#include "FastLED.h"

#define LED_PIN     9
#define LED_TYPE    WS2812B
#define COLOR_ORDER GRB
#define NUM_LEDS    48
CRGB leds[NUM_LEDS];
int location =0;
unsigned long time;

void setup() {
  FastLED.addLeds<LED_TYPE,LED_PIN,COLOR_ORDER>(leds, NUM_LEDS);
  time=millis();
}

void loop()
{
 if (millis()>=time) { // update leds every 10ms(=50hz)
  time=time+10;
  chase();
 }
}

void chase()
{
   if (location==255) location=0;
   location=location+1;
   CHSV start_col = CHSV(location, 255, 255);
   CHSV end_col = CHSV(255+location, 255, 255);
   fill_gradient(leds,0,end_col,NUM_LEDS,start_col,BACKWARD_HUES);
   int brightest=location/255.0*NUM_LEDS;
   for (byte w=0;w<NUM_LEDS;w++) {
     leds[brightest].fadeLightBy(w*5.3);
     brightest=brightest-1;
     if (brightest<0) brightest=NUM_LEDS;
   }
   FastLED.show();
}