#include <avr/io.h>

// #define FIRST_LETTER_IMPL(x) #x[0]
// #define FIRST_LETTER(x) FIRST_LETTER_IMPL(x)
// #define PORT_LETTER(PIN) FIRST_LETTER(PIN)

// global registers
#define SET(register, bit)			register |= 1 << bit
#define CLEAR(register, bit)		register &= ~(1 << bit)
#define TEST(register, bit)			register & 1 << bit
#define TOGGLE(register, bit)		register ^= 1 << bit

// actions on ports
#define TEST_PIN(port, bit)			TEST(PIN ## port, bit)
#define TOGGLE_PIN(port, bit)		SET(PIN ## port, bit)
#define MODE_INPUT(port, bit)		CLEAR(DDR ## port, bit)
#define MODE_OUTPUT(port, bit)		SET(DDR ## port, bit)
#define IS_PIN_SET(port, bit)		(TEST_PIN(port, bit)) == 0
#define IS_PIN_CLEAR(port, bit)		(TEST_PIN(port, bit)) == 1
#define TURN_ON(_PORT, BIT)			SET(PORT ## _PORT, BIT)
#define TURN_OFF(_PORT, BIT)		CLEAR(PORT ## _PORT, BIT)

// LEDs
#define TURN_ON_LED(bit)			SET(PORTB, bit)
#define TURN_OFF_LED(bit)			CLEAR(PORTB, bit)
#define TOGGLE_LED(led)				TOGGLE_PIN(B, led)

// ELEMENTS
#define D1 0
#define D2 1
#define D3 2
#define D4 4
#define SW1 2
#define SW2 4

// TIME
#define PRESCALE_VALUE				1024
#if (PRESCALE_VALUE == 1)
	#define PRESCALE_SET			(1 << CS10)
#elif (PRESCALE_VALUE == 8)
	#define PRESCALE_SET			(1 << CS11)
#elif (PRESCALE_VALUE == 64)
	#define PRESCALE_SET			(1 << CS10) | (1 << CS11)
#elif (PRESCALE_VALUE == 256)
	#define PRESCALE_SET			(1 << CS12)
#elif (PRESCALE_VALUE == 1024)
	#define PRESCALE_SET			(1 << CS10) | (1 << CS12)
#endif
#define TIME_MS(ms)					(((F_CPU / PRESCALE_VALUE) * ms) / 1000)

int main() {
	MODE_OUTPUT(B, D2);

	TCCR1B |= (1 << WGM12);						// set timer in CTC (Clear Time on Compare) mode
												//   -> Table 16-4 : use bit WGM12 to set mode ctc
												//   -> 16.11.2 : bit WGM12 is located in register TCCR1B

	OCR1A = TIME_MS(500);						// set CTC compare value
												//   -> Table 16-4 : the value to reset the timer is the Output Compare Registers OCR1A

	TCCR1B |= (PRESCALE_SET);

	while(1) {
		if (TIFR1 & (1 << OCF1A)) {				// when comparison has occured, bit OCF1A is set
												//    -> 16.2.1 : "The compare match event will also set the Compare Match Flag (OCF1A/B) which can be used to generate an Output Compare interrupt request"
												//    -> 16.11.9 : bit OCF1A is in register TIFR1

			TOGGLE_LED(D2);						// toggle using PINx register

			TIFR1 = (1 << OCF1A);				// clear the CTC flag
												//    -> 16.11.9 : "OCF1A can be cleared by writing a logic one to its bit location"
		}
	}
}