Arduino 4-digit-7-segment display counter

Tried a single digit 7-segment display previously, this time tried a 12-pins 4-digit-7-segment display. Using 7 segments to display a digit, for four digits, it's controlled by selecting the digit to HIGH, and segments to LOW (since it's common anode like the previous one).
Using multiplexing at one time only one digit is active(e.g. for 2ms). All digits is turned on is serial, but because human’s eye is inert we have illusion, that all digits are lighting at same time. (ref: https://www.electronicsblog.net/4-digits-7-segments-led-display-multiplexing-with-arduino/)

The circuit can be built as following: (ref: http://ecotronics.ch.honorius.sui-inter.net/wordpress/2013/multiplexed-4-digit-7-segment-led-display-mit-arduino-ansteuern/)

To make the counter, actually the display is rapidly refreshing to display the digits.

the programming will be as following: (adapted from the codes: https://www.electronicsblog.net/4-digits-7-segments-led-display-multiplexing-with-arduino/)

 // segment | Arduino board PIN number   
 #define segG 12  
 #define segF 11  
 #define segA 6  
 #define segB 7  
 #define segE 10  
 #define segD 9  
 #define segC 8  
 #define DP 13  
 #define GND1 2  
 #define GND2 3  
 #define GND3 4  
 #define GND4 5  
 int timer=0;   
 int i=0;  
 // functions to display digits  
 void digit0 () {  
  digitalWrite(segA, LOW);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, LOW);  
  digitalWrite(segF, LOW);  
  digitalWrite(segG, HIGH);  
 };  
 void digit1 () {  
  digitalWrite(segA, HIGH);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, HIGH);  
  digitalWrite(segE, HIGH);  
  digitalWrite(segF, HIGH);  
  digitalWrite(segG, HIGH);  
 };  
 void digit2 () {  
  digitalWrite(segA,LOW);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, HIGH);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, LOW);  
  digitalWrite(segF, HIGH);  
  digitalWrite(segG, LOW);  
 };  
 void digit3 () {  
  digitalWrite(segA,LOW);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, HIGH);  
  digitalWrite(segF, HIGH);  
  digitalWrite(segG, LOW);  
 };  
 void digit4 () {  
  digitalWrite(segA,HIGH);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, HIGH);  
  digitalWrite(segE, HIGH);  
  digitalWrite(segF, LOW);  
  digitalWrite(segG, LOW);  
 };  
 void digit5 () {  
  digitalWrite(segA,LOW);  
  digitalWrite(segB, HIGH);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, HIGH);  
  digitalWrite(segF, LOW);  
  digitalWrite(segG, LOW);  
 };  
 void digit6 () {  
  digitalWrite(segA,LOW);  
  digitalWrite(segB, HIGH);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, LOW);  
  digitalWrite(segF, LOW);  
  digitalWrite(segG, LOW);  
 };  
 void digit7 () {  
  digitalWrite(segA,LOW);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, HIGH);  
  digitalWrite(segE, HIGH);  
  digitalWrite(segF, HIGH);  
  digitalWrite(segG, HIGH);  
 };  
 void digit8 () {  
  digitalWrite(segA, LOW);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, LOW);  
  digitalWrite(segF, LOW);  
  digitalWrite(segG, LOW);  
 };  
 void digit9 () {  
  digitalWrite(segA, LOW);  
  digitalWrite(segB, LOW);  
  digitalWrite(segC, LOW);  
  digitalWrite(segD, LOW);  
  digitalWrite(segE, HIGH);  
  digitalWrite(segF, LOW);  
  digitalWrite(segG, LOW);  
 };  
 //function to display digit from inputed int  
 void showdigit (int digit)  
 {  
  switch (digit) {  
  case 0:  
   digit0 ();  
   break;  
  case 1:  
   digit1 ();  
   break;  
  case 2:  
   digit2 ();  
   break;  
  case 3:  
   digit3 ();  
   break;  
  case 4:  
   digit4 ();  
   break;  
  case 5:  
   digit5 ();  
   break;  
  case 6:  
   digit6 ();  
   break;  
  case 7:  
   digit7 ();  
   break;  
  case 8:  
   digit8 ();  
   break;  
  case 9:  
   digit9 ();  
   break;  
  default:  
   break;  
  };   
 };  
 // showing 4 digits  
 void showdigits (int number)  
 {  
  // e.g. we have "1234"  
  showdigit(number/1000); // segments are set to display "1"  
  digitalWrite(GND1, HIGH); // first digit on,  
  digitalWrite(GND2, LOW); // other off  
  digitalWrite(GND3, LOW);  
  digitalWrite(GND4, LOW);  
  delay (1);  
  number = number%1000; // remainder of 1234/1000 is 234  
  digitalWrite(GND1, LOW); // first digit is off  
  showdigit(number/100); //// segments are set to display "2"  
  digitalWrite(GND2, HIGH); // second digit is on  
  delay (1); // and so on....  
  number =number%100;    
  digitalWrite(GND2, LOW);  
  showdigit(number/10);  
  digitalWrite(GND3, HIGH);  
  delay (1);  
  number =number%10;   
  digitalWrite(GND3, LOW);  
  showdigit(number);   
  digitalWrite(GND4, HIGH);  
  delay (1);  
 };   
 void setup()  
 {  
  pinMode(segA, OUTPUT);  
  pinMode(segB, OUTPUT);  
  pinMode(segC, OUTPUT);  
  pinMode(segD, OUTPUT);  
  pinMode(segE, OUTPUT);  
  pinMode(segF, OUTPUT);  
  pinMode(segG, OUTPUT);  
  pinMode(DP, OUTPUT);  
  pinMode(GND1, OUTPUT);  
  pinMode(GND2, OUTPUT);  
  pinMode(GND3, OUTPUT);  
  pinMode(GND4, OUTPUT);  
 };  
 void loop ()  
 {   
  timer++;  
  showdigits (i);  
  if (timer==10) {  
   timer=0;  
   i++;  
   if (i>10000) {  
    i=0;  
   };  
  };   
 };