#define __AVR_ATmega2560__
#define F_CPU 16000000UL

#include <avr/io.h>
#include <util/delay.h>
#include <stdbool.h>
#include <avr/interrupt.h>

uint16_t freq_high = 1200;
uint16_t freq_low = 800;
volatile uint8_t speed = 25;
volatile uint8_t tick_count = 0;
volatile uint16_t beep_time = 0;
volatile int debounce_int2 = 10;
volatile int debounce_int3 = 10;
volatile int tick_timer_start =  0xffff;

void start_timer() {
    TCCR1B |= _BV(CS10) | _BV(CS12); // clk/1024
    TIMSK1 |= _BV(TOIE1);

    TCCR4B |= _BV(CS41); // clk/8
    TIMSK4 |= _BV(OCIE4A);
    TCNT1 = tick_timer_start;

    // Enable interrupts
    sei();
}

ISR(INT2_vect) {
    if (debounce_int2) return;
    debounce_int2 = 100;

    if (speed < 100) speed++;
}

ISR(INT3_vect) {
    if (debounce_int3) return;
    debounce_int3 = 100;

    if (speed > 1) speed--;
}

ISR(TIMER1_OVF_vect) {
    tick_count++;
    if (tick_count > 3) tick_count = 0;

    beep_time = 50;

    TCNT1 = tick_timer_start;
}

ISR(TIMER4_COMPA_vect) {
    if (beep_time > 0) {
        PORTK ^= 0b10000000;
        beep_time--;
    }
    TCNT4 = 0;
}

void main() {
    DDRK = 0b10000000;
    DDRD = 0b00000000;
    PORTD |= 0b00001100;

    // EIMSK controls which ports can trigger interrupts
    EIMSK |= _BV(INT2) | _BV(INT3);
    // Trigger on falling edge on INT2 and INT3
    EICRA |= _BV(ISC21) | _BV(ISC31);

    start_timer();
    while (true) {
        OCR4A = (F_CPU / 8) / (tick_count == 0 ? freq_high : freq_low);
        tick_timer_start =  0xffff - (F_CPU / 1024 / 1000) * (100 + speed * 10);

        if (debounce_int2 > 0) debounce_int2--;
        if (debounce_int3 > 0) debounce_int3--;
        _delay_ms(1);
    }
}