dbc_decoder.cpp

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#include "dbc_decoder.h"
 
const DecodeFrame *DbcDecoder::FindFrame(const DecodeDatabase &database,
                                         std::uint32_t canId,
                                         bool isExtended) const {
    const FrameKey key(canId, isExtended);
    const std::unordered_map<FrameKey, std::size_t, FrameKeyHasher>::const_iterator it =
            database.frameIndexByKey.find(key);
 
    if (it == database.frameIndexByKey.end())
        return nullptr;
 
    const std::size_t index = it->second;
    if (index >= database.frames.size())
        return nullptr;
 
    return &database.frames[index];
}
 
DecodedFrameValue DbcDecoder::Decode(const DecodeDatabase &database,
                                     const RawCanFrame &frame) const {
    DecodedFrameValue result;
    const DecodeFrame *definition = FindFrame(database, frame.canId, frame.isExtended);
 
    if (definition == nullptr)
        return result;
 
    result.definition = definition;
    result.valid = true;
 
    const SelectorState selector = ReadSelector(*definition, frame);
    for (const DecodeSignal &signal : definition->signals)
        result.signals.push_back(DecodeOneSignal(signal, frame, selector));
 
    return result;
}
 
DbcDecoder::SelectorState DbcDecoder::ReadSelector(const DecodeFrame &definition,
                                                  const RawCanFrame &frame) {
    SelectorState selector;
    std::size_t count = 0U;
    for (const DecodeSignal &signal : definition.signals) {
        if (signal.multiplexing.role == MultiplexRole::Selector) {
            selector.definition = &signal;
            ++count;
        }
    }
    if (count != 1U)
        return selector;
 
    const DecodeSignal &signal = *selector.definition;
    selector.valid = signal.valueType == ValueType::Unsigned &&
                     signal.length > 0U && signal.length <= 64U;
    selector.available = selector.valid && ExtractUnsigned(frame.data, signal, selector.value);
    return selector;
}
 
DecodedSignalValue DbcDecoder::DecodeOneSignal(const DecodeSignal &signal,
                                              const RawCanFrame &frame,
                                              const SelectorState &selector) {
    DecodedSignalValue decoded;
    std::uint64_t unsignedValue = 0U;
 
    decoded.definition = &signal;
 
    if (signal.length == 0U || signal.length > 64U ||
        (signal.multiplexing.role != MultiplexRole::Unconditional && !selector.valid)) {
        return decoded;
    }
    if (signal.multiplexing.role == MultiplexRole::Branch) {
        if (selector.definition->length < 64U && signal.multiplexing.value >= (std::uint64_t(1U) << selector.definition->length)) {
            return decoded;
        }
        if (!selector.available) {
            decoded.status = SignalDecodeStatus::SelectorUnavailable;
            return decoded;
        }
        if (signal.multiplexing.value != selector.value) {
            decoded.status = SignalDecodeStatus::Inactive;
            return decoded;
        }
    }
    if (!ExtractUnsigned(frame.data, signal, unsignedValue)) {
        decoded.status = SignalDecodeStatus::InsufficientPayload;
        return decoded;
    }
 
    if (signal.valueType == ValueType::Signed)
        decoded.rawValue = SignExtend(unsignedValue, signal.length);
    else
        decoded.rawValue = unsignedValue;
 
    double numericValue;
    if (const std::int64_t *signedValue = std::get_if<std::int64_t>(&decoded.rawValue))
        numericValue = static_cast<double>(*signedValue);
    else
        numericValue = static_cast<double>(std::get<std::uint64_t>(decoded.rawValue));
    decoded.physicalValue = numericValue * signal.factor + signal.offset;
    decoded.valid = true;
    decoded.status = SignalDecodeStatus::Valid;
 
    return decoded;
}
 
bool DbcDecoder::ExtractUnsigned(const std::vector<std::uint8_t> &data,
                                 const DecodeSignal &signal,
                                 std::uint64_t &value) {
    if ((signal.length == 0U) || (signal.length > 64U))
        return false;
 
    if (signal.byteOrder == ByteOrder::Intel)
        return ExtractIntel(data, signal.startBit, signal.length, value);
 
    return ExtractMotorola(data, signal.startBit, signal.length, value);
}
 
bool DbcDecoder::ExtractIntel(const std::vector<std::uint8_t> &data,
                              std::uint32_t startBit,
                              std::uint32_t length,
                              std::uint64_t &value) {
    value = 0U;
 
    for (std::uint32_t bitIndex = 0U; bitIndex < length; ++bitIndex) {
        const std::uint32_t absoluteBit = startBit + bitIndex;
        const std::uint32_t byteIndex = absoluteBit / 8U;
        const std::uint32_t bitInByte = absoluteBit % 8U;
 
        if (byteIndex >= data.size())
            return false;
 
        const std::uint64_t bitValue =
                (static_cast<std::uint64_t>((data[byteIndex] >> bitInByte) & 0x01U) << bitIndex);
 
        value |= bitValue;
    }
 
    return true;
}
 
bool DbcDecoder::ExtractMotorola(const std::vector<std::uint8_t> &data,
                                 std::uint32_t startBit,
                                 std::uint32_t length,
                                 std::uint64_t &value) {
    /*
   * DBC Motorola bit numbering:
   * - startBit points to the most significant bit of the signal
   * - inside a byte, bit numbering goes 7..0
   * - crossing byte boundary moves to the next byte, bit 7
   * Example: a signal starting at 7 follows 7..0, 15..8, 23..16.
   */
 
    value = 0U;
 
    std::size_t byteIndex = startBit / 8U;
    std::uint32_t bitInByte = startBit % 8U;
 
    for (std::uint32_t bitIndex = 0U; bitIndex < length; ++bitIndex) {
        if (byteIndex >= data.size())
            return false;
 
        value <<= 1U;
        value |= static_cast<std::uint64_t>((data[byteIndex] >> bitInByte) & 0x01U);
 
        if (bitInByte == 0U) {
            ++byteIndex;
            bitInByte = 7U;
        } else
            --bitInByte;
    }
 
    return true;
}
 
std::int64_t DbcDecoder::SignExtend(std::uint64_t value, std::uint32_t bitLength) {
    if ((bitLength == 0U) || (bitLength > 64U))
        return 0;
 
    const std::uint64_t signMask = std::uint64_t(1U) << (bitLength - 1U);
    const std::uint64_t valueMask = (bitLength == 64U)
                                            ? ~std::uint64_t(0U)
                                            : (std::uint64_t(1U) << bitLength) - 1U;
    value &= valueMask;
 
    if ((value & signMask) == 0U)
        return static_cast<std::int64_t>(value);
 
    // The complemented magnitude fits int64_t, including for INT64_MIN.
    return -1 - static_cast<std::int64_t>((~value) & valueMask);
}