// LIMITATION: the first and last wheels in your train must NOT be 9v train
// motor bogies.  See below.

// tune DARK_HIGHWATER by placing a wheel in front of the sensor and adding
// about 5.  LIGHT_LOWATER can be just under your no-train light reading,
// but the closer you can get it to DARK_HIGHWATER (without false readings)
// the faster you can run a train and still properly detect standard bogies.
#define DARK_HIGHWATER	35
#define LIGHT_LOWATER	40

// connect the sensor described at:
// http://www.ben.com/LEGO/rcx/train-sensor.html
#define TRAIN_SENSOR	SENSOR_2

// assumes the powered bogie plate is in the back
#define ENGINE_FORWARD 1
#define ENGINE_REVERSE 0

// Called just after a complete engine passes (assumes 1 real motor bogie
// in the front and one "fake" one in the front.  Examples:
//
// engine from the #4564 Freight Rail Runner, MOTOR IN BACK
// engine from the #4565 Freight & Crane Railway, MOTOR IN BACK
// engine from the #4558 Metroliner, MOTOR IN BACK
//
// NOT the engine from the #3224 Classic Train or the #4561 Railway Express.
// Both use only the motor bogie in the engine.  To allow for only these
// engines would be possible, but they could not pass the sensor first
// without redesigning the detection algorithm.
//
void Engine(const int &dir)
{
	// example: play a tone
	PlayTone(2000, 10 + dir * 10);
}

// Called just after any car with two fixed axles passes.  Examples:
//
// any freight car in #4564 Freight Rail Runner
// the Octan Tanker
// 
void Boxcar()
{
	// example: play a tone
	PlayTone(500, 10);
}

// Called just after any car with two standard bogie plates passes.  Examples:
// 
// 4557 Club Car
// any freight car in #4565 Freight & Crane Railway
// *maybe* the coaches from the #4558 Metroliner, but I don't know if the
// descender in the middle would block the light sensor.
//
void Double()
{
	// example play a tone
	PlayTone(1000, 10);

	// example: stop after first long car
	SetPower(OUT_A, 7);
	// stop hard
	Toggle(OUT_A);
	Wait(30);
	Off(OUT_A);
	StopAllTasks();
}

int in_long, light, dark, axle, single;

#define countlight() light = 0; until(TRAIN_SENSOR < DARK_HIGHWATER) light +=1;
#define countdark() dark = 0; until(TRAIN_SENSOR > LIGHT_LOWATER) dark +=1;

task main()
{
	SetSensor(TRAIN_SENSOR, SENSOR_LIGHT);
	SelectDisplay(DISPLAY_SENSOR_2);

	// example: stop after first long car
	SetPower(OUT_A, 6);
	OnFwd(OUT_A);

	single = 0;

	while (true) {
		countlight();
		// you might have to make this larger if you have huge
		// and drive them really slowly.  keep it as small as
		// possible to ensure it is always reset by a new train.
		if (light > single * 10) {
			// new train.  insist that this is a SINGLE axle,
			// not a motor plate
			countdark();
			single = dark;
			axle = 0;
			continue;
		} else {
			countdark();
		}
		// motor plates block light all the way across, not just
		// at the wheels
		if (dark > single * 3) {
			if (in_long == 0) {
				in_long = 1; // start
			} else {
				in_long = 0;
				Engine(ENGINE_FORWARD);
			}
			continue;
		}

		// adapt to new train speed
		single = (single * 2 + dark) / 3;

		if (axle == 1) {
			// axles come in pairs, either cars or bogies.
			// we want the distance between pairs
			axle = 0;
			continue;
		} else {
			axle = 1;
		}

		// the gap between two wheels on a normal bogie plate is
		// less than one wheel across.
		if (light < single) {
			if (in_long == 0) {
				in_long = 1;
			} else {
				in_long = 0;
				Double();
			}
		} else if (light > single * 2) {
			Boxcar();
			in_long = 0;
		} else {
			if (in_long == 0) {
				in_long = 1;	// this is a fake power bogie
			} else {
				in_long = 0;
				Engine(ENGINE_REVERSE);
			}
		}
	}
}