sdrangel/modems/m17/ax25_frame.h

// Copyright 2012-2021 Rob Riggs <rob@modemm17.com>
// All rights reserved.

#pragma once

#include <algorithm>
#include <cassert>
#include <cctype>
#include <cstdint>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <optional>
#include <sstream>
#include <stdexcept>
#include <string>
#include <vector>


namespace modemm17 {

struct ax25_frame
{
    using repeaters_type = std::vector<std::string>;
    using pid_type = uint8_t;
    enum frame_type {UNDEFINED, INFORMATION, SUPERVISORY, UNNUMBERED};

private:

    static const std::string::size_type DEST_ADDRESS_POS = 0;
    static const std::string::size_type SRC_ADDRESS_POS = 7;
    static const std::string::size_type LAST_ADDRESS_POS = 13;
    static const std::string::size_type FIRST_REPEATER_POS = 14;
    static const std::string::size_type ADDRESS_LENGTH = 7;

    std::string destination_;
    std::string source_;
    repeaters_type repeaters_;
    frame_type type_;
    uint8_t raw_type_;
    std::string info_;
    uint16_t fcs_;
    uint16_t crc_;
    pid_type pid_;

    static std::string removeAddressExtensionBit(const std::string& address)
    {
        std::string result = address;
        for (size_t i = 0; i != result.size(); i++)
        {
            result[i] = (uint8_t(result[i]) >> 1);
        }
        return result;
    }

    static int getSSID(const std::string& address)
    {
        assert(address.size() == ADDRESS_LENGTH);

        return (address[6] & 0x0F);
    }

    static std::string appendSSID(const std::string& address, int ssid)
    {
        std::string result = address;

        if (ssid)
        {
            result += '-';
            result += std::to_string(ssid);
        }
        return result;
    }

    static bool fixup_address(std::string& address)
    {
        assert(address.size() == ADDRESS_LENGTH);

        bool result = (address[ADDRESS_LENGTH - 1] & 1) == 0;

        address = removeAddressExtensionBit(address);

        const int ssid = getSSID(address);

        // Remove trailing spaces and SSID.
        size_t pos = address.find_first_of(' ');
        if (pos == std::string::npos) pos = 6;
        address.erase(pos);

        address = appendSSID(address, ssid);

        return result;
    }

    static frame_type parse_type(const std::string& frame, size_t pos)
    {
        uint8_t c(frame[pos]);
        switch (c & 0x03)
        {
        case 0:
            return INFORMATION;
        case 1:
            return SUPERVISORY;
        case 2:
            return INFORMATION;
        default:
            return UNNUMBERED;
        }
    }

    static std::string parse_info(const std::string& frame, size_t pos)
    {
        std::ostringstream output;

        for (int i = pos; i < ((int) frame.size()) - 2; i++)
        {
            char c = frame[i];
            if (std::isprint(c))
            {
                output << c;
            }
            else
            {
                output << "0x" << std::setw(2)
                    << std::setbase(16) << int(uint8_t(c)) << ' ';
            }
        }
        return output.str();
    }

    static uint16_t parse_fcs(const std::string& frame)
    {
        size_t checksum_pos = frame.size() - 2;

        uint16_t tmp =
            ((uint8_t(frame[checksum_pos + 1]) << 8) |
                uint8_t(frame[checksum_pos]));

        uint16_t checksum = 0;
        for (size_t i = 1; i != 0x10000; i <<= 1)
        {
            checksum <<= 1;
            checksum |= ((tmp & i) ? 1 : 0);
        }

        return checksum;
    }

    static std::string parse_destination(const std::string& frame)
    {
        assert(frame.size() > DEST_ADDRESS_POS + ADDRESS_LENGTH);
        return frame.substr(DEST_ADDRESS_POS, ADDRESS_LENGTH);
    }

    static std::string parse_source(const std::string& frame)
    {
        assert(frame.size() > SRC_ADDRESS_POS + ADDRESS_LENGTH);
        return frame.substr(SRC_ADDRESS_POS, ADDRESS_LENGTH);
    }

    static repeaters_type parse_repeaters(const std::string& frame)
    {
        repeaters_type result;
        std::string::size_type index = FIRST_REPEATER_POS;
        bool more = (index + ADDRESS_LENGTH) < frame.length();

        while (more)
        {
            std::string repeater = frame.substr(index, ADDRESS_LENGTH);
            index += ADDRESS_LENGTH;
            more = fixup_address(repeater)
                and (index + ADDRESS_LENGTH) < frame.length();
            result.push_back(repeater);
        }

        return result;
    }

    void parse(const std::string& frame)
    {
        if (frame.length() < 17) return;

        fcs_ = parse_fcs(frame);

        destination_ = parse_destination(frame);
        fixup_address(destination_);

        source_ = parse_source(frame);
        bool have_repeaters = fixup_address(source_);

        if (have_repeaters)
        {
            repeaters_ = parse_repeaters(frame);
        }

        size_t index = ADDRESS_LENGTH * (repeaters_.size() + 2);

        if (frame.length() < index + 5) return;

        type_ = parse_type(frame, index);
        raw_type_ = uint8_t(frame[index++]);

        if (type_ == UNNUMBERED) pid_ = uint8_t(frame[index++]);

        info_.assign(frame.begin() + index, frame.end() - 2);
    }

public:

    ax25_frame(const std::string& frame) :
        destination_(),
        source_(),
        repeaters_(),
        type_(UNDEFINED),
        info_(),
        fcs_(-1),
        crc_(0),
        pid_()
    {
        parse(frame);
    }

    std::string destination() const { return destination_; }

    std::string source() const { return source_; }

    repeaters_type repeaters() const { return repeaters_; }

    frame_type type() const { return type_; }

    std::string info() const { return info_; }

    uint16_t fcs() const { return fcs_; }

    uint16_t crc() const { return crc_; }

    pid_type pid() const { return pid_; }
};


void write(std::ostream& os, const ax25_frame& frame)
{
    typedef typename ax25_frame::repeaters_type repeaters_type;

    os << "Dest: " << frame.destination() << std::endl
        << "Source: " << frame.source() << std::endl;

    repeaters_type repeaters = frame.repeaters();
    if (!repeaters.empty())
    {
        os << "Via: ";
        std::copy(
            repeaters.begin(), repeaters.end(),
            std::ostream_iterator<std::string>(os, " "));
        os << std::endl;
    }

    if (frame.pid())
    {
        os << "PID: " << std::setbase(16) << int(frame.pid()) << std::endl;
    }
    os << "Info: " << std::endl << frame.info() << std::endl;
}

} // modemm17