#include <FastLED.h>

#define DATA_PIN 4                          
#define NUM_LEDS 194

CRGB leds[NUM_LEDS];

int index;
int direction = 1;       // +1 or -1
int speed = 2;           // 1 to 10 or so

void setup()
{
  FastLED.addLeds<WS2812B, DATA_PIN, GRB>(leds, NUM_LEDS);
  fill_solid(leds, NUM_LEDS, CRGB( 0, 0, 0));
  FastLED.show();
  FastLED.setBrightness(255);
}

void loop() 
{
  EVERY_N_MILLISECONDS(20)   // every 20ms should be enough for human eye
  {
    myRainbow( index);
    index += speed * direction;

    if( index >= NUM_LEDS)
    {
      index -= NUM_LEDS;
    }
    if( index < 0)
    {
      index += NUM_LEDS;
    }
  }

  EVERY_N_SECONDS(3)      // change direction, just for fun
  {
    direction = -direction;
  }
}

// Fill 194 leds with a hue from 0 to 255.
void myRainbow( int startLed)
{
  const float hue_increment = float( 255) / float( NUM_LEDS);

  int j = startLed;
  for( int i=0; i<NUM_LEDS; i++, j++)
  {
    if( j >= NUM_LEDS)        // index rolls beyond the array ?
    {
      j = 0;                  // wrap around
    }
    float thisHue = float( i) * hue_increment;
    leds[j] = CHSV( int(thisHue), 255, 255);
  }
  FastLED.show();
}