#include <FastLED.h>

#define WIDTH 8
#define HEIGHT 8
#define NUM_LEDS ((WIDTH) * (HEIGHT))

CRGB leds[NUM_LEDS + 1];

uint16_t XY(uint8_t x, uint8_t y) {
  if (x >= WIDTH) return NUM_LEDS;
  if (y >= HEIGHT) return NUM_LEDS;
  if (y & 1)
    return (y + 1) * WIDTH - 1 - x;
  else
    return y * WIDTH + x;
}

void setup() {
  FastLED.addLeds<NEOPIXEL, PB1>(leds, NUM_LEDS);
  FastLED.setCorrection(UncorrectedColor);
  FastLED.setTemperature(UncorrectedTemperature);
  FastLED.setDither(DISABLE_DITHER);
}

struct Ball {
  float x_speed, y_speed, radius, transition;
  CRGB colour;
};

Ball balls[] = {
  {8, 9.58, 1.5, 5, CRGB::Red},
  {7, 7.2, 1.5, 5, CRGB::Green},
  {6, 6.5, 1.5, 5, CRGB::Blue},
};

void loop() {
  for (auto b = 0; b < sizeof(balls) / sizeof(*balls); b++) {
    float offset_x = sinf(balls[b].x_speed * millis() / 8192.f) * WIDTH / 2 - WIDTH / 2;
    float offset_y = cosf(balls[b].y_speed * millis() / 8192.f) * HEIGHT / 2 - HEIGHT / 2;
    float radius = balls[b].radius;
    float transition = balls[b].transition;
    float max_sum_squares = powf(radius + transition, 2);

    for (int y = 0; y < HEIGHT; y++) {
      for (int x = 0; x < WIDTH; x++) {
        float sum_squares = powf(x + offset_x, 2) + powf(y + offset_y, 2);
        if (sum_squares > max_sum_squares)
          continue;
        CRGB faded = balls[b].colour;
        float distance = sqrtf(sum_squares) - radius;
        if (distance >= 0)
          faded %= 255 - 255.f * distance / transition;
        leds[XY(x, y)] += faded;
      }
    }
  }

  FastLED.show();
  FastLED.clear();
}