audio/chmap: add mp_iterate_builtin_layouts

Mostly useful for unit tests in order to access the channel
layouts from chmap which have mapped channels.

audio/chmap.c

@@ -470,6 +470,23 @@ char *mp_chmap_to_str_hr_buf(char *buf, size_t buf_size, const struct mp_chmap *
     return mp_chmap_to_str_buf(buf, buf_size, &map);
 }
 
+mp_ch_layout_tuple *mp_iterate_builtin_layouts(void **opaque)
+{
+    uintptr_t i = (uintptr_t)*opaque;
+
+    if (i >= MP_ARRAY_SIZE(std_layout_names) ||
+        !std_layout_names[i][0])
+        return NULL;
+
+    *opaque = (void *)(i + 1);
+
+    if (std_layout_names[i][1][0] == '\0') {
+        return mp_iterate_builtin_layouts(opaque);
+    }
+
+    return &std_layout_names[i];
+}
+
 void mp_chmap_print_help(struct mp_log *log)
 {
     mp_info(log, "Speakers:\n");

audio/chmap.h

@@ -75,6 +75,8 @@ struct mp_chmap {
     uint8_t speaker[MP_NUM_CHANNELS];
 };
 
+typedef const char * const (mp_ch_layout_tuple)[2];
+
 #define MP_SP(speaker) MP_SPEAKER_ID_ ## speaker
 
 #define MP_CHMAP2(a, b) \
@@ -129,6 +131,19 @@ char *mp_chmap_to_str_hr_buf(char *buf, size_t buf_size, const struct mp_chmap *
 
 bool mp_chmap_from_str(struct mp_chmap *dst, bstr src);
 
+/**
+ * Iterate over all built-in channel layouts which have mapped channels.
+ *
+ * @param opaque a pointer where the iteration state is stored. Must point
+ *               to nullptr to start the iteration.
+ *
+ * @return nullptr when the iteration is finished.
+ *         Otherwise a pointer to an array of two char pointers.
+ *         - [0] being the human-readable layout name.
+ *         - [1] being the string representation of the layout.
+ */
+mp_ch_layout_tuple *mp_iterate_builtin_layouts(void **opaque);
+
 struct mp_log;
 void mp_chmap_print_help(struct mp_log *log);