#include "FastLED.h"
const size_t NUM_LEDS = 28;
CRGB leds[NUM_LEDS];
CRGB background[NUM_LEDS];

const uint8_t basesStart[] = { 5, 10, 15, 20 };
const uint8_t basesLength = 3;
const uint8_t basesCount = sizeof( basesStart ) / sizeof( basesStart[0] );

void setup()
{
  FastLED.addLeds< WS2812, 6, GRB >( leds, NUM_LEDS );
  FastLED.setBrightness( 50 );
  fill_gradient_RGB( leds, NUM_LEDS, CRGB::Magenta, CRGB::Yellow );
  memcpy( background, leds, sizeof( background ) );

  for ( uint8_t i = 0; i < basesCount; i++ )
  {
    for ( uint8_t j = basesStart[i], k = j + basesLength; j < k; j++ )
    {
      leds[j] = CRGB::White;
    }
  }

  FastLED.show();
  delay( 2000 );
}

void loop()
{
  uint32_t now = millis();
  static uint32_t past = now;

  if ( now - past >= 1000UL )
  {
    past = now;

    static int8_t offset = 0;

    for ( uint8_t i = 0; i < basesCount; i++ )
    {
      for ( uint8_t j = basesStart[i] + offset, k = j + basesLength; j < k; j++ )
      {
        leds[j] = background[j];
      }
    }
    
    offset = offset == 3 ? -3 : 3;

    for ( uint8_t i = 0; i < basesCount; i++ )
    {
      for ( uint8_t j = basesStart[i] + offset, k = j + basesLength; j < k; j++ )
      {
        leds[j] = CRGB::White;
      }
    }

    FastLED.show();
  }
}