+ * Set up channel positions based on a default channel configuration
+ * as specified in table 1.17.
+ *
+ * @return Returns error status. 0 - OK, !0 - error
+ */
+static int set_default_channel_config(AVCodecContext *avctx,
+ uint8_t (*layout_map)[3],
+ int *tags,
+ int channel_config)
+{
+ if (channel_config < 1 || channel_config > 7) {
+ av_log(avctx, AV_LOG_ERROR,
+ "invalid default channel configuration (%d)\n",
+ channel_config);
+ return AVERROR_INVALIDDATA;
+ }
+ *tags = tags_per_config[channel_config];
+ memcpy(layout_map, aac_channel_layout_map[channel_config - 1],
+ *tags * sizeof(*layout_map));
+ return 0;
+}
+
+static ChannelElement *get_che(AACContext *ac, int type, int elem_id)
+{
+ /* For PCE based channel configurations map the channels solely based
+ * on tags. */
+ if (!ac->oc[1].m4ac.chan_config) {
+ return ac->tag_che_map[type][elem_id];
+ }
+ // Allow single CPE stereo files to be signalled with mono configuration.
+ if (!ac->tags_mapped && type == TYPE_CPE &&
+ ac->oc[1].m4ac.chan_config == 1) {
+ uint8_t layout_map[MAX_ELEM_ID*4][3];
+ int layout_map_tags;
+ push_output_configuration(ac);
+
+ if (set_default_channel_config(ac->avctx, layout_map,
+ &layout_map_tags, 2) < 0)
+ return NULL;
+ if (output_configure(ac, layout_map, layout_map_tags,
+ OC_TRIAL_FRAME, 1) < 0)
+ return NULL;
+
+ ac->oc[1].m4ac.chan_config = 2;
+ ac->oc[1].m4ac.ps = 0;
+ }
+ // And vice-versa
+ if (!ac->tags_mapped && type == TYPE_SCE &&
+ ac->oc[1].m4ac.chan_config == 2) {
+ uint8_t layout_map[MAX_ELEM_ID * 4][3];
+ int layout_map_tags;
+ push_output_configuration(ac);
+
+ if (set_default_channel_config(ac->avctx, layout_map,
+ &layout_map_tags, 1) < 0)
+ return NULL;
+ if (output_configure(ac, layout_map, layout_map_tags,
+ OC_TRIAL_FRAME, 1) < 0)
+ return NULL;
+
+ ac->oc[1].m4ac.chan_config = 1;
+ if (ac->oc[1].m4ac.sbr)
+ ac->oc[1].m4ac.ps = -1;
+ }
+ /* For indexed channel configurations map the channels solely based
+ * on position. */
+ switch (ac->oc[1].m4ac.chan_config) {
+ case 7:
+ if (ac->tags_mapped == 3 && type == TYPE_CPE) {
+ ac->tags_mapped++;
+ return ac->tag_che_map[TYPE_CPE][elem_id] = ac->che[TYPE_CPE][2];
+ }
+ case 6:
+ /* Some streams incorrectly code 5.1 audio as
+ * SCE[0] CPE[0] CPE[1] SCE[1]
+ * instead of
+ * SCE[0] CPE[0] CPE[1] LFE[0].
+ * If we seem to have encountered such a stream, transfer
+ * the LFE[0] element to the SCE[1]'s mapping */
+ if (ac->tags_mapped == tags_per_config[ac->oc[1].m4ac.chan_config] - 1 && (type == TYPE_LFE || type == TYPE_SCE)) {
+ ac->tags_mapped++;
+ return ac->tag_che_map[type][elem_id] = ac->che[TYPE_LFE][0];
+ }
+ case 5:
+ if (ac->tags_mapped == 2 && type == TYPE_CPE) {
+ ac->tags_mapped++;
+ return ac->tag_che_map[TYPE_CPE][elem_id] = ac->che[TYPE_CPE][1];
+ }
+ case 4:
+ if (ac->tags_mapped == 2 &&
+ ac->oc[1].m4ac.chan_config == 4 &&
+ type == TYPE_SCE) {
+ ac->tags_mapped++;
+ return ac->tag_che_map[TYPE_SCE][elem_id] = ac->che[TYPE_SCE][1];
+ }
+ case 3:
+ case 2:
+ if (ac->tags_mapped == (ac->oc[1].m4ac.chan_config != 2) &&
+ type == TYPE_CPE) {
+ ac->tags_mapped++;
+ return ac->tag_che_map[TYPE_CPE][elem_id] = ac->che[TYPE_CPE][0];
+ } else if (ac->oc[1].m4ac.chan_config == 2) {
+ return NULL;
+ }
+ case 1:
+ if (!ac->tags_mapped && type == TYPE_SCE) {
+ ac->tags_mapped++;
+ return ac->tag_che_map[TYPE_SCE][elem_id] = ac->che[TYPE_SCE][0];
+ }
+ default:
+ return NULL;
+ }
+}
+
+/**
+ * Decode an array of 4 bit element IDs, optionally interleaved with a
+ * stereo/mono switching bit.