Started work on APRS (AX25)

firmware/application/protocols/ax25.cpp

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+/*
+ * Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
+ * Copyright (C) 2017 Furrtek
+ *
+ * This file is part of PortaPack.
+ *
+ * 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, 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; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "ax25.hpp"
+
+#include "portapack_shared_memory.hpp"
+
+namespace ax25 {
+
+void AX25Frame::make_extended_field(char * const data, size_t length) {
+	size_t i = 0;
+	
+	for (i = 0; i < length - 1; i++)
+		add_data(data[i] << 1);
+	
+	add_data((data[i] << 1) | 1);
+}
+
+void AX25Frame::add_byte(uint8_t byte, bool is_flag, bool is_data) {
+	size_t i;
+	
+	for (i = 0; i < 8; ) {
+		
+		if (!(byte & 1)) {
+			current_bit ^= 1;		// Zero: flip
+			ones_counter = 0;
+			byte >>= 1;
+			i++;
+		} else {
+			ones_counter++;
+			if ((ones_counter == 5) && (!is_flag)) {
+				current_bit ^= 1;	// Stuff zero: flip
+				ones_counter = 0;
+			} else {
+				byte >>= 1;
+				i++;
+			}
+		}
+		
+		if (is_data)
+			crc_ccitt.process_bit(current_bit);
+		current_byte <<= 1;
+		current_byte |= current_bit;
+		
+		if (bit_counter == 7) {
+			bit_counter = 0;
+			*bb_data_ptr = current_byte;
+			bb_data_ptr++;
+		} else {
+			bit_counter++;
+		}
+	}
+}
+
+void AX25Frame::flush() {
+	if (bit_counter)
+		*bb_data_ptr = current_byte << (7 - bit_counter);
+};
+
+void AX25Frame::add_flag() {
+	add_byte(AX25_FLAG, true, false);
+};
+
+void AX25Frame::add_data(uint8_t byte) {
+	add_byte(byte, false, true);
+};
+
+void AX25Frame::add_checksum() {
+	auto checksum = crc_ccitt.checksum();
+	add_byte(checksum >> 8, false, false);
+	add_byte(checksum, false, false);
+}
+
+void AX25Frame::make_ui_frame(char * const address, const uint8_t control,
+	const uint8_t protocol, uint8_t * const info, size_t length) {
+	
+	size_t i;
+	
+	bb_data_ptr = shared_memory.bb_data.data;
+	bit_counter = 0;
+	current_bit = 0;
+	current_byte = 0;
+	ones_counter = 0;
+	crc_ccitt.reset();
+	
+	add_flag();
+	
+	make_extended_field(address, 14);
+	add_data(control);
+	add_data(protocol);
+	
+	for (i = 0; i < length; i++)
+		add_data(info[i]);
+	
+	add_checksum();
+	add_flag();
+	
+	flush();
+}
+
+} /* namespace ax25 */