#include <Adafruit_NeoPixel.h>

#define PIN 2
#define NUM_PIXELS 32
#define NUM_PER_RING 16

Adafruit_NeoPixel pixels = Adafruit_NeoPixel(NUM_PIXELS, PIN, NEO_GRB + NEO_KHZ800);

int offset = 0;             // the position of the red LEDs
int speed = 50;             // initial delay in milliseconds
int speedStep = 1;          // how much to change speed each loop
int minSpeed = 5;           // fastest
int maxSpeed = 100;         // slowest
int direction = 1;           // 1 = forward, -1 = backward

void setup() {
  pixels.begin();
}

void loop() {
  // update LED rings
  for (int i = 0; i < NUM_PER_RING; i++) {
    int brightness = 0; // default off

    if (i == offset || i == (offset + 8) % NUM_PER_RING) {
      brightness = 255; // red LEDs
    }

    // left ring
    pixels.setPixelColor(i, pixels.Color(brightness, brightness, 0));

    // right ring
    pixels.setPixelColor(i + NUM_PER_RING, pixels.Color(brightness, brightness, 0));
  }

  pixels.show();

  // move offset according to direction
  offset += direction;
  if (offset >= NUM_PER_RING) offset = 0;
  if (offset < 0) offset = NUM_PER_RING - 1;

  // update speed
  speed += speedStep;

  // reverse direction and speedStep if limits reached
  if (speed >= maxSpeed || speed <= minSpeed) {
    speedStep = -speedStep;
    direction = -direction; // reverse LED movement
  }

  delay(speed); // delay controls speed
}