--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmfrp/sidclass.c	Sat Nov 19 07:21:26 2022 +0000
@@ -0,0 +1,50 @@
+/*
+ * gsmfr_preproc_sid_classify() utility function classifies
+ * a GSM 06.10 frame in RTP encoding according to the rules
+ * of GSM 06.31 (or 3GPP TS 46.031) section 6.1.1, judging it
+ * as SID=0, SID=1 or SID=2.
+ */
+
+#include "gsm_fr_preproc.h"
+
+static const unsigned short sid_field_bits[95] = {
+	57, 58, 60, 61, 63, 64, 66, 67, 69, 70, 72, 73,
+	75, 76, 78, 79, 81, 82, 84, 85, 87, 88, 90, 91,
+	93, 94, 113, 114, 116, 117, 119, 120, 122, 123,
+	125, 126, 128, 129, 131, 132, 134, 135, 137,
+	138, 140, 141, 143, 144, 146, 147, 149, 150,
+	169, 170, 172, 173, 175, 176, 178, 179, 181,
+	182, 184, 185, 187, 188, 190, 191, 193, 194,
+	196, 197, 199, 200, 202, 203, 205, 206, 225,
+	226, 228, 229, 231, 232, 234, 235, 237, 240,
+	243, 246, 249, 252, 255, 258, 261
+};
+
+static inline int get_bit(const gsm_byte *frame, unsigned bitnum)
+{
+	unsigned bytenum = bitnum >> 3;
+	unsigned bit_in_byte = 7 - (bitnum & 7);
+	unsigned bitmask = 1 << bit_in_byte;
+
+	if (frame[bytenum] & bitmask)
+		return 1;
+	else
+		return 0;
+}
+
+int gsmfr_preproc_sid_classify(const gsm_byte *frame)
+{
+	unsigned idx, n;
+
+	n = 0;
+	for (idx = 0; idx < 95; idx++) {
+		if (get_bit(frame, sid_field_bits[idx]))
+			n++;
+		if (n >= 16)
+			return 0;
+	}
+	if (n < 2)
+		return 2;
+	else
+		return 1;
+}