqueue.h"

#include "glib-compat-private.h"

/**

 * SECTION:gstatomicqueue

 * @title: GstAtomicQueue

 * @short_description: An atomic queue implementation

 *

 * The #GstAtomicQueue object implements a queue that can be used from multiple

 * threads without performing any blocking operations.

 */

G_DEFINE_BOXED_TYPE (GstAtomicQueue, gst_atomic_queue,

    (GBoxedCopyFunc) gst_atomic_queue_ref,

    (GBoxedFreeFunc) gst_atomic_queue_unref);

/* By default the queue uses 2 * sizeof(gpointer) * clp2 (max_items) of

 * memory. clp2(x) is the next power of two >= than x.

 *

 * The queue can operate in low memory mode, in which it consumes almost

 * half the memory at the expense of extra overhead in the readers. This

 * is disabled by default because even without LOW_MEM mode, the memory

 * consumption is still lower than a plain GList.

 */

#undef LOW_MEM

typedef struct _GstAQueueMem GstAQueueMem;

struct _GstAQueueMem

{

  gint size;

  gpointer *array;

  volatile gint head;

  volatile gint tail_write;

  volatile gint tail_read;

  GstAQueueMem *next;

  GstAQueueMem *free;

};

static guint

clp2 (guint n)

{

  guint res = 1;

  while (res < n)

    res <<= 1;

  return res;

}

static GstAQueueMem *

new_queue_mem (guint size, gint pos)

{

  GstAQueueMem *mem;

  mem = g_new (GstAQueueMem, 1);

  /* we keep the size as a mask for performance */

  mem->size = clp2 (MAX (size, 16)) - 1;

  mem->array = g_new0 (gpointer, mem->size + 1);

  mem->head = pos;

  mem->tail_write = pos;

  mem->tail_read = pos;

  mem->next = NULL;

  mem->free = NULL;

  return mem;

}

static void

free_queue_mem (GstAQueueMem * mem)

{

  g_free (mem->array);

  g_free (mem);

}

struct _GstAtomicQueue

{

  volatile gint refcount;

#ifdef LOW_MEM

  gint num_readers;

#endif

  GstAQueueMem *head_mem;

  GstAQueueMem *tail_mem;

  GstAQueueMem *free_list;

};

static void

add_to_free_list (GstAtomicQueue * queue, GstAQueueMem * mem)

{

  do {

    mem->free = g_atomic_pointer_get (&queue->free_list);

  } while (!g_atomic_pointer_compare_and_exchange (&queue->free_list,

          mem->free, mem));

}

static void

clear_free_list (GstAtomicQueue * queue)

{

  GstAQueueMem *free_list;

  /* take the free list and replace with NULL */

  do {

    free_list = g_atomic_pointer_get (&queue->free_list);

    if (free_list == NULL)

      return;

  } while (!g_atomic_pointer_compare_and_exchange (&queue->free_list, free_list,

          NULL));

  while (free_list) {

    GstAQueueMem *next = free_list->free;

    free_queue_mem (free_list);

    free_list = next;

  }

}

/**

 * gst_atomic_queue_new:

 * @initial_size: initial queue size

 *

 * Create a new atomic queue instance. @initial_size will be rounded up to the

 * nearest power of 2 and used as the initial size of the queue.

 *

 * Returns: a new #GstAtomicQueue

 */

GstAtomicQueue *

gst_atomic_queue_new (guint initial_size)

{

  GstAtomicQueue *queue;

  queue = g_new (GstAtomicQueue, 1);

  queue->refcount = 1;

#ifdef LOW_MEM

  queue->num_readers = 0;

#endif

  queue->head_mem = queue->tail_mem = new_queue_mem (initial_size, 0);

  queue->free_list = NULL;

  return queue;

}

/**

 * gst_atomic_queue_ref:

 * @queue: a #GstAtomicQueue

 *

 * Increase the refcount of @queue.

 */

void

gst_atomic_queue_ref (GstAtomicQueue * queue)

{

  g_return_if_fail (queue != NULL);

  g_atomic_int_inc (&queue->refcount);

}

static void

gst_atomic_queue_free (GstAtomicQueue * queue)

{

  free_queue_mem (queue->head_mem);

  if (queue->head_mem != queue->tail_mem)

    free_queue_mem (queue->tail_mem);

  clear_free_list (queue);

  g_free (queue);

}

/**

 * gst_atomic_queue_unref:

 * @queue: a #GstAtomicQueue

 *

 * Unref @queue and free the memory when the refcount reaches 0.

 */

void

gst_atomic_queue_unref (GstAtomicQueue * queue)

{

  g_return_if_fail (queue != NULL);

  if (g_atomic_int_dec_and_test (&queue->refcount))

    gst_atomic_queue_free (queue);

}

/**

 * gst_atomic_queue_peek:

 * @queue: a #GstAtomicQueue

 *

 * Peek the head element of the queue without removing it from the queue.

 *

 * Returns: (transfer none) (nullable): the head element of @queue or

 * %NULL when the queue is empty.

 */

gpointer

gst_atomic_queue_peek (GstAtomicQueue * queue)

{

  GstAQueueMem *head_mem;

  gint head, tail, size;

  g_return_val_if_fail (queue != NULL, NULL);

  while (TRUE) {

    GstAQueueMem *next;

    head_mem = g_atomic_pointer_get (&queue->head_mem);

    head = g_atomic_int_get (&head_mem->head);

    tail = g_atomic_int_get (&head_mem->tail_read);

    size = head_mem->size;

    /* when we are not empty, we can continue */

    if (G_LIKELY (head != tail))

      break;

    /* else array empty, try to take next */

    next = g_atomic_pointer_get (&head_mem->next);

    if (next == NULL)

      return NULL;

    /* now we try to move the next array as the head memory. If we fail to do that,

     * some other reader managed to do it first and we retry */

    if (!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,

            next))

      continue;

    /* when we managed to swing the head pointer the old head is now

     * useless and we add it to the freelist. We can't free the memory yet

     * because we first need to make sure no reader is accessing it anymore. */

    add_to_free_list (queue, head_mem);

  }

  return head_mem->array[head & size];

}

/**

 * gst_atomic_queue_pop:

 * @queue: a #GstAtomicQueue

 *

 * Get the head element of the queue.

 *

 * Returns: (transfer full): the head element of @queue or %NULL when

 * the queue is empty.

 */

gpointer

gst_atomic_queue_pop (GstAtomicQueue * queue)

{

  gpointer ret;

  GstAQueueMem *head_mem;

  gint head, tail, size;

  g_return_val_if_fail (queue != NULL, NULL);

#ifdef LOW_MEM

  g_atomic_int_inc (&queue->num_readers);

#endif

  do {

    while (TRUE) {

      GstAQueueMem *next;

      head_mem = g_atomic_pointer_get (&queue->head_mem);

      head = g_atomic_int_get (&head_mem->head);

      tail = g_atomic_int_get (&head_mem->tail_read);

      size = head_mem->size;

      /* when we are not empty, we can continue */

      if G_LIKELY

        (head != tail)

            break;

      /* else array empty, try to take next */

      next = g_atomic_pointer_get (&head_mem->next);

      if (next == NULL)

        return NULL;

      /* now we try to move the next array as the head memory. If we fail to do that,

       * some other reader managed to do it first and we retry */

      if G_UNLIKELY

        (!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,

                next))

            continue;

      /* when we managed to swing the head pointer the old head is now

       * useless and we add it to the freelist. We can't free the memory yet

       * because we first need to make sure no reader is accessing it anymore. */

      add_to_free_list (queue, head_mem);

    }

    ret = head_mem->array[head & size];

  } while G_UNLIKELY

  (!g_atomic_int_compare_and_exchange (&head_mem->head, head, head + 1));

#ifdef LOW_MEM

  /* decrement number of readers, when we reach 0 readers we can be sure that

   * none is accessing the memory in the free list and we can try to clean up */

  if (g_atomic_int_dec_and_test (&queue->num_readers))

    clear_free_list (queue);

#endif

  return ret;

}

/**

 * gst_atomic_queue_push:

 * @queue: a #GstAtomicQueue

 * @data: the data

 *

 * Append @data to the tail of the queue.

 */

void

gst_atomic_queue_push (GstAtomicQueue * queue, gpointer data)

{

  GstAQueueMem *tail_mem;

  gint head, tail, size;

  g_return_if_fail (queue != NULL);

  do {

    while (TRUE) {

      GstAQueueMem *mem;

      tail_mem = g_atomic_pointer_get (&queue->tail_mem);

      head = g_atomic_int_get (&tail_mem->head);

      tail = g_atomic_int_get (&tail_mem->tail_write);

      size = tail_mem->size;

      /* we're not full, continue */

      if G_LIKELY

        (tail - head <= size)

            break;

      /* else we need to grow the array, we store a mask so we have to add 1 */

      mem = new_queue_mem ((size << 1) + 1, tail);

      /* try to make our new array visible to other writers */

      if G_UNLIKELY

        (!g_atomic_pointer_compare_and_exchange (&queue->tail_mem, tail_mem,

                mem)) {

        /* we tried to swap the new writer array but something changed. This is

         * because some other writer beat us to it, we free our memory and try

         * again */

        free_queue_mem (mem);

        continue;

        }

      /* make sure that readers can find our new array as well. The one who

       * manages to swap the pointer is the only one who can set the next

       * pointer to the new array */

      g_atomic_pointer_set (&tail_mem->next, mem);

    }

  } while G_UNLIKELY

  (!g_atomic_int_compare_and_exchange (&tail_mem->tail_write, tail, tail + 1));

  tail_mem->array[tail & size] = data;

  /* now wait until all writers have completed their write before we move the

   * tail_read to this new item. It is possible that other writers are still

   * updating the previous array slots and we don't want to reveal their changes

   * before they are done. FIXME, it would be nice if we didn't have to busy

   * wait here. */

  while G_UNLIKELY

    (!g_atomic_int_compare_and_exchange (&tail_mem->tail_read, tail, tail + 1));

}

/**

 * gst_atomic_queue_length:

 * @queue: a #GstAtomicQueue

 *

 * Get the amount of items in the queue.

 *

 * Returns: the number of elements in the queue.

 */

guint

gst_atomic_queue_length (GstAtomicQueue * queue)

{

  GstAQueueMem *head_mem, *tail_mem;

  gint head, tail;

  g_return_val_if_fail (queue != NULL, 0);

#ifdef LOW_MEM

  g_atomic_int_inc (&queue->num_readers);

#endif

  head_mem = g_atomic_pointer_get (&queue->head_mem);

  head = g_atomic_int_get (&head_mem->head);

  tail_mem = g_atomic_pointer_get (&queue->tail_mem);

  tail = g_atomic_int_get (&tail_mem->tail_read);

#ifdef LOW_MEM

  if (g_atomic_int_dec_and_test (&queue->num_readers))

    clear_free_list (queue);

#endif

  return tail - head;

}