gstreamer/gst/gsttag.c

/* GStreamer
 * Copyright (C) 2003 Benjamin Otte <in7y118@public.uni-hamburg.de>
 *
 * gsttag.c: tag support (aka metadata)
 *
 * 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., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

#ifdef HAVE_CONFIG_H
#  include "config.h"
#endif

#include "gst_private.h"
#include "gst-i18n-lib.h"
#include "gsttag.h"
#include "gstinfo.h"
#include "gstvalue.h"

#include <gobject/gvaluecollector.h>
#include <string.h>

#define GST_TAG_IS_VALID(tag)		(gst_tag_get_info (tag) != NULL)

typedef struct
{
  GType type;                   /* type the data is in */

  gchar *nick;                  /* translated name */
  gchar *blurb;                 /* translated description of type */

  GstTagMergeFunc merge_func;   /* functions to merge the values */
  GstTagFlag flag;              /* type of tag */
}
GstTagInfo;

#define TAGLIST "taglist"
static GQuark gst_tag_list_quark;
static GMutex *__tag_mutex;
static GHashTable *__tags;

#define TAG_LOCK g_mutex_lock (__tag_mutex)
#define TAG_UNLOCK g_mutex_unlock (__tag_mutex)

GType
gst_tag_list_get_type (void)
{
  static GType _gst_tag_list_type;

  if (_gst_tag_list_type == 0) {
    _gst_tag_list_type = g_boxed_type_register_static ("GstTagList",
        (GBoxedCopyFunc) gst_tag_list_copy, (GBoxedFreeFunc) gst_tag_list_free);

#if 0
    g_value_register_transform_func (_gst_tag_list_type, G_TYPE_STRING,
        _gst_structure_transform_to_string);
#endif
  }

  return _gst_tag_list_type;
}

void
_gst_tag_initialize (void)
{
  gst_tag_list_quark = g_quark_from_static_string (TAGLIST);
  __tag_mutex = g_mutex_new ();
  __tags = g_hash_table_new (g_direct_hash, g_direct_equal);
  gst_tag_register (GST_TAG_TITLE, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("title"), _("commonly used title"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_ARTIST, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("artist"),
      _("person(s) responsible for the recording"),
      gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_ALBUM, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("album"),
      _("album containing this data"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_DATE, GST_TAG_FLAG_META, G_TYPE_UINT,       /* FIXME: own data type for dates? */
      _("date"),
      _("date the data was created (in Julian calendar days)"), NULL);
  gst_tag_register (GST_TAG_GENRE, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("genre"),
      _("genre this data belongs to"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_COMMENT, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("comment"),
      _("free text commenting the data"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_TRACK_NUMBER, GST_TAG_FLAG_META,
      G_TYPE_UINT,
      _("track number"),
      _("track number inside a collection"), gst_tag_merge_use_first);
  gst_tag_register (GST_TAG_TRACK_COUNT, GST_TAG_FLAG_META,
      G_TYPE_UINT,
      _("track count"),
      _("count of tracks inside collection this track belongs to"),
      gst_tag_merge_use_first);
  gst_tag_register (GST_TAG_ALBUM_VOLUME_NUMBER, GST_TAG_FLAG_META,
      G_TYPE_UINT,
      _("disc number"),
      _("disc number inside a collection"), gst_tag_merge_use_first);
  gst_tag_register (GST_TAG_ALBUM_VOLUME_COUNT, GST_TAG_FLAG_META,
      G_TYPE_UINT,
      _("disc count"),
      _("count of discs inside collection this disc belongs to"),
      gst_tag_merge_use_first);
  gst_tag_register (GST_TAG_LOCATION, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("location"),
      _("original location of file as a URI"),
      gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_DESCRIPTION, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("description"),
      _("short text describing the content of the data"),
      gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_VERSION, GST_TAG_FLAG_META,
      G_TYPE_STRING, _("version"), _("version of this data"), NULL);
  gst_tag_register (GST_TAG_ISRC, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("ISRC"),
      _
      ("International Standard Recording Code - see http://www.ifpi.org/isrc/"),
      NULL);
  gst_tag_register (GST_TAG_ORGANIZATION, GST_TAG_FLAG_META, G_TYPE_STRING, _("organization"), _("organization"),       /* FIXME */
      gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_COPYRIGHT, GST_TAG_FLAG_META,
      G_TYPE_STRING, _("copyright"), _("copyright notice of the data"), NULL);
  gst_tag_register (GST_TAG_CONTACT, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("contact"), _("contact information"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_LICENSE, GST_TAG_FLAG_META,
      G_TYPE_STRING, _("license"), _("license of data"), NULL);
  gst_tag_register (GST_TAG_PERFORMER, GST_TAG_FLAG_META,
      G_TYPE_STRING,
      _("performer"),
      _("person(s) performing"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_DURATION, GST_TAG_FLAG_DECODED,
      G_TYPE_UINT64,
      _("duration"), _("length in GStreamer time units (nanoseconds)"), NULL);
  gst_tag_register (GST_TAG_CODEC, GST_TAG_FLAG_ENCODED,
      G_TYPE_STRING,
      _("codec"),
      _("codec the data is stored in"), gst_tag_merge_strings_with_comma);
  gst_tag_register (GST_TAG_VIDEO_CODEC, GST_TAG_FLAG_ENCODED,
      G_TYPE_STRING,
      _("video codec"), _("codec the video data is stored in"), NULL);
  gst_tag_register (GST_TAG_AUDIO_CODEC, GST_TAG_FLAG_ENCODED,
      G_TYPE_STRING,
      _("audio codec"), _("codec the audio data is stored in"), NULL);
  gst_tag_register (GST_TAG_BITRATE, GST_TAG_FLAG_ENCODED,
      G_TYPE_UINT, _("bitrate"), _("exact or average bitrate in bits/s"), NULL);
  gst_tag_register (GST_TAG_NOMINAL_BITRATE, GST_TAG_FLAG_ENCODED,
      G_TYPE_UINT, _("nominal bitrate"), _("nominal bitrate in bits/s"), NULL);
  gst_tag_register (GST_TAG_MINIMUM_BITRATE, GST_TAG_FLAG_ENCODED,
      G_TYPE_UINT, _("minimum bitrate"), _("minimum bitrate in bits/s"), NULL);
  gst_tag_register (GST_TAG_MAXIMUM_BITRATE, GST_TAG_FLAG_ENCODED,
      G_TYPE_UINT, _("maximum bitrate"), _("maximum bitrate in bits/s"), NULL);
  gst_tag_register (GST_TAG_ENCODER, GST_TAG_FLAG_ENCODED,
      G_TYPE_STRING,
      _("encoder"), _("encoder used to encode this stream"), NULL);
  gst_tag_register (GST_TAG_ENCODER_VERSION, GST_TAG_FLAG_ENCODED,
      G_TYPE_UINT,
      _("encoder version"),
      _("version of the encoder used to encode this stream"), NULL);
  gst_tag_register (GST_TAG_SERIAL, GST_TAG_FLAG_ENCODED,
      G_TYPE_UINT, _("serial"), _("serial number of track"), NULL);
  gst_tag_register (GST_TAG_TRACK_GAIN, GST_TAG_FLAG_META,
      G_TYPE_DOUBLE, _("replaygain track gain"), _("track gain in db"), NULL);
  gst_tag_register (GST_TAG_TRACK_PEAK, GST_TAG_FLAG_META,
      G_TYPE_DOUBLE, _("replaygain track peak"), _("peak of the track"), NULL);
  gst_tag_register (GST_TAG_ALBUM_GAIN, GST_TAG_FLAG_META,
      G_TYPE_DOUBLE, _("replaygain album gain"), _("album gain in db"), NULL);
  gst_tag_register (GST_TAG_ALBUM_PEAK, GST_TAG_FLAG_META,
      G_TYPE_DOUBLE, _("replaygain album peak"), _("peak of the album"), NULL);
}

/**
 * gst_tag_merge_use_first:
 * @dest: uninitialized GValue to store result in
 * @src: GValue to copy from
 *
 * This is a convenience function for the func argument of gst_tag_register(). 
 * It creates a copy of the first value from the list.
 */
void
gst_tag_merge_use_first (GValue * dest, const GValue * src)
{
  const GValue *ret = gst_value_list_get_value (src, 0);

  g_value_init (dest, G_VALUE_TYPE (ret));
  g_value_copy (ret, dest);
}

/**
 * gst_tag_merge_strings_with_comma:
 * @dest: uninitialized GValue to store result in
 * @src: GValue to copy from
 * 
 * This is a convenience function for the func argument of gst_tag_register().
 * It concatenates all given strings using a comma. The tag must be registered
 * as a G_TYPE_STRING or this function will fail.
 */
void
gst_tag_merge_strings_with_comma (GValue * dest, const GValue * src)
{
  GString *str;
  gint i, count;

  count = gst_value_list_get_size (src);
  str = g_string_new (g_value_get_string (gst_value_list_get_value (src, 0)));
  for (i = 1; i < count; i++) {
    /* seperator between two string */
    str = g_string_append (str, _(", "));
    str =
        g_string_append (str, g_value_get_string (gst_value_list_get_value (src,
                1)));
  }

  g_value_init (dest, G_TYPE_STRING);
  g_value_set_string_take_ownership (dest, str->str);
  g_string_free (str, FALSE);
}
static GstTagInfo *
gst_tag_lookup (GQuark entry)
{
  GstTagInfo *ret;

  TAG_LOCK;
  ret = g_hash_table_lookup (__tags, GUINT_TO_POINTER (entry));
  TAG_UNLOCK;

  return ret;
}

/**
 * gst_tag_register:
 * @name: the name or identifier string
 * @flag: a flag describing the type of tag info
 * @type: the type this data is in
 * @nick: human-readable name
 * @blurb: a human-readable description about this tag
 * @func: function for merging multiple values of this tag
 *
 * Registers a new tag type for the use with GStreamer's type system. If a type
 * with that name is already registered, that one is used.
 * The old registration may have used a different type however. So don't rely
 * on your supplied values.
 * This function takes ownership of all supplied variables.
 */
void
gst_tag_register (gchar * name, GstTagFlag flag, GType type,
    gchar * nick, gchar * blurb, GstTagMergeFunc func)
{
  GQuark key;
  GstTagInfo *info;

  g_return_if_fail (name != NULL);
  g_return_if_fail (nick != NULL);
  g_return_if_fail (blurb != NULL);
  g_return_if_fail (type != 0 && type != GST_TYPE_LIST);

  key = g_quark_from_string (name);
  info = gst_tag_lookup (key);
  g_return_if_fail (info == NULL);

  info = g_new (GstTagInfo, 1);
  info->flag = flag;
  info->type = type;
  info->nick = nick;
  info->blurb = blurb;
  info->merge_func = func;

  TAG_LOCK;
  g_hash_table_insert (__tags, GUINT_TO_POINTER (key), info);
  TAG_UNLOCK;
}

/**
 * gst_tag_exists:
 * @tag: name of the tag
 *
 * Checks if the given type is already registered.
 *
 * Returns: TRUE if the type is already registered
 */
gboolean
gst_tag_exists (const gchar * tag)
{
  g_return_val_if_fail (tag != NULL, FALSE);

  return gst_tag_lookup (g_quark_from_string (tag)) != NULL;
}

/**
 * gst_tag_get_type:
 * @tag: the tag
 *
 * Gets the #GType used for this tag.
 *
 * Returns: the #GType of this tag
 */
GType
gst_tag_get_type (const gchar * tag)
{
  GstTagInfo *info;

  g_return_val_if_fail (tag != NULL, 0);
  info = gst_tag_lookup (g_quark_from_string (tag));
  g_return_val_if_fail (info != NULL, 0);

  return info->type;
}

/**
 * gst_tag_get_nick
 * @tag: the tag
 *
 * Returns the human-readable name of this tag, You must not change or free 
 * this string.
 *
 * Returns: the human-readable name of this tag
 */
const gchar *
gst_tag_get_nick (const gchar * tag)
{
  GstTagInfo *info;

  g_return_val_if_fail (tag != NULL, NULL);
  info = gst_tag_lookup (g_quark_from_string (tag));
  g_return_val_if_fail (info != NULL, NULL);

  return info->nick;
}

/**
 * gst_tag_get_description:
 * @tag: the tag
 *
 * Returns the human-readable description of this tag, You must not change or 
 * free this string.
 *
 * Return the human-readable description of this tag
 */
const gchar *
gst_tag_get_description (const gchar * tag)
{
  GstTagInfo *info;

  g_return_val_if_fail (tag != NULL, NULL);
  info = gst_tag_lookup (g_quark_from_string (tag));
  g_return_val_if_fail (info != NULL, NULL);

  return info->blurb;
}

/**
 * gst_tag_get_flag:
 * @tag: the tag
 *
 * Returns the flag of this tag.
 */
GstTagFlag
gst_tag_get_flag (const gchar * tag)
{
  GstTagInfo *info;

  g_return_val_if_fail (tag != NULL, GST_TAG_FLAG_UNDEFINED);
  info = gst_tag_lookup (g_quark_from_string (tag));
  g_return_val_if_fail (info != NULL, GST_TAG_FLAG_UNDEFINED);

  return info->flag;
}

/**
 * gst_tag_list_is_fixed:
 * @tag: tag to check
 *
 * Checks if the given tag is fixed. A fixed tag can only contain one value.
 * Unfixed tags can contain lists of values.
 *
 * Returns: TRUE, if the given tag is fixed.
 */
gboolean
gst_tag_is_fixed (const gchar * tag)
{
  GstTagInfo *info;

  g_return_val_if_fail (tag != NULL, FALSE);
  info = gst_tag_lookup (g_quark_from_string (tag));
  g_return_val_if_fail (info != NULL, FALSE);

  return info->merge_func == NULL;
}

/**
 * gst_tag_list_new:
 *
 * Creates a new empty GstTagList.
 *
 * Returns: An empty tag list
 */
GstTagList *
gst_tag_list_new (void)
{
  return GST_TAG_LIST (gst_structure_new (TAGLIST, NULL));
}

/**
 * gst_is_tag_list:
 * @p: Object that might be a taglist
 *
 * Checks if the given pointer is a taglist.
 *
 * Returns: TRUE, if the given pointer is a taglist
 */
gboolean
gst_is_tag_list (gconstpointer p)
{
  g_return_val_if_fail (p != NULL, FALSE);

  return ((GstStructure *) p)->name == gst_tag_list_quark;
}
typedef struct
{
  GstStructure *list;
  GstTagMergeMode mode;
}
GstTagCopyData;
static void
gst_tag_list_add_value_internal (GstStructure * list, GstTagMergeMode mode,
    GQuark tag, GValue * value)
{
  GstTagInfo *info = gst_tag_lookup (tag);
  const GValue *value2;

  g_assert (info != NULL);

  if (info->merge_func
      && (value2 = gst_structure_id_get_value (list, tag)) != NULL) {
    GValue dest = { 0, };

    switch (mode) {
      case GST_TAG_MERGE_REPLACE_ALL:
      case GST_TAG_MERGE_REPLACE:
        gst_structure_id_set_value (list, tag, value);
        break;
      case GST_TAG_MERGE_PREPEND:
        gst_value_list_concat (&dest, value, value2);
        gst_structure_id_set_value (list, tag, &dest);
        g_value_unset (&dest);
        break;
      case GST_TAG_MERGE_APPEND:
        gst_value_list_concat (&dest, value2, value);
        gst_structure_id_set_value (list, tag, &dest);
        g_value_unset (&dest);
        break;
      case GST_TAG_MERGE_KEEP:
      case GST_TAG_MERGE_KEEP_ALL:
        break;
      default:
        g_assert_not_reached ();
        break;
    }
  } else {
    switch (mode) {
      case GST_TAG_MERGE_APPEND:
      case GST_TAG_MERGE_KEEP:
        if (gst_structure_id_get_value (list, tag) != NULL)
          break;
        /* fall through */
      case GST_TAG_MERGE_REPLACE_ALL:
      case GST_TAG_MERGE_REPLACE:
      case GST_TAG_MERGE_PREPEND:
        gst_structure_id_set_value (list, tag, value);
        break;
      case GST_TAG_MERGE_KEEP_ALL:
        break;
      default:
        g_assert_not_reached ();
        break;
    }
  }
}
static gboolean
gst_tag_list_copy_foreach (GQuark tag, GValue * value, gpointer user_data)
{
  GstTagCopyData *copy = (GstTagCopyData *) user_data;

  gst_tag_list_add_value_internal (copy->list, copy->mode, tag, value);

  return TRUE;
}

/**
 * gst_tag_list_insert:
 * @into: list to merge into
 * @from: list to merge from
 * @mode: the mode to use
 * 
 * Inserts the tags of the second list into the first list using the given mode.
 */
void
gst_tag_list_insert (GstTagList * into, const GstTagList * from,
    GstTagMergeMode mode)
{
  GstTagCopyData data;

  g_return_if_fail (GST_IS_TAG_LIST (into));
  g_return_if_fail (GST_IS_TAG_LIST (from));
  g_return_if_fail (GST_TAG_MODE_IS_VALID (mode));

  data.list = (GstStructure *) into;
  data.mode = mode;
  if (mode == GST_TAG_MERGE_REPLACE_ALL) {
    gst_structure_remove_all_fields (data.list);
  }
  gst_structure_foreach ((GstStructure *) from, gst_tag_list_copy_foreach,
      &data);
}

/**
 * gst_tag_list_copy:
 * @list: list to copy
 *
 * Copies a given #GstTagList.
 *
 * Returns: copy of the given list
 */
GstTagList *
gst_tag_list_copy (const GstTagList * list)
{
  g_return_val_if_fail (GST_IS_TAG_LIST (list), NULL);

  return GST_TAG_LIST (gst_structure_copy ((GstStructure *) list));
}

/**
 * gst_tag_list_merge:
 * @list1: first list to merge
 * @list2: second list to merge
 * @mode: the mode to use
 * 
 * Merges the two given lists into a new list. If one of the lists is NULL, a
 * copy of the other is returned. If both lists are NULL, NULL is returned.
 *
 * Returns: the new list
 */
GstTagList *
gst_tag_list_merge (const GstTagList * list1, const GstTagList * list2,
    GstTagMergeMode mode)
{
  g_return_val_if_fail (list1 == NULL || GST_IS_TAG_LIST (list1), NULL);
  g_return_val_if_fail (list2 == NULL || GST_IS_TAG_LIST (list2), NULL);
  g_return_val_if_fail (GST_TAG_MODE_IS_VALID (mode), NULL);

  if (!list1 && !list2) {
    return NULL;
  } else if (!list1) {
    return gst_tag_list_copy (list2);
  } else if (!list2) {
    return gst_tag_list_copy (list1);
  } else {
    GstTagList *ret;

    ret = gst_tag_list_copy (list1);
    gst_tag_list_insert (ret, list2, mode);
    return ret;
  }
}

/**
 * gst_tag_list_free:
 * @list: the list to free
 *
 * Frees the given list and all associated values.
 */
void
gst_tag_list_free (GstTagList * list)
{
  g_return_if_fail (GST_IS_TAG_LIST (list));
  gst_structure_free ((GstStructure *) list);
}

/**
 * gst_tag_list_get_tag_size:
 * @list: a taglist
 * @tag: the tag to query
 *
 * Checks how many value are stored in this tag list for the given tag.
 *
 * Returns: The number of tags stored
 */
guint
gst_tag_list_get_tag_size (const GstTagList * list, const gchar * tag)
{
  const GValue *value;

  g_return_val_if_fail (GST_IS_TAG_LIST (list), 0);

  value = gst_structure_get_value ((GstStructure *) list, tag);
  if (value == NULL)
    return 0;
  if (G_VALUE_TYPE (value) != GST_TYPE_LIST)
    return 1;

  return gst_value_list_get_size (value);
}

/**
 * gst_tag_list_add:
 * @list: list to set tags in
 * @mode: the mode to use
 * @tag: tag
 * @...: NULL-terminated list of values to set
 *
 * Sets the values for the given tags using the specified mode.
 */
void
gst_tag_list_add (GstTagList * list, GstTagMergeMode mode, const gchar * tag,
    ...)
{
  va_list args;

  g_return_if_fail (GST_IS_TAG_LIST (list));
  g_return_if_fail (GST_TAG_MODE_IS_VALID (mode));
  g_return_if_fail (tag != NULL);

  va_start (args, tag);
  gst_tag_list_add_valist (list, mode, tag, args);
  va_end (args);
}

/**
 * gst_tag_list_add_values:
 * @list: list to set tags in
 * @mode: the mode to use
 * @tag: tag
 * @...: GValues to set
 *
 * Sets the GValues for the given tags using the specified mode.
 */
void
gst_tag_list_add_values (GstTagList * list, GstTagMergeMode mode,
    const gchar * tag, ...)
{
  va_list args;

  g_return_if_fail (GST_IS_TAG_LIST (list));
  g_return_if_fail (GST_TAG_MODE_IS_VALID (mode));
  g_return_if_fail (tag != NULL);

  va_start (args, tag);
  gst_tag_list_add_valist_values (list, mode, tag, args);
  va_end (args);
}

/**
 * gst_tag_list_add_valist:
 * @list: list to set tags in
 * @mode: the mode to use
 * @tag: tag
 * @var_args: tag / value pairs to set
 *
 * Sets the values for the given tags using the specified mode.
 */
void
gst_tag_list_add_valist (GstTagList * list, GstTagMergeMode mode,
    const gchar * tag, va_list var_args)
{
  GstTagInfo *info;
  GQuark quark;
  gchar *error = NULL;

  g_return_if_fail (GST_IS_TAG_LIST (list));
  g_return_if_fail (GST_TAG_MODE_IS_VALID (mode));
  g_return_if_fail (tag != NULL);

  while (tag != NULL) {
    GValue value = { 0, };

    quark = g_quark_from_string (tag);
    info = gst_tag_lookup (quark);
    if (info == NULL)
      g_warning ("no GstTag for %s", tag);
    g_return_if_fail (info != NULL);
    g_value_init (&value, info->type);
    G_VALUE_COLLECT (&value, var_args, 0, &error);
    if (error) {
      g_warning ("%s: %s", G_STRLOC, error);
      g_free (error);
      /* we purposely leak the value here, it might not be
       * in a sane state if an error condition occoured
       */
      return;
    }
    gst_tag_list_add_value_internal (list, mode, quark, &value);
    g_value_unset (&value);
    tag = va_arg (var_args, gchar *);
  }
}

/**
 * gst_tag_list_add_valist_values:
 * @list: list to set tags in
 * @mode: the mode to use
 * @tag: tag
 * @var_args: tag / GValue pairs to set
 *
 * Sets the GValues for the given tags using the specified mode.
 */
void
gst_tag_list_add_valist_values (GstTagList * list, GstTagMergeMode mode,
    const gchar * tag, va_list var_args)
{
  GstTagInfo *info;
  GQuark quark;

  g_return_if_fail (GST_IS_TAG_LIST (list));
  g_return_if_fail (GST_TAG_MODE_IS_VALID (mode));
  g_return_if_fail (tag != NULL);

  while (tag != NULL) {
    quark = g_quark_from_string (tag);
    info = gst_tag_lookup (quark);
    g_return_if_fail (info != NULL);
    gst_tag_list_add_value_internal (list, mode, quark, va_arg (var_args,
            GValue *));
    tag = va_arg (var_args, gchar *);
  }
}

/**
 * gst_tag_list_remove_tag:
 * @list: list to remove tag from
 * @tag: tag to remove
 *
 * Removes the goven tag from the taglist.
 */
void
gst_tag_list_remove_tag (GstTagList * list, const gchar * tag)
{
  g_return_if_fail (GST_IS_TAG_LIST (list));
  g_return_if_fail (tag != NULL);

  gst_structure_remove_field ((GstStructure *) list, tag);
}
typedef struct
{
  GstTagForeachFunc func;
  GstTagList *tag_list;
  gpointer data;
}
TagForeachData;
static int
structure_foreach_wrapper (GQuark field_id, GValue * value, gpointer user_data)
{
  TagForeachData *data = (TagForeachData *) user_data;

  data->func (data->tag_list, g_quark_to_string (field_id), data->data);
  return TRUE;
}

/**
 * gst_tag_list_foreach:
 * @list: list to iterate over
 * @func: function to be called for each tag
 * @user_data: user specified data
 *
 * Calls the given function for each tag inside the tag list. Note that if there
 * is no tag, the function won't be called at all.
 */
void
gst_tag_list_foreach (GstTagList * list, GstTagForeachFunc func,
    gpointer user_data)
{
  TagForeachData data;

  g_return_if_fail (GST_IS_TAG_LIST (list));
  g_return_if_fail (func != NULL);

  data.func = func;
  data.tag_list = list;
  data.data = user_data;
  gst_structure_foreach ((GstStructure *) list, structure_foreach_wrapper,
      &data);
}

/***** tag events *****/

/**
 * gst_event_new_tag:
 * @list: the tag list to put into the event or NULL for an empty list
 *
 * Creates a new tag event with the given list and takes ownership of it.
 *
 * Returns: a new tag event
 */
GstEvent *
gst_event_new_tag (GstTagList * list)
{
  GstEvent *ret;

  g_return_val_if_fail (list == NULL || GST_IS_TAG_LIST (list), NULL);

  ret = gst_event_new (GST_EVENT_TAG);
  if (!list)
    list = gst_tag_list_new ();
  ret->event_data.structure.structure = (GstStructure *) list;

  return ret;
}

/**
 * get_event_tag_get_list:
 * @tag_event: a tagging #GstEvent
 *
 * Gets the taglist from a given tagging event.
 * 
 * Returns: The #GstTagList of the event
 */
GstTagList *
gst_event_tag_get_list (GstEvent * tag_event)
{
  g_return_val_if_fail (GST_IS_EVENT (tag_event), NULL);
  g_return_val_if_fail (GST_EVENT_TYPE (tag_event) == GST_EVENT_TAG, NULL);

  return GST_TAG_LIST (tag_event->event_data.structure.structure);
}

/**
 * gst_tag_list_get_value_index:
 * @list: a #GStTagList
 * @tag: tag to read out
 * @index: number of entry to read out
 *
 * Gets the value that is at the given index for the given tag in the given 
 * list.
 * 
 * Returns: The GValue for the specified entry or NULL if the tag wasn't available
 *	    or the tag doesn't have as many entries
 */
G_CONST_RETURN GValue *
gst_tag_list_get_value_index (const GstTagList * list, const gchar * tag,
    guint index)
{
  const GValue *value;

  g_return_val_if_fail (GST_IS_TAG_LIST (list), NULL);
  g_return_val_if_fail (tag != NULL, NULL);

  value = gst_structure_get_value ((GstStructure *) list, tag);
  if (value == NULL)
    return NULL;

  if (GST_VALUE_HOLDS_LIST (value)) {
    if (index >= gst_value_list_get_size (value))
      return NULL;
    return gst_value_list_get_value (value, index);
  } else {
    if (index > 0)
      return NULL;
    return value;
  }
}

/**
 * gst_tag_list_copy_value:
 * @dest: uninitialized #GValue to copy into
 * @list: list to get the tag from
 * @tag: tag to read out
 *
 * Copies the contents for the given tag into the value, merging multiple values 
 * into one if multiple values are associated with the tag.
 * You must g_value_unset() the value after use.
 *
 * Returns: TRUE, if a value was copied, FALSE if the tag didn't exist in the 
 *	    given list.
 */
gboolean
gst_tag_list_copy_value (GValue * dest, const GstTagList * list,
    const gchar * tag)
{
  const GValue *src;

  g_return_val_if_fail (GST_IS_TAG_LIST (list), FALSE);
  g_return_val_if_fail (tag != NULL, FALSE);
  g_return_val_if_fail (dest != NULL, FALSE);
  g_return_val_if_fail (G_VALUE_TYPE (dest) == 0, FALSE);

  src = gst_structure_get_value ((GstStructure *) list, tag);
  if (!src)
    return FALSE;

  if (G_VALUE_TYPE (src) == GST_TYPE_LIST) {
    GstTagInfo *info = gst_tag_lookup (g_quark_from_string (tag));

    /* must be there or lists aren't allowed */
    g_assert (info->merge_func);
    info->merge_func (dest, src);
  } else {
    g_value_init (dest, G_VALUE_TYPE (src));
    g_value_copy (src, dest);
  }
  return TRUE;
}

/***** evil macros to get all the gst_tag_list_get_*() functions right *****/

#define TAG_MERGE_FUNCS(name,type)						\
gboolean									\
gst_tag_list_get_ ## name (const GstTagList *list, const gchar *tag,		\
			   type *value)						\
{										\
  GValue v = { 0, };								\
										\
  g_return_val_if_fail (GST_IS_TAG_LIST (list), FALSE);				\
  g_return_val_if_fail (tag != NULL, FALSE);					\
  g_return_val_if_fail (value != NULL, FALSE);					\
										\
  if (!gst_tag_list_copy_value (&v, list, tag))					\
      return FALSE;								\
  *value = COPY_FUNC (g_value_get_ ## name (&v));				\
  g_value_unset (&v);								\
  return TRUE;									\
}										\
										\
gboolean									\
gst_tag_list_get_ ## name ## _index (const GstTagList *list, const gchar *tag, 	\
			   guint index, type *value)			  	\
{										\
  const GValue *v;    								\
										\
  g_return_val_if_fail (GST_IS_TAG_LIST (list), FALSE);				\
  g_return_val_if_fail (tag != NULL, FALSE);					\
  g_return_val_if_fail (value != NULL, FALSE);					\
										\
  if ((v = gst_tag_list_get_value_index (list, tag, index)) == NULL)		\
      return FALSE;								\
  *value = COPY_FUNC (g_value_get_ ## name (v));			      	\
  return TRUE;									\
}

#define COPY_FUNC /**/
TAG_MERGE_FUNCS (char, gchar)
TAG_MERGE_FUNCS (uchar, guchar)
TAG_MERGE_FUNCS (boolean, gboolean)
TAG_MERGE_FUNCS (int, gint)
TAG_MERGE_FUNCS (uint, guint)
TAG_MERGE_FUNCS (long, glong)
TAG_MERGE_FUNCS (ulong, gulong)
TAG_MERGE_FUNCS (int64, gint64)
TAG_MERGE_FUNCS (uint64, guint64)
TAG_MERGE_FUNCS (float, gfloat)
TAG_MERGE_FUNCS (double, gdouble)
#undef COPY_FUNC
#define COPY_FUNC g_strdup
TAG_MERGE_FUNCS (string, gchar *)