Closes #24

core/j1939.h

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+#ifndef J1939_H
+#define J1939_H
+
+#include <cstdint>
+
+/** Extract an 18-bit PGN from a normalized 29-bit ID explicitly interpreted as J1939.
+ * Priority and source address are excluded. PDU1 also excludes destination address.
+ * Both data-page bits are retained; this does not validate the J1939 protocol.
+ */
+inline std::uint32_t ExtractJ1939Pgn(std::uint32_t canId) {
+    const std::uint32_t pf = (canId >> 16U) & 0xFFU;
+    const std::uint32_t pgn = (canId >> 8U) & 0x3FFFFU;
+    return pf < 240U ? pgn & 0x3FF00U : pgn;
+}
+
+#endif /* J1939_H */