Closes #11

core/dbc_decoder.cpp

@@ -0,0 +1,158 @@
+/**
+ * @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"
+
+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;
+
+    for (std::size_t signalIndex = 0U; signalIndex < definition->signals.size(); ++signalIndex) {
+        const DecodeSignal &signal = definition->signals[signalIndex];
+        DecodedSignalValue decoded;
+        std::uint64_t unsignedValue = 0U;
+
+        decoded.definition = &signal;
+
+        if (!ExtractUnsigned(frame.data, signal, unsignedValue)) {
+            decoded.valid = false;
+            result.signals.push_back(decoded);
+            continue;
+        }
+
+        if (signal.valueType == ValueType::Signed)
+            decoded.rawValue = SignExtend(unsignedValue, signal.length);
+        else
+            decoded.rawValue = unsignedValue;
+
+        double numericValue;
+        if (const auto *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;
+
+        result.signals.push_back(decoded);
+    }
+
+    return result;
+}
+
+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);
+}