// IR packet
int hh,hl;
byte d1h,d1l;
byte d0h,d0l;
byte th,tl;
int len;
byte dat[50];

// last receive time
unsigned long last = 0;

int read_byte(){
  while(Serial.available() == 0);
  return Serial.read();
}

boolean rcv_pkt(){
  int i;
  byte cs = 0;
  
  if(read_byte() != 0xff) return false;
  cs += (i = read_byte());  hh = i * 256;
  cs += (i = read_byte());  hh += i;
  cs += (i = read_byte());  hl = i * 256;  
  cs += (i = read_byte());  hl += i;
  cs += (d1h = read_byte());
  cs += (d1l = read_byte());
  cs += (d0h = read_byte());
  cs += (d0l = read_byte());
  cs += (th = read_byte());
  cs += (tl = read_byte());
  cs += (i = read_byte());  len = i * 256;
  cs += (i = read_byte());  len += i;
  if (len > 400 || len <= 0) return false;
  for(i = 0;i < bytes(); i++){
    cs += (dat[i] = read_byte());
  }
  if(read_byte() != cs) return false;
  
  return true;
}

int bytes(){
  if(len == 0) return 0;
  return (len - 1) / 8 + 1;
}

byte checksum(){
  byte cs = 0;
  int i;
  
  cs += (byte)(hh >> 8) + (byte)(hh & 0xff);
  cs += (byte)(hl >> 8) + (byte)(hl & 0xff);
  cs += d1h + d1l + d0h + d0l + th + tl;
  cs += (byte)(len >> 8) + (byte)(len & 0xff);
  for(i = 0;i < bytes(); i++) cs += dat[i];

  return cs;
}

void snd_pkt(){
  Serial.write((byte)0xff);
  Serial.write((byte)(hh >> 8));
  Serial.write((byte)(hh & 0xff));
  Serial.write((byte)(hl >> 8));
  Serial.write((byte)(hl & 0xff));
  Serial.write(d1h);
  Serial.write(d1l);
  Serial.write(d0h);
  Serial.write(d0l);
  Serial.write(th);
  Serial.write(tl);
  Serial.write((byte)(len >> 8));
  Serial.write((byte)(len & 0xff));
  Serial.write(dat,bytes());
  Serial.write(checksum());
  read_byte();  // receive ACK
}

void setup(){
  pinMode(13,OUTPUT);
  Serial.begin(19200);
  len = 0;
}

void loop(){
  if (Serial.available() > 0){
    digitalWrite(13,HIGH);
    if(rcv_pkt()){
      last = millis();
    }
    digitalWrite(13,LOW);
  }
  
  if(millis() - last > 1000 && len > 0){
    digitalWrite(13,HIGH);
    snd_pkt();
    len = 0;
    digitalWrite(13,LOW);
  }
}