#include <stdint.h>
#include <string.h>

#define PORT 0x3f8          // COM1
 
static int init_serial() {
   outb(PORT + 1, 0x00);    // Disable all interrupts
   outb(PORT + 3, 0x80);    // Enable DLAB (set baud rate divisor)
   outb(PORT + 0, 0x03);    // Set divisor to 3 (lo byte) 38400 baud
   outb(PORT + 1, 0x00);    //                  (hi byte)
   outb(PORT + 3, 0x03);    // 8 bits, no parity, one stop bit
   outb(PORT + 2, 0xC7);    // Enable FIFO, clear them, with 14-byte threshold
   outb(PORT + 4, 0x0B);    // IRQs enabled, RTS/DSR set
   outb(PORT + 4, 0x1E);    // Set in loopback mode, test the serial chip
   outb(PORT + 0, 0xAE);    // Test serial chip (send byte 0xAE and check if serial returns same byte)
 
   // Check if serial is faulty (i.e: not same byte as sent)
   if(inb(PORT + 0) != 0xAE) {
      return 1;
   }
 
   // If serial is not faulty set it in normal operation mode
   // (not-loopback with IRQs enabled and OUT#1 and OUT#2 bits enabled)
   outb(PORT + 4, 0x0F);
   return 0;
}

int serial_received() {
   return inb(PORT + 5) & 1;
}
 
char read_serial() {
   while (serial_received() == 0);
 
   return inb(PORT);
}

int is_transmit_empty() {
   return inb(PORT + 5) & 0x20;
}
 
void write_serial(char a) {
   while (is_transmit_empty() == 0);
   
   outb(PORT, a);
}

void serial_print(const char* str, ...) {
  size_t len = strlen(str);
  for (size_t i=0; i < len; i++) {
    if (str[i] == '\n') {
	write_serial('\r');
	write_serial('\n');
    } else {
      write_serial(str[i]);
    }
  }
}

inline void outb(uint16_t port, uint8_t val)
{
    asm volatile ( "outb %0, %1" : : "a"(val), "Nd"(port) );
    /* There's an outb %al, $imm8  encoding, for compile-time constant port numbers that fit in 8b.  (N constraint).
     * Wider immediate constants would be truncated at assemble-time (e.g. "i" constraint).
     * The  outb  %al, %dx  encoding is the only option for all other cases.
     * %1 expands to %dx because  port  is a uint16_t.  %w1 could be used if we had the port number a wider C type */
}

inline int inb(int port)
{
    uint8_t ret;
    asm volatile ( "inb %1, %0"
                   : "=a"(ret)
                   : "Nd"(port) );
    return ret;
}