#include <FastLED.h>

#define LED_PIN     2
#define NUM_LEDS    60
#define LED_TYPE    WS2812B
#define COLOR_ORDER GRB
#define BRIGHTNESS  200

CRGB leds[NUM_LEDS];

// ------------------- Particle Class -------------------
struct Particle {
  CRGB color;
  float position;
  float velocity;
  unsigned long birthTime;
  unsigned long lastUpdate;
  bool active;

  Particle() {
    active = false;
  }

  void init(CRGB c, float pos, float maxSpeed) {
    color = c;
    position = pos;
    velocity = (random(1000, 10000) / 10000.0f) * maxSpeed * 2 - maxSpeed;
    birthTime = millis();
    lastUpdate = millis();
    active = true;
  }

  float age() {
    return (millis() - birthTime) / 1000.0f; // seconds
  }

  void update() {
    float deltaTime = (millis() - lastUpdate) / 1000.0f;
    position += velocity * deltaTime;
    lastUpdate = millis();
    velocity -= (2 * velocity * deltaTime);
    color.fadeToBlackBy(random(10, 25));  // random fading
  }
};

// ------------------- Settings -------------------
float MaxSpeed = 30.0f;     // max pixels per sec velocity
float NewParticleProb = 0.05f; // chance per frame of new burst
float ParticlePreignition = 0.0f;
float ParticleIgnition = 0.2f;
float ParticleHold = 0.0f;
float ParticleFade = 2.0f;

#define MAX_PARTICLES 60
Particle particles[MAX_PARTICLES];

// ------------------- Setup -------------------
void setup() {
  FastLED.addLeds<LED_TYPE, LED_PIN, COLOR_ORDER>(leds, NUM_LEDS);
  FastLED.setBrightness(BRIGHTNESS);
  randomSeed(analogRead(0));
}

// ------------------- Loop -------------------
void loop() {
  // Maybe spawn a new burst
  if (random(0, 1000) < (NewParticleProb * 1000)) {
    int startPos = random(0, NUM_LEDS);
    CRGB c = CHSV(random8(), 255, 255);
    int count = random(10, 50);
    float multiplier = (random(0, 1000) / 1000.0f) * 3;

    for (int i = 0; i < count; i++) {
      // find a free slot
      for (int j = 0; j < MAX_PARTICLES; j++) {
        if (!particles[j].active) {
          particles[j].init(c, startPos, MaxSpeed * multiplier);
          break;
        }
      }
    }
  }

  // Clear background
  fill_solid(leds, NUM_LEDS, CRGB::Black);

  // Update & draw particles
  for (int i = 0; i < MAX_PARTICLES; i++) {
    if (!particles[i].active) continue;

    particles[i].update();

    float age = particles[i].age();
    CRGB c = particles[i].color;
    float fade = 0;

    if (age > ParticlePreignition && age < ParticleIgnition + ParticlePreignition) {
      c = CRGB::White;  // flash white
    } else {
      if (age < ParticlePreignition) {
        fade = 1.0 - (age / ParticlePreignition);
      } else {
        age -= ParticlePreignition;
        if (age < ParticleHold + ParticleIgnition) {
          fade = 0.0;
        } else if (age > ParticleHold + ParticleIgnition + ParticleFade) {
          fade = 1.0;
        } else {
          age -= (ParticleHold + ParticleIgnition);
          fade = (age / ParticleFade);
        }
      }
      c.fadeToBlackBy((uint8_t)(fade * 255));
    }

    int pos = constrain((int)round(particles[i].position), 0, NUM_LEDS - 1);
    leds[pos] += c;

    // Kill particle if too old
    if (age > (ParticleHold + ParticleIgnition + ParticleFade)) {
      particles[i].active = false;
    }
  }

  FastLED.show();
  delay(16); // ~60fps
}