Fix WMath's map() implementation for inverse/round-trip mapping

[dok-net]

Inspired by Servo.cpp's improved_map, this superseded that code, and has better performance (identical to prior WMath implementation).
I expect this is better used to replace the core implementation, then drop "improved_map" from Servo.cpp, than keep it local and have a map() function that may be compatible but works horribly ;-)

[dok-net]

Here's the sketch for testing correctness and timing:

uint32_t expired;
uint32_t iterations;

void setup()
{
	Serial.begin(115200);
	while (!Serial);
	Serial.println("check map");
	expired = 0;
	iterations = 0;
}

constexpr int imin = 0;
constexpr int imax = 180;
constexpr int omin = 1000;
constexpr int omax = 2000;

int i = 0;
int fails = 0;

void loop()
{
	++iterations;
	i = (i + 1) % (imax + 1);

#if defined(ESP8266) || defined(ESP32)
	cli();
	volatile auto start = ESP.getCycleCount();
#else
	// Set Timer 1 to normal mode at F_CPU.
	TCCR1A = 0;
	TCCR1B = 1;
	cli();
	volatile uint16_t start = TCNT1;
#endif

	volatile auto o = map(i, imin, imax, omin, omax);

#if defined(ESP8266) || defined(ESP32)
	sei();
	volatile auto cycles = ESP.getCycleCount() - start;
#else
	sei();
	volatile uint16_t finish = TCNT1;
	uint16_t cycles = finish - start;
#endif

	expired += cycles;
	
	if (i != map(o, omin, omax, imin, imax))
	{
		++fails;
	}

	if (iterations > 100000)
	{
		Serial.print("Cycles per map(...) = ");	Serial.print(expired / iterations);
		Serial.print(", fails = ");	Serial.print(fails);
		Serial.println();
		expired = 0;
		iterations = 0;
		fails = 0;
	}
}

[dok-net]

arduino/ArduinoCore-avr#309

[earlephilhower]

LGTM.

[devyte]

This doesn't handle divisor == 0.

[dok-net]

This today got merged into Arduino Core for ESP32, espressif/arduino-esp32#3655