#include <avr/io.h>
#include <avr/interrupt.h>

#define BUFFER_SIZE 4
#define MOD(n)                ((n) & (BUFFER_SIZE-1))
#define INC(idx)              idx = (MOD(idx+1))
#define IS_BUFFER_EMPTY(buf)  (buf.inIdx == buf.outIdx)
#define IS_BUFFER_FULL(buf)   (MOD(buf.inIdx-1) == buf.outIdx)

typedef struct {
  uint8_t buffer[BUFFER_SIZE];
  uint8_t inIdx;
  uint8_t outIdx;
} CircularBuffer_t;

//UART. 527000, 7 ,O, 2
CircularBuffer_t txBuffer;
CircularBuffer_t rxBuffer;

void UART_Init(void) {
  UBRR0 = 1;
  UCSR0A = 0;
  UCSR0B = (1 << RXCIE0) | (3 << TXEN0);
  UCSR0C = (3 << UPM00) | (1 << USBS0) | (2 << UCSZ00);
  sei();
}

ISR (USART0_UDRE_vect) {
  if (!(IS_BUFFER_EMPTY(txBuffer))) { //if not empty
    UDR0 = txBuffer.buffer[txBuffer.outIdx];
    INC(txBuffer.outIdx);
  } else {
    UCSR0B &= ~(1 << UDRIE0);
  }
}

ISR(USART0_RX_vect) {
  rxBuffer.buffer[rxBuffer.inIdx] = UDR0;
  INC(rxBuffer.inIdx);
}

void delay(void) {
  volatile uint16_t cnt = 0xFFFF;
  while (cnt--);
}

void UART_putchar(char data) {
  txBuffer.buffer[txBuffer.inIdx] = data;
  INC(txBuffer.inIdx);
  UCSR0B |= (1 << UDRIE0);
}

int main(void) {
  txBuffer.inIdx = 0;
  txBuffer.outIdx = 0;
  UART_Init();
  //delay();
  UART_putchar('U');
  delay();
  UART_putchar('A');
  UART_putchar('B');
  UART_putchar('C');
  while(1);
  return 0;
}