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#define TIME_DIFFERENCE     (9)
#define FONT_WIDTH_1        (4)
#define FONT_WIDTH          (6)
#define DISPLAY_WIDTH       (28)
 
#define XY5_HEADER          (0x80)
#define XY5_WRITE_COMMAND   (0x83)
#define XY5_ENDBYTE         (0x8f)
#define XY5_ADDRESS         (0x01)
 
 
uint8_t xy5Buffer[DISPLAY_WIDTH];
 
const uint8_t fontData[][FONT_WIDTH] = {
  {0x7f, 0x7f, 0x41, 0x41, 0x7f, 0x7f}, // 0
  {0x00, 0x00, 0x42, 0x7f, 0x7f, 0x40}, // 1
  {0x79, 0x79, 0x49, 0x49, 0x4f, 0x4f}, // 2
  {0x41, 0x49, 0x49, 0x49, 0x7f, 0x7f}, // 3
  {0x1f, 0x1f, 0x10, 0x7f, 0x7f, 0x10}, // 4
  {0x4f, 0x4f, 0x49, 0x49, 0x79, 0x79}, // 5
  {0x7f, 0x7f, 0x49, 0x49, 0x79, 0x79}, // 6
  {0x01, 0x01, 0x71, 0x7d, 0x0f, 0x03}, // 7
  {0x7f, 0x7f, 0x49, 0x49, 0x7f, 0x7f}, // 8
  {0x4f, 0x4f, 0x49, 0x49, 0x7f, 0x7f}, // 9
  {0x42, 0x7f, 0x7f, 0x40, 0x00, 0x00}, // extra 1
};
 
void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}
 
void loop() {
  // put your main code here, to run repeatedly:
  long time;
 
  if ((time = getTimeFromGPS()) >= 0) {
    printTime(time);
  }
}
 
void clearBuffer() {
  for (int i = 0; i < DISPLAY_WIDTH; i++) {
    xy5Buffer[i] = 0;
  }
}
 
void writeANumberToBuffer(int xAxis, int c) {
  for (int i = 0; i < FONT_WIDTH; i++) {
    xy5Buffer[xAxis++] = fontData[c][i];
  }
}
 
void flushBuffer(int address) {
  Serial.write(XY5_HEADER);
  Serial.write(XY5_WRITE_COMMAND);
  Serial.write(address);
 
  for (int i = 0; i < DISPLAY_WIDTH; i++) {
    Serial.write(xy5Buffer[i]);
  }
 
  Serial.write(XY5_ENDBYTE);
}
 
void printTime(long time) {
  int xAxis[] = {22, 15, 5};  // where to print digits
  long localTime;
 
  clearBuffer();
 
  // GMT to local time and make it a 12-hour clock
  localTime = (time + TIME_DIFFERENCE * 10000L + 240000L) % 240000L;
  if (localTime >= 120000L) {
    localTime -= 120000L;
  }
 
  // colon
  if (time % 2) {
    xy5Buffer[12] = xy5Buffer[13] = 0x7f;
    xy5Buffer[12] = xy5Buffer[13] &= ~(1 << (6 - ((time / 10) % 10)));
  }
   
  localTime /= 10;
 
  // hour and minute
  for (int i = 0; i < 3; i++) {
    localTime /= 10;
    writeANumberToBuffer(xAxis[i], localTime % 10);
  }
 
  // leading 1
  if (localTime / 10) {
    for (int i = 0; i < FONT_WIDTH_1; i++) {
      xy5Buffer[i] = fontData[10][i];
    }
  }
 
  flushBuffer(XY5_ADDRESS);
}
 
long getTimeFromGPS() {
  char gpsData[80];
  int position = 0;
 
  // get GPS data
  while (1) {
    if (Serial.available()) {
      gpsData[position] = Serial.read();
      if (gpsData[position] == '\n') {
        gpsData[position - 1] = '\0';
        position = 0;
        break;
      } else {
        position++;
      }
    }
  }
 
  // get time
  if (strcmp(strtok(gpsData, ","), "$GPRMC") == 0) {
    return atol(strtok(NULL, "."));
  } else {
    return -1;
  }
}