avformat/avc: Add functions to split access unit into list of NALUs

This will allow to avoid the temporary buffer and memcpys
when repacketing annex B to mp4-style H.264/H.265 without
searching twice for start codes.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/avc.h

@@ -25,6 +25,35 @@
 #include <stdint.h>
 #include "avio.h"
 
+typedef struct NALU {
+    int offset;
+    uint32_t size;
+} NALU;
+
+typedef struct NALUList {
+    NALU *nalus;
+    unsigned nalus_array_size;
+    unsigned nb_nalus;          ///< valid entries in nalus
+} NALUList;
+
+/* This function will parse the given annex B buffer and create
+ * a NALUList from it. This list can be passed to ff_nal_units_write_list()
+ * to write the access unit reformatted to mp4.
+ *
+ * @param list A NALUList. The list->nalus and list->nalus_array_size
+ *             must be valid when calling this function and may be updated.
+ *             nb_nalus is set by this function on success.
+ * @param buf  buffer containing annex B H.264 or H.265. Must be padded.
+ * @param size size of buf, excluding padding.
+ * @return < 0 on error, the size of the mp4-style packet on success.
+ */
+int ff_nal_units_create_list(NALUList *list, const uint8_t *buf, int size);
+
+/* Writes a NALUList to the specified AVIOContext. The list must originate
+ * from ff_nal_units_create_list() with the same buf. */
+void ff_nal_units_write_list(const NALUList *list, AVIOContext *pb,
+                             const uint8_t *buf);
+
 int ff_avc_parse_nal_units(AVIOContext *s, const uint8_t *buf, int size);
 int ff_avc_parse_nal_units_buf(const uint8_t *buf_in, uint8_t **buf, int *size);
 int ff_isom_write_avcc(AVIOContext *pb, const uint8_t *data, int len);