/*
Sketch uses 354 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 0 bytes (0%) of dynamic memory, leaving 2048 bytes for local variables. Maximum is 2048 bytes.

FUNCTION: void set_led()
Step 1 resets all 3 pins to input.
Step 2 clears the PORTD bits, which disables any internal pull-up resistors.
Step 3 re-enables only the two needed output pins.
Step 4 sets HIGH/LOW levels.
*/

#define F_CPU 16000000UL
#include <avr/io.h>
#include <util/delay.h>

// Define bit positions
#define PIN_A PD2
#define PIN_B PD3
#define PIN_C PD4

void setup() {
  // Initially set all pins as input
  //DDRD &= ~(1 << PIN_A) & ~(1 << PIN_B) & ~(1 << PIN_C);
  //DDRD = 0x00;
  DDRD &= ~((1 << PIN_A) | (1 << PIN_B) | (1 << PIN_C));
}

void set_led(uint8_t high, uint8_t low, uint8_t z) {
  // Step 1: Set all pins as input first
  // This works to set DDRD as 0, but not best practice
  DDRD &= ~(1 << PIN_A) & ~(1 << PIN_B) & ~(1 << PIN_C);
  // This works to set DDRD as 0, but clears all of DDRB, be careful!
  DDRD = 0x00;
  // This works to set DDRD as 0, !! best practice method !!
  DDRD &= ~((1 << PIN_A) | (1 << PIN_B) | (1 << PIN_C));

  // Step 2: Disable pull-ups (ensure Hi-Z) on all three
  PORTD &= ~((1 << PIN_A) | (1 << PIN_B) | (1 << PIN_C));

  // Step 3: Set high and low pins as output
  DDRD |= (1 << high) | (1 << low);

  // Step 4: Drive the pins
  PORTD |= (1 << high);   // High
  PORTD &= ~(1 << low);   // Low
}

int main(void) {
  setup();

  while(1) {
    set_led(PIN_A, PIN_B, PIN_C); // LED1: A -> B
    _delay_ms(500);
        
    set_led(PIN_B, PIN_A, PIN_C); // LED2: B -> A
    _delay_ms(500);
       
    set_led(PIN_A, PIN_C, PIN_B); // LED3: A -> C
    _delay_ms(500);

    set_led(PIN_C, PIN_A, PIN_B); // LED4: C -> A
    _delay_ms(500);

    set_led(PIN_B, PIN_C, PIN_A); // LED5: B -> C
    _delay_ms(500);

    set_led(PIN_C, PIN_B, PIN_A); // LED6: C -> B
    _delay_ms(500);
  }
}