package com.company;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Calculator {
    private String _baseNumber;

    public Calculator() {
        this._baseNumber = "0";
    }

    public Calculator(String _baseNumber) {
        this._baseNumber = _baseNumber;
    }

    public Calculator(int _baseNumber) {
        this._baseNumber = Integer.toString(_baseNumber);
    }

    public String add(String numToAdd) {
        if(_baseNumber.length()==0 || numToAdd.length() == 0)
            return "";

        int overflow = 0;
        List<String> finalResult = new ArrayList<String>();

        int num1CurrentIndex = _baseNumber.length() - 1;
        int num2CurrentIndex = numToAdd.length() - 1;

        while(true){

            int num1IndexInt;
            if(num1CurrentIndex >= 0)
                num1IndexInt = Integer.parseInt(Character.toString(_baseNumber.charAt(num1CurrentIndex)));
            else
                num1IndexInt = 0;

            num1CurrentIndex--;

            int num2IndexInt;
            if(num2CurrentIndex >= 0)
                num2IndexInt = Integer.parseInt(Character.toString(numToAdd.charAt(num2CurrentIndex)));
            else
                num2IndexInt = 0;

            num2CurrentIndex--;



            int result  = num1IndexInt + num2IndexInt + overflow;
            if(result > 9){
                overflow = 1;
                result -= 10;
            }
            else
                overflow = 0;

            finalResult.add(Integer.toString(result));



            if((num1CurrentIndex < 0) && (num2CurrentIndex < 0)){
                finalResult.add(Integer.toString(overflow));
                break;
            }
        }

        finalResult.remove(finalResult.size() - 1);
        Collections.reverse(finalResult);
        return String.join("", finalResult);
    }

    public String multiply(String numToMultiply) {

        if(_baseNumber.length()==0 || numToMultiply.length() == 0)
            return "";
        else if(_baseNumber.equals("0") || numToMultiply.equals("0"))
            return "0";

        _baseNumber = new StringBuffer(_baseNumber).reverse().toString();
        numToMultiply = new StringBuffer(numToMultiply).reverse().toString();

        int[] partialResults = new int[_baseNumber.length() + numToMultiply.length()];


        for (int i = 0; i < _baseNumber.length(); i++)
        {
            for (int j = 0; j < numToMultiply.length(); j++)
                partialResults[i + j] = partialResults[i + j] + (_baseNumber.charAt(i) - '0') * (numToMultiply.charAt(j) - '0');
        }

        StringBuilder finalResult = new StringBuilder();
        for (int i = 0; i < partialResults.length; i++)
        {
            int digit = partialResults[i] % 10;
            int overflow = partialResults[i] / 10;

            if(i < partialResults.length -1)
            {
                partialResults[i + 1] = partialResults[i + 1] + overflow;
            }
            finalResult.insert(0, digit);

        }

        if(finalResult.charAt(0) == '0')
            finalResult = new StringBuilder(finalResult.substring(1));

        return finalResult.toString();
    }

    public int Factorial(int n){
        if (n < 0)
            throw new IllegalArgumentException("n must be larger or equal to 0");

        int result = 1;
        for (int i = 1; i <= n ; i++)
            result *= i;

        return result;
    }
}