--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmfr2/tfo_main.c	Thu Sep 19 20:35:10 2024 +0000
@@ -0,0 +1,54 @@
+/*
+ * This module implements a wrapper around the main processing functions
+ * of our Rx DTX or TFO preprocessor, handling RTP input per TW-TS-001.
+ */
+
+#include <stdint.h>
+#include <string.h>
+#include "tw_gsmfr.h"
+
+int gsmfr_tfo_xfrm_main(struct gsmfr_preproc_state *st, const uint8_t *rtp_pl,
+			unsigned rtp_pl_len, uint8_t *out)
+{
+	switch (rtp_pl_len) {
+	case 0:
+		/* BFI-no-data, but not an invalid RTP input per se */
+		gsmfr_preproc_bfi(st, 0, out);
+		return 0;
+	case 1:
+		if ((rtp_pl[0] & 0xF6) != 0xE6)
+			goto bad_rtp_input;
+		/* TW-TS-001 No_Data frame */
+		gsmfr_preproc_bfi(st, rtp_pl[0] & 1, out);
+		return 0;
+	case GSMFR_RTP_FRAME_LEN:
+		if ((rtp_pl[0] & 0xF0) != 0xD0)
+			goto bad_rtp_input;
+		/* basic RTP format */
+		memcpy(out, rtp_pl, GSMFR_RTP_FRAME_LEN);
+		gsmfr_preproc_good_frame_hm(st, out);
+		return 0;
+	case GSMFR_RTP_FRAME_LEN+1:
+		if ((rtp_pl[0] & 0xF4) != 0xE0)
+			goto bad_rtp_input;
+		if ((rtp_pl[1] & 0xF0) != 0xD0)
+			goto bad_rtp_input;
+		/* extended RTP format (TW-TS-001) */
+		if (rtp_pl[0] & 0x02)
+			gsmfr_preproc_bfi_bits(st, rtp_pl + 1, rtp_pl[0] & 1,
+						out);
+		else {
+			memcpy(out, rtp_pl + 1, GSMFR_RTP_FRAME_LEN);
+			gsmfr_preproc_good_frame_hm(st, out);
+		}
+		return 0;
+	default:
+	bad_rtp_input:
+		/*
+		 * Treat it like BFI-no-data, and tell the caller
+		 * that we received an invalid RTP payload.
+		 */
+		gsmfr_preproc_bfi(st, 0, out);
+		return -1;
+	}
+}