dbc/core/dbc_decoder.cpp

/**
 * @file dbc_decoder.cpp
 * @brief
 *
 * Created: 2026-03-13
 * Author: Deeaitch (Dim. Himro)
 *
 * Licensed under the MIT License.
 * See LICENSE file in the project root for full license text.
 */

#include "dbc_decoder.h"
#include "signal_layout.h"
#include <cmath>
#include <utility>

std::shared_ptr<const DecodeFrame> DbcDecoder::FindFrame(const DecodeDatabase &database,
                                                         std::uint32_t canId,
                                                         bool isExtended) const {
    return database.FindFrame(FrameKey(canId, isExtended));
}

DecodedFrameValue DbcDecoder::Decode(const DecodeDatabase &database,
                                     const RawCanFrame &frame) const {
    DecodedFrameValue result;
    result.key = FrameKey(frame.canId, frame.isExtended);
    const std::shared_ptr<const DecodeFrame> definition = FindFrame(database, frame.canId, frame.isExtended);

    if (definition == nullptr)
        return result;

    result.definition = definition;
    result.valid = true;
    result.status = frame.data.size() < definition->dlc
                        ? FrameDecodeStatus::InsufficientPayload
                        : FrameDecodeStatus::Valid;

    const SelectorState selector = ReadSelector(*definition, frame);
    result.signals.reserve(definition->signals.size());
    for (const DecodeSignal &signal: definition->signals) {
        DecodedSignalValue decoded = DecodeOneSignal(signal, definition->dlc, frame, selector);
        if (!decoded.valid && decoded.status != SignalDecodeStatus::Inactive &&
            result.status == FrameDecodeStatus::Valid)
            result.status = FrameDecodeStatus::SignalErrors;
        decoded.definition = std::shared_ptr<const DecodeSignal>(definition, &signal);
        decoded.key = SignalKey(result.key, signal.name);
        result.signals.push_back(std::move(decoded));
    }

    return result;
}

bool DbcDecoder::IsValidDefinition(const DecodeSignal &signal, std::uint8_t dlc) {
    if (signal.length == 0U || signal.length > 64U ||
        (signal.byteOrder != ByteOrder::Intel && signal.byteOrder != ByteOrder::Motorola) ||
        (signal.valueType != ValueType::Unsigned && signal.valueType != ValueType::Signed) ||
        (signal.multiplexing.role != MultiplexRole::Unconditional &&
         signal.multiplexing.role != MultiplexRole::Selector &&
         signal.multiplexing.role != MultiplexRole::Branch))
        return false;

    return IsValidSignalLayout(signal.startBit, signal.length,
                              signal.byteOrder == ByteOrder::Intel, dlc);
}

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 &&
                     IsValidDefinition(signal, definition.dlc);
    selector.available = selector.valid && ExtractUnsigned(frame.data, signal, selector.value);
    return selector;
}

DecodedSignalValue DbcDecoder::DecodeOneSignal(const DecodeSignal &signal,
                                               std::uint8_t dlc,
                                               const RawCanFrame &frame,
                                               const SelectorState &selector) {
    DecodedSignalValue decoded;
    std::uint64_t unsignedValue = 0U;

    if (!IsValidDefinition(signal, dlc) ||
        (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;
    if (!std::isfinite(decoded.physicalValue)) {
        decoded.status = SignalDecodeStatus::PhysicalConversionFailed;
        return decoded;
    }
    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::uint64_t absoluteBit = std::uint64_t(startBit) + bitIndex;
        const std::uint64_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);
}