Add the CRC and callsign encoding.

M17CRC.cpp

@@ -0,0 +1,69 @@
+/*
+ *   Copyright (C) 2018,2020 by Jonathan Naylor G4KLX
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "M17CRC.h"
+
+#include <cstdio>
+#include <cassert>
+
+const uint8_t  BIT_MASK_TABLE1[] = { 0x80U, 0x40U, 0x20U, 0x10U, 0x08U, 0x04U, 0x02U, 0x01U };
+
+#define READ_BIT(p,i)    (p[(i)>>3] & BIT_MASK_TABLE1[(i)&7])
+
+bool CM17CRC::checkCRC(const unsigned char* in, unsigned int nBytes)
+{
+	assert(in != NULL);
+	assert(nBytes > 2U);
+
+	uint16_t crc = createCRC(in, nBytes);
+
+	uint8_t temp[2U];
+	temp[0U] = (crc >> 8) & 0xFFU;
+	temp[1U] = (crc >> 0) & 0xFFU;
+
+	return temp[0U] == in[nBytes - 2U] && temp[1U] == in[nBytes - 1U];
+}
+
+void CM17CRC::encodeCRC(unsigned char* in, unsigned int nBytes)
+{
+	assert(in != NULL);
+	assert(nBytes > 2U);
+
+	uint16_t crc = createCRC(in, nBytes);
+
+	in[nBytes - 2U] = (crc >> 8) & 0xFFU;
+	in[nBytes - 1U] = (crc >> 0) & 0xFFU;
+}
+uint16_t CM17CRC::createCRC(const unsigned char* in, unsigned int nBytes)
+{
+	assert(in != NULL);
+
+	uint16_t crc = 0xFFFFU;
+
+	for (unsigned int i = 0U; i < nBytes; i++) {
+		bool bit1 = READ_BIT(in, i) != 0x00U;
+		bool bit2 = (crc & 0x8000U) == 0x8000U;
+
+		crc <<= 1;
+
+		if (bit1 ^ bit2)
+			crc ^= 0x5935U;
+	}
+
+	return crc;
+}