init commit

engine/transmission.h

@@ -0,0 +1,22 @@
+#pragma once
+
+// Gear ratio + final drive coupling between engine and wheels.
+// Torque flows engine -> wheels; rpm flows wheels -> engine (reflected).
+class Transmission {
+public:
+    void set_gear(int gear); // 0 = neutral, 1..N = forward gears
+    int gear() const;
+
+    double output_torque_nm(double engine_torque_nm) const;
+    double reflected_engine_rpm(double wheel_rpm) const;
+
+private:
+    double ratio() const;
+
+    int m_gear = 1;
+
+    static constexpr double kFinalDrive = 3.42;
+    static constexpr double kGearRatios[] = {0.0, 3.5, 2.1, 1.4, 1.0, 0.8};
+    static constexpr int kNumGears =
+        static_cast<int>(sizeof(kGearRatios) / sizeof(kGearRatios[0])) - 1;
+};