// two-bit up/down counter FSM: B0 = count down, B1 = count up

volatile int B0, B1, tick; // button and timer flags

void C0(void) { tick = 1; } // FSM clock

Timer t0( 1000, C0); // 1 second

void setup() {
  pinMode( D0, OUTPUT); pinMode( D7, OUTPUT); // LEDs
  pinMode( A0, INPUT_PULLDOWN); pinMode( A1, INPUT_PULLDOWN); // buttons
  t0.start(); // timer
}

enum states { s00, s01, s10, s11 }; // enum creates distinct integer constants

enum states state = s00, next; // current and next state variables

void FSM(void) {
  B0 = digitalRead( A0); B1 = digitalRead( A1); // inputs
  next = state; // stay in current state by default
  switch( state) {
    case s00: digitalWrite( D0, 0); digitalWrite( D7, 0);
      if( B0) next = s11; else if( B1) next = s01;
      break;
    case s01: digitalWrite( D0, 0); digitalWrite( D7, 1);
      if( B0) next = s00; else if( B1) next = s10;
      break;
    case s10: digitalWrite( D0, 1); digitalWrite( D7, 0);
      if( B0) next = s01; else if( B1) next = s11;
      break;
    case s11: digitalWrite( D0, 1); digitalWrite( D7, 1);
      if( B0) next = s10; else if( B1) next = s00;
      break;
  }
  state = next; // update state
}

void loop() {
  if(tick) { FSM(); tick = 0; }
}