void init_port();                           // port initialization function
void init_lcd();                            // lcd initialization function
void delay1(volatile int time);             // delay function
void lcd_cmd(volatile char cmd);            // lcd command function
void lcd_print(volatile long printNumber);  // lcd number print function
void lcd_print_char(volatile char ptr);     // lcd character print function
volatile char checkKey();                   // keyboard scanning function
void calculateNumber(volatile char key);    // function for calculate the pressed number
void calculateResult();                     // function for the result calculation

volatile char *dirf, *outf, *dirk, *outk, *dira, *outa, *dirc, *inc;
volatile long number, num1, num2, result, temp;
volatile char action, resultFlag;   // 'action' is the operation type variable





int main() {     //////////////////// MAIN FUNCION ///////////////////////////
  Serial.begin(9600);
  Serial.println("         key A = '+'          key B = '-'          key C = '*'");
  Serial.println("         key D = '/'          key # = '='          key * = 'Clear'");
  Serial.println("It performs only two number operation at a time. To do multiple operations, perform operation between result and another number:" );
  Serial.println("   Ex-   1st stage: 3+5=8" );
  Serial.println("         2nd stage: *5=40" );
  Serial.println("         3rd stage: -25=15 and so on....." );
  
  volatile char key;
  init_port();
  init_lcd();
 
while(1){
  key=checkKey();
   if(key){
    calculateNumber(key);

    lcd_cmd(0x01);         // to clear the screen
    
     if(num1>0) lcd_print(num1);
else if(num1<0){lcd_print_char('-'); temp=num1-1; temp=~temp; lcd_print(temp); temp=0;}
     if(action) lcd_print_char(action);
     if(num2) lcd_print(num2);
              lcd_print(number);
     if(resultFlag){
      lcd_cmd(0xc0);
      lcd_print_char('=');
      
           if(result>0) lcd_print(result);
      else if(result<0){lcd_print_char('-'); temp=result-1; temp=~temp; lcd_print(temp); temp=0;}
      else {lcd_print_char('0'); result=num1=num2=0;}
      resultFlag=0;
     }
   }
 }
}


void calculateNumber(volatile char key){ //////// NUMBER CALCULATION FUNCTION
  
  if((key=='+')||(key=='-')||(key=='*')||(key=='/')){
    if(result){num1=result; number=result=num2=0; action=key;}
    else {num1=number; number=0; action=key;}
    }
  else if(key=='='){num2=number; number=0; resultFlag=1; calculateResult();}
  else if(key=='c'){number=num1=num2=result=action=0;}
  else{
    if(number==0) number=key-48;
    else number=(number*10) + (key-48);
    }
    delay1(1500);                           // key debouncing delay
}



void calculateResult(){      //////////////////// RESULT CALCULATION FUNCTION
       if(action=='+') result=num1+num2;
  else if(action=='-') result=num1-num2;
  else if(action=='*') result=num1*num2;
  else if(action=='/') result=num1/num2;
}



void lcd_print(volatile long printNumber){////////////// NUMBER PRINT FUNCION
  volatile char i=0, j, num[20], revNum[20], length=0;

 while(printNumber){
   revNum[i++]=printNumber%10;     // the reverse number
   printNumber/=10;
   length++;
  }
  length--;                      // number length

 for(i=0, j=length; i<=length; i++, j--){  // the actual number
  num[i]=revNum[j] + 48;
 }

  for(i=0; i<=length; i++){    // print the number
    *outf=num[i];
    *outk=2;  delay1(1);
    *outk=3;  delay1(1);
    *outk=2;  delay1(1);
  }
}



void lcd_print_char(volatile char ptr){///////////// CHARACTER PRINT FUNCTION
  *outf=ptr;
  *outk=2;  delay1(1);
  *outk=3;  delay1(1);
  *outk=2;  delay1(1);
}



volatile char checkKey(){///////////////////////// KEYBOARD SCANNING FUNCTION
   volatile char row, col, key;
   for(row=0; row<4; row++){
    *outa= 1 << row;  
    if(*inc){ col=*inc; break;}
} 

  if(row==0){
    if(col==1) key='1';
    else if(col==2) key='2';
    else if(col==4) key='3';
    else if(col==8) key='+';
  }
  else if(row==1){
    if(col==1) key='4';
    else if(col==2) key='5';
    else if(col==4) key='6';
    else if(col==8) key='-';
  }
  else if(row==2){
    if(col==1) key='7';
    else if(col==2) key='8';
    else if(col==4) key='9';
    else if(col==8) key='*';
  }
  else if(row==3){
    if(col==1) key='c';
    else if(col==2) key='0';
    else if(col==4) key='=';
    else if(col==8) key='/';
  }
  else key=0;
  return key;
}



////////////////////////////////DISPLAY AND PORT FUNCTIONS/////////////////////////////

void init_lcd(){
  lcd_cmd(0x38);
  lcd_cmd(0x0c);
  lcd_cmd(0x01);
}

void lcd_cmd(volatile char cmd){
  *outf=cmd;   
  *outk=0;  delay1(1); 
  *outk=1;  delay1(1);    
  *outk=0;  delay1(1);
}

void init_port(){
  dira=0x21; *dira=0xff; outa=0x22;
  dirc=0x27; *dirc=0; inc=0x26;
  dirf=0x30; *dirf=0xff; outf=0x31;
  dirk=0x107; *dirk=0x03; outk=0x108;
}

void delay1(volatile int time){
  volatile long i;
  while(time--)
  for(i=0; i<100; i++);
}