void uart_init() {
  // set baud rate 9600
  UBRR0 = (F_CPU / 16 / 9600 - 1);

  // enable USART transmitter
  UCSR0B = 1 << TXEN0;

  UCSR0C =  0
            | 0b11 << UCSZ00 // 8 data bits
            | 0b1 << USBS0   // 2 stop bits
            ;
}

ISR(USART_RX_vect) {
  // copy received UART byte to pins 2..9
  uint8_t c = UDR0;
  if (c != '\n' && c != '\r') {
    PORTD = c << 2;
    PORTB = c >> 6;
  }
}

void uart_putc(char c) {
  while (!(UCSR0A & 1 << UDRE0)) {}
  UDR0 = c;
}

void uart_puts(char const *s) {
  while (*s) uart_putc(*s++);
}

void uart_putn(int n, bool neg = 1) {
  char buf[5];
  neg = neg && n < 0;
  if (neg) n = -n;
  char *s = buf[5];
  *--s = 0;
  do {
    *--s = n % 10 + '0';
    n /= 10;
  } while (n);
  uart_puts(s);
}

void adc_init() {
  // CTC mode
  uint8_t wgm1 = 0b1111;

  TCCR1A = 0
           // set waveform generation mode
           | (wgm1 & 0b11) << WGM10
           ;

  TCCR1B = 0
           // set timer 1 prescaler to 1/8
           | 0b010 << CS10
           // set waveform generation mode
           | ((wgm1 >> 2) & 0b11) << WGM12
           ;

  // overflow every 400 ticks
  OCR1A = 400;

  // enable timer 1 overflow interrupt
  TIMSK1 = 1 << TOIE1;


  ADMUX = 0
          // use ADC0 as input
          | 0 << MUX0
          ;

  ADCSRA = 0
           // enable the ADC
           | 1 << ADEN
           // enable conversion on trigger
           | 1 << ADATE
           // enable conversion done interrupt
           | 1 << ADIE
           ;

  ADCSRB = 0
           // trigger conversion on timer 1 overflow
           | 0b110 << ADTS0
           ;
}

ISR(TIMER1_OVF_vect) {
  // start ADC conversion
  ADCSRA |= 1 << ADSC;
}

volatile uint16_t adc__result;
volatile uint8_t adc__result_changed = 0;

ISR(ADC_vect) {
  // read the conversion result.
  // ADCL must be read before ADCH
  adc__result = ADC;
  adc__result_changed = 1;

  uint16_t leds = 0;
  adc__result /= 102; // map adc__result from 0..1023 to 0..9
  for (int i = 0; i < 10; i++)
    leds |= (adc__result > i) << i;
  PORTD = leds << 2;
  PORTB = leds >> 6;
}

uint16_t adc_get() {
  uint16_t ret;
  do {
    adc__result_changed = 0;
    ret = adc__result;
  } while (adc__result_changed);
  return ret;
}

int main() {
  // mark pins 2..13 as output and pin 1 as TX
  DDRD = ~0b11;
  DDRB = ~0;

  adc_init();
  uart_init();

  // enable interrupts
  sei();

  uart_putc('0');

  uint16_t old_result = 11;
  for (;;) {
    uint16_t result = adc_get();
    if (old_result != result) {
      uart_putn(result);
      uart_putc('\n');
    }
    old_result = result;
  }
}