// https://www.reddit.com/r/FastLED/comments/1am4bn5/trouble_wrapping_around_a_ring_with_crgbset/

#include <FastLED.h>

#define DATA_PIN    PB1
#define NUM_LEDS    99
#define BRIGHTNESS  255
CRGB realLEDS[NUM_LEDS];
CRGBSet leds(realLEDS, NUM_LEDS);

const long outerInterval = 20;
int dot = 0;
int increment = 12;

void setup() {
  FastLED.addLeds<WS2811, DATA_PIN, GRB>(leds, NUM_LEDS);
  FastLED.setCorrection(TypicalPixelString);
  FastLED.setBrightness(BRIGHTNESS);
}

void loop()
{
  EVERY_N_MILLIS(outerInterval) {
      FastLED.clear();
      safe_fill(dot, dot+increment, CRGB::Blue);
      safe_fill(dot+increment*2, dot+increment*3, CRGB::Blue);
      safe_fill(dot+increment*4, dot+increment*5, CRGB::Blue);
      safe_fill(dot+increment*6, dot+increment*7, CRGB::Blue);

      FastLED.show();

      if (++dot >= NUM_LEDS) dot = 0;
  }
}


void safe_fill(int p_start, int p_end, CRGB colour) {
  // constrain the start and end positions to the length of the array
  p_start %= NUM_LEDS; 
  p_end %= NUM_LEDS;

  if (p_end >= p_start) {
    // the whole fill fits within the leds[] array
    leds(p_start, p_end) = colour;
  } else {
    // the fill wraps, so fill the head...
    leds(p_start, NUM_LEDS - 1) = colour;
    // ...and the tail separately
    leds(0, p_end) = colour;
  }
}