--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmhr1/rtp_in.c	Sat Jun 15 06:52:37 2024 +0000
@@ -0,0 +1,70 @@
+/*
+ * The function implemented in this module takes whatever HRv1 payload format
+ * came in from RTP (can be RFC 5993 with or without TW-TS-002 extensions,
+ * can be bare TS 101 318, can be zero-length or missing payload) and turns it
+ * into canonical TW-TS-002 format, be it for storage or for immediate
+ * further processing.
+ */
+
+#include <stdint.h>
+#include <string.h>
+#include "tw_gsmhr.h"
+
+int gsmhr_rtp_in_preen(const uint8_t *rtp_in, unsigned rtp_in_len,
+			uint8_t *canon_pl)
+{
+	int ft;
+
+	switch (rtp_in_len) {
+	case 0:
+		/* BFI-no-data, but not an invalid RTP input per se */
+		canon_pl[0] = 0x70;
+		return 0;
+	case 1:
+		/*
+		 * This length is valid only if the payload is in
+		 * RFC 5993 or TW-TS-002 format with FT=1 or FT=7.
+		 */
+		if (rtp_in[0] & 0x80)
+			goto bad_rtp_input;
+		ft = rtp_in[0] >> 4;
+		if (ft == 1 || ft == 7) {
+			canon_pl[0] = rtp_in[0];
+			return 0;
+		} else
+			goto bad_rtp_input;
+	case GSMHR_FRAME_LEN_RPF:
+		/*
+		 * The length is that of a TS 101 318 payload.
+		 * No further checks can be done: every possible
+		 * bit pattern is a valid payload in this format.
+		 * But we do need to check for a perfect SID
+		 * (the only kind of SID this format allows)
+		 * and mark it accordingly in the output.
+		 */
+		canon_pl[0] = gsmhr_ts101318_is_perfect_sid(rtp_in) << 4;
+		memcpy(canon_pl + 1, rtp_in, GSMHR_FRAME_LEN_RPF);
+		return 0;
+	case GSMHR_FRAME_LEN_5993:
+		/*
+		 * This length is valid only if the payload is in
+		 * RFC 5993 or TW-TS-002 format with FT=0, FT=2 or FT=6.
+		 */
+		if (rtp_in[0] & 0x80)
+			goto bad_rtp_input;
+		ft = rtp_in[0] >> 4;
+		if (ft == 0 || ft == 2 || ft == 6) {
+			memcpy(canon_pl, rtp_in, GSMHR_FRAME_LEN_5993);
+			return 0;
+		} else
+			goto bad_rtp_input;
+	default:
+	bad_rtp_input:
+		/*
+		 * Treat it like BFI-no-data, and tell the caller
+		 * that we received an invalid RTP payload.
+		 */
+		canon_pl[0] = 0x70;
+		return -1;
+	}
+}