#include <Adafruit_NeoPixel.h>
#ifdef __AVR__
  #include <avr/power.h>
#endif

#define PIN 3
#define LEDS 16
#define PACKET_SZ ( (LEDS * 3) + 3 )

// Parameter 1 = number of pixels in strip
// Parameter 2 = Arduino pin number (most are valid)
// Parameter 3 = pixel type flags, add together as needed:
//   NEO_KHZ800  800 KHz bitstream (most NeoPixel products w/WS2812 LEDs)
//   NEO_KHZ400  400 KHz (classic 'v1' (not v2) FLORA pixels, WS2811 drivers)
//   NEO_GRB     Pixels are wired for GRB bitstream (most NeoPixel products)
//   NEO_RGB     Pixels are wired for RGB bitstream (v1 FLORA pixels, not v2)
Adafruit_NeoPixel strip = Adafruit_NeoPixel(LEDS, PIN, NEO_GRB + NEO_KHZ800);

unsigned char serial_buffer[PACKET_SZ];
unsigned int head = 0;
unsigned int start;
unsigned int checksum_1;
unsigned int checksum_0;

void setup() {
  Serial.begin(115200);
  strip.begin();
  strip.setPixelColor(0, 0x110000);
  strip.show();
}

void loop() {
  
  if ( Serial.available() ) {
    serial_buffer[head] = Serial.read();

  
    if ( head >= (PACKET_SZ - 1) ) {
      start = 0;
      checksum_1 = head;
      checksum_0 = head - 1;
      head = 0;
    }
    else {
      start = head + 1;
      checksum_1 = head;
      if ( head == 0 ) {
        checksum_0 = PACKET_SZ - 1;
      }
      else {
        checksum_0 = head - 1;
      }
      head++;
    }
  
    if ( serial_buffer[start] == 0xAA ) {
      unsigned short sum = 0;

      for ( int i = 0; i < checksum_0; i++ ) {
        sum += serial_buffer[i];
      }

      if ( start > 0 ) {
        for ( int i = start; i < PACKET_SZ; i++ ) {
          sum += serial_buffer[i];
        }
      }
      
      //Test if valid write packet
      if ( ( ( (unsigned short)serial_buffer[checksum_0] << 8 ) | serial_buffer[checksum_1] ) == sum ) {
        noInterrupts();
        for( int i = 0; i < LEDS; i++ ) {
          int idx = start + 1 + ( 3 * i );
  
          if ( idx >= (PACKET_SZ - 1) ) {
            idx = idx - PACKET_SZ;
          }

          strip.setPixelColor(i, strip.Color(serial_buffer[idx], serial_buffer[idx+1], serial_buffer[idx+2]));
        }

        strip.show();
        interrupts();
      }
    }
  }
}