// https://forum.arduino.cc/t/digital-hourglass/1177746/
// https://arduinoplusplus.wordpress.com/2023/11/19/led-matrix-hourglass-timer/#more-11996

#include <FastLED.h>

#define DATA_PIN   2
#define NUM_LEDS 128
#define BTTN_PIN   3
int counter;

CRGB leds[NUM_LEDS];

void setup() {
  Serial.begin(115200); // for debug
  pinMode(BTTN_PIN, INPUT_PULLUP); // change direction 

  FastLED.addLeds<WS2812B, DATA_PIN, GRB>(leds, NUM_LEDS);
  FastLED.show();
  // randomSeed(analogRead(0)); // for future use

  for (int i = 0; i < NUM_LEDS / 2; i++) { // only color top square
    leds[i] = CRGB(CRGB::CRGB::DeepSkyBlue);
  }
  FastLED.show();

  // pixels falling out of top square
  for (int i = 0; i < NUM_LEDS / 2; i += 9) {
    // counter++; // LED counter
    leds[i] = CRGB(CRGB::Black);
    FastLED.show();
    delay(250); // visual
    if (i > 0)
    leds[i] = CRGB(CRGB::CRGB::DeepSkyBlue);
    FastLED.show();
  }

  // pixels falling into bottom square
  for (int i = NUM_LEDS / 2 + .5; i < NUM_LEDS; i += 9) {
    // counter++; // LED counter
    leds[i] = CRGB(CRGB::CRGB::DeepSkyBlue);
    FastLED.show();
    delay(250);
    if ((i > 0) && (i < NUM_LEDS-1))
      leds[i] = CRGB(CRGB::Black);
    FastLED.show();
  }
}

void loop() {}

/*
  (line 1 has 1 LED)
    LED #1 location in each square is 0,0

  (line 2 has 2 LEDs)
    LED 2 = 1, 0
    LED 3 = 0, 1

  (line 3 has 3 LEDs)
    LED 4 = 2, 0
    LED 5 = 1,1
    LED 6 = 0, 2

  (line 4)
    7 = 3, 0
    8 = 2, 1
    9 = 1, 2
    10 = 0, 3

  See the pattern?

    Next line number (L) increases by 1 (toward a MAX of 7)
    First LED in line number is (L-1, 0)
    Next LEDs - "x" decreases toward 0, "y" increases to L-1
    Last LED in line number is (0, L-1)
    After you get midway (7,0) (1, 6) (2, 5) (3, 4) (4, 3) (5, 2) ( (6, 1) (0,7)... decrease the number of "x,y" pairs
    (line 14 has 2 LEDs (7, 6)(6, 7)
    (line 15 has 1 LED (7,7)

  Each time you set "x, y" to OFF on the top square, you set "x, y" ON on the bottom square.
*/