#define F_CPU 8'000'000UL

#include <stdint.h>
#include <avr/io.h>
#include <util/delay.h>


#define SNAKE_SIZE 4
#if SNAKE_SIZE > 7
#error "SNAKE_SIZE too big"
#elif SNAKE_SIZE == 0
#error "SNAKE_SIZE must be greater than 0"
#endif

int main() {
    // Leds
    DDRB = 0xff;
    PORTB = 0x00;

    // Buttons
    DDRD = 0x00;
    PORTD = 0x01;

    uint8_t head = SNAKE_SIZE - 1;
    PORTB = (1 << (SNAKE_SIZE + 1)) - 1;
    while(true) {
        if(PIND & 0x01) {
            PORTB |= (1 << (head + 1));
            PORTB &= ~(1 << (head + 8 - SNAKE_SIZE + 1) % 8);
            ++head;
            head %= 8;
        } else {
            PORTB |= (1 << (head + 8 - SNAKE_SIZE) % 8);
            PORTB &= ~(1 << head);
            head += 7;
            head %= 8;
        }

        _delay_ms(1000);
    }
}