/*-data-type------size---------description----------------------
  boolean        (8 bit)   -  [true/false]
  byte           (8 bit)   -  [0-255] unsigned number
  char           (8 bit)   -  [-128 to 127] signed number
  unsigned char  (8 bit)   -  [-128 to 127] signed number
  word           (16 bit)  -  [0-65535] unsigned number
  unsigned int   (16 bit)  -  [0-65535] unsigned number
  int            (16 bit)  -  [-32768 to 32767] signed number
  unsigned long  (32 bit)  -  [0-4,294,967,295] unsigned number usually for millis
  long           (32 bit)  -  [-2,147,483,648 to 2,147,483,647] signed number
  float          (32 bit)  -  [-3.4028235E38 to 3.4028235E38] signed number
  uint8_t        (8 bit)   -  [0-255] unsigned number
  int8_t         (8 bit)   -  [-127 - 127] signed number
  uint16_t       (16 bit)  -  [0-65,535] unsigned number
  int16_t        (16 bit)  -  [-32,768 - 32,767] signed number
  uint32_t       (32 bit)  -  [0-4,294,967,295] unsigned number
  int32_t        (32 bit)  -  [-2,147,483,648 - 2,147,483,647] signed number
  uint64_t       (64 bit)  -  [0-18,446,744,073,709,551,615] unsigned number
  int64_t        (64 bit)  -  [−9,223,372,036,854,775,808 - 9,223,372,036,854,775,807] signed number
  --------------------------------------------------------------
  camelCase                -  anything that changes
  snake_case               -  variable's that are exclusive in a function
  Snake_Case               -  CLASS/struct exclusave varables/functions
  iNVERTEDcAMELcASE        -  outside code that is being accessed [database]
  SNake_CAse               -  duplicate varables inside the case function [frequently used in library names]
  ALL_CAPS                 -  const varable names or defines
  ------------- by jediRick & RefreshMyMind --------------------
*/

#define PIN_QTY (3)

unsigned long currentMillis;
const unsigned long onTime = 250;
unsigned long previousMillis = 0;
unsigned long interval = onTime;

uint8_t BUTTON_PINS[PIN_QTY] = {18, 19, 20};
volatile bool BUTTON_FLAGS[PIN_QTY] = {false, false, false};

void setup() {
  Serial.begin(115200);
  for (uint8_t i = 0; i < PIN_QTY; i++) {
    pinMode(BUTTON_PINS[i], INPUT_PULLUP);
    delay(5);
  }
  attachInterrupt(digitalPinToInterrupt(BUTTON_PINS[0]), buttonPinInterrupt0, RISING);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PINS[1]), buttonPinInterrupt1, RISING);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PINS[2]), buttonPinInterrupt2, RISING);
  delay(5);
}

void loop() {
  printValues();
}

void buttonPinInterrupt0() {
  BUTTON_FLAGS[0] = true;
}

void buttonPinInterrupt1() {
  BUTTON_FLAGS[1] = true;
}

void buttonPinInterrupt2() {
  BUTTON_FLAGS[2] = true;
}

void printValues() {
  if (BUTTON_FLAGS[0] == true) {
    checkTimer();
  }
  if (BUTTON_FLAGS[1] == true) {
    checkTimer();
  }
  if (BUTTON_FLAGS[2] == true) {
    checkTimer();
  }
}

void checkTimer() {
  currentMillis = millis();
  if ((unsigned long)(currentMillis - previousMillis) >= interval) {
    if (BUTTON_FLAGS[0]) {
      interval = onTime;
      Serial.println("buttonFlag0: " + (String)BUTTON_FLAGS[0]);
    }
    if (BUTTON_FLAGS[1]) {
      interval = onTime;
      Serial.println("buttonFlag1: " + (String)BUTTON_FLAGS[1]);
    }
    if (BUTTON_FLAGS[2]) {
      interval = onTime;
      Serial.println("buttonFlag2: " + (String)BUTTON_FLAGS[2]);
    }
  }
  BUTTON_FLAGS[0] = false;
  BUTTON_FLAGS[1] = false;
  BUTTON_FLAGS[2] = false;
  previousMillis = currentMillis;
}