/* GObject - GLib Type, Object, Parameter and Signal Library

 * Copyright (C) 2009 Benjamin Otte <otte@gnome.org>

 *

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

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

 * License as published by the Free Software Foundation; either

 * version 2.1 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

 * Lesser General Public License for more details.

 *

 * You should have received a copy of the GNU Lesser General

 * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.

 */

#include "config.h"

#include <string.h>

#include "gatomicarray.h"

/* A GAtomicArray is a growable, mutable array of data

 * generally of the form of a header of a specific size and

 * then a array of items of a fixed size.

 *

 * It is possible to do lock-less read transactions from the

 * array without any protection against other reads or writes,

 * but such read operation must be aware that the data in the

 * atomic array can change at any time during the transaction,

 * and only at the end can we verify if the transaction succeeded

 * or not. Thus the reading transaction cannot for instance

 * dereference a pointer in the array inside the transaction.

 *

 * The size of an array however cannot change during a read

 * transaction.

 *

 * Writes to the array is done in a copy-update style, but there

 * is no real protection against multiple writers overwriting each

 * others updates, so writes must be protected by an external lock.

 */

G_LOCK_DEFINE_STATIC (array);

typedef struct _FreeListNode FreeListNode;

struct _FreeListNode {

  FreeListNode *next;

};

/* This is really a list of array memory blocks, using the

 * first item as the next pointer to chain them together.

 * Protected by array lock */

static FreeListNode *freelist = NULL;

/* must hold array lock */

static gpointer

freelist_alloc (gsize size, gboolean reuse)

{

  gpointer mem;

  FreeListNode *free, **prev;

  gsize real_size;

  if (reuse)

    {

      for (free = freelist, prev = &freelist; free != NULL; prev = &free->next, free = free->next)

	{

	  if (G_ATOMIC_ARRAY_DATA_SIZE (free) == size)

	    {

	      *prev = free->next;

	      return (gpointer)free;

	    }

	}

    }

  real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));

  mem = g_slice_alloc (real_size);

  mem = ((char *) mem) + sizeof (gsize);

  G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;

  return mem;

}

/* must hold array lock */

static void

freelist_free (gpointer mem)

{

  FreeListNode *free;

  free = mem;

  free->next = freelist;

  freelist = free;

}

void

_g_atomic_array_init (GAtomicArray *array)

{

  array->data = NULL;

}

/* Get a copy of the data (if non-NULL) that

 * can be changed and then re-applied with

 * g_atomic_array_update().

 *

 * If additional_element_size is > 0 then

 * then the new memory chunk is that much

 * larger, or there were no data we return

 * a chunk of header_size + additional_element_size.

 * This means you can use this to grow the

 * array part and it handles the first element

 * being added automatically.

 *

 * We don't support shrinking arrays, as if

 * we then re-grow we may reuse an old pointer

 * value and confuse the transaction check.

 */

gpointer

_g_atomic_array_copy (GAtomicArray *array,

		      gsize header_size,

		      gsize additional_element_size)

{

  guint8 *new, *old;

  gsize old_size, new_size;

  G_LOCK (array);

  old = g_atomic_pointer_get (&array->data);

  if (old)

    {

      old_size = G_ATOMIC_ARRAY_DATA_SIZE (old);

      new_size = old_size + additional_element_size;

      /* Don't reuse if copying to same size, as this may end

	 up reusing the same pointer for the same array thus

	 confusing the transaction check */

      new = freelist_alloc (new_size, additional_element_size != 0);

      memcpy (new, old, old_size);

    }

  else if (additional_element_size != 0)

    {

      new_size = header_size + additional_element_size;

      new = freelist_alloc (new_size, TRUE);

    }

  else

    new = NULL;

  G_UNLOCK (array);

  return new;

}

/* Replace the data in the array with the new data,

 * freeing the old data (for reuse). The new data may

 * not be smaller than the current data.

 */

void

_g_atomic_array_update (GAtomicArray *array,

			gpointer new_data)

{

  guint8 *old;

  G_LOCK (array);

  old = g_atomic_pointer_get (&array->data);

  g_assert (old == NULL || G_ATOMIC_ARRAY_DATA_SIZE (old) <= G_ATOMIC_ARRAY_DATA_SIZE (new_data));

  g_atomic_pointer_set (&array->data, new_data);

  if (old)

    freelist_free (old);

  G_UNLOCK (array);

}