#include <iostream>
using namespace std;

const int maxLenghtOfArray = 100; const int maxInputLenght = 1000;
bool inputValidation(char input[]) {
    if (!((input[0] >= '0' && input[0] <= '9')||input[0]=='-')) {
        return false;
    }
    for (int i = 0;input[i] != '\0';i++) {
        if ((input[i] >= '0' && input[i] <= '9') || input[i] == ' ' || input[i] == '+' || input[i] == '-' || input[i] == '*' || input[i] == '/' || input[i] == '!') {
            if ((input[i] >= '0' && input[i] <= '9') && ((input[i + 1] >= '0' && input[i + 1] <= '9') || input[i + 1] == ' ' || input[i + 1] == '!' || input[i + 1] == '\0')) {
                continue; //if the number after the symbol is correct
            }
            else if (input[i] == '!' && (input[i + 1] == ' ' || input[i + 1] == '\0')) {
                continue; //if the input after the factorial is valid
            }
            else if ((input[i] == '-')&& (input[i+1] >= '0' && input[i+1] <= '9')) {
                //if the typed number is negative
                int i_test = i+1;
                while ((input[i_test] >= '0' && input[i_test] <= '9')|| (input[i_test] == '!')) {
                    //is '!' correctly used
                    if (input[i_test] == '!') {
                        return false;
                    }
                    i_test++;
                }
                continue;
            }
            else if ((input[i] == '+' || input[i] == '-' || input[i] == '*' || input[i] == '/') && (input[i + 1] == ' ') && ((input[i + 2] >= '0' && input[i + 2] <= '9')||input[i+2]=='-') && (input[i + 1] != '\0')) {
                continue; //if the symbol after the operation is correct
            }
            else if (input[i] == ' ') {
                if (!(input[i - 1] == '+' || input[i - 1] == '-' || input[i - 1] == '*' || input[i - 1] == '/') && !(input[i + 1] == '+' || input[i + 1] == '-' || input[i + 1] == '*' || input[i + 1] == '/')) {
                    return false; //if after space the input is correct
                }
            }
            else {
                return false;
            }
        }
        else {
            return false;
        }
    }
    return true;
}
bool exit(char input[]) {
    //exit from the console
    if (input[0] == 'e' && input[1] == 'x' && input[2] == 'i' && input[3] == 't') {
        return true;
    }
    else {
        return false;
    }
}
int convertDigitsIntoNumber(int digits[], int lenght, bool negativeNumber) {
    int copyOfLenght = lenght - 1;
    int number = 0;
    for (int i = 0;i < lenght;copyOfLenght--, i++) {
        number += digits[i] * pow(10, copyOfLenght);
    }
    if (negativeNumber == true) {
        number *= -1;
        return number;
    }
    return number;
}
int factorialCalculation(int number) {
    int factorial = 1;
    for (int i = 1;i <= number;i++) {
        factorial *= i;
    }
    return factorial;
}
int* convertInputNumbersFromCharToInt(char input[], int& numbersTyped) {
    numbersTyped = 0;
    bool factorial = false;
    bool negativeNumber = false;
    int intigerOfInputCharArray = 0;               //input[intigerOfInputCharArray]
    int digits = 0;                                //how many digits a number has
    int digitsOfNumber[maxLenghtOfArray];          //saving the digits of a number when it's broken down form the char array
    int arrayOfNumbers[maxLenghtOfArray];          //save the actual numbers
    bool endOfInput = false;                       //when it reaches the end of the input
    while (input[intigerOfInputCharArray] != '\0') {
        digits = 0;
        factorial = false;
        if ((input[intigerOfInputCharArray] == '+' || input[intigerOfInputCharArray] == '-' || input[intigerOfInputCharArray] == '*' || input[intigerOfInputCharArray] == '/')) {
            if (input[intigerOfInputCharArray] == '-' && (input[intigerOfInputCharArray+1] >= '0' && input[intigerOfInputCharArray+1] <= '9')) {
                //is the number negative
                negativeNumber = true;
                intigerOfInputCharArray++;
            }
            else {
                //skip the operation symbols
                intigerOfInputCharArray += 2;
            }
        }
        else {
            while (input[intigerOfInputCharArray] != ' ') {
                //save the digits of the number until it reaches space
                if (input[intigerOfInputCharArray] == '\0') {
                    endOfInput = true;
                    break;
                }
                else if (input[intigerOfInputCharArray] == '!') {
                    factorial = true;
                    intigerOfInputCharArray++;
                }
                else {
                    digitsOfNumber[digits] = input[intigerOfInputCharArray] - '0';
                    intigerOfInputCharArray++; digits++;
                }
            }
            if (factorial == true) {
                //save the number after it's calculated as factorial
                arrayOfNumbers[numbersTyped] = factorialCalculation(convertDigitsIntoNumber(digitsOfNumber, digits,negativeNumber));
                numbersTyped++; intigerOfInputCharArray++;
            }
            else {
                //save the actual number in the array of numbers
                arrayOfNumbers[numbersTyped] = convertDigitsIntoNumber(digitsOfNumber, digits,negativeNumber);
                numbersTyped++; intigerOfInputCharArray++;
                negativeNumber = false;
            }
        }
        if (endOfInput == true) {
            break;
        }
    }
    return arrayOfNumbers;
}
char* operatorsFunction(char input[]) {
    char operators[maxLenghtOfArray];
    int intigerOfChar = 0;  //operations [intigerOfChar]
    for (int i = 0;input[i] != '\0';i++) {
        if ((input[i] == '+' || input[i] == '-' || input[i] == '*' || input[i] == '/')) {
            if (input[i] == '-' && (input[i + 1] >= '0' && input[i + 1] <= '9')) {
                //if the '-' is for negative number
                continue;
            }
            operators[intigerOfChar] = input[i];
            intigerOfChar++;
        }
    }
    operators[intigerOfChar] = '\0';
    return operators;
}
void calculationOfInput(int numbers[], char operators[], int& numbersTyped) {
    int answer = 0;
    bool error = false;
    if (numbersTyped == 1) {
        //if it's typed only one number
        cout<<"Answer: "<<numbers[0]<<endl;
    }
    else {
        for (int i = 0; i < numbersTyped - 1; i++) {
            switch (operators[i]){
            case '+':
                if (i == 0) {
                    answer = numbers[i] + numbers[i + 1];
                }
                else {
                    answer += numbers[i + 1];
                }
                break;
            case '-':
                if (i == 0) {
                    answer = numbers[i] - numbers[i + 1];
                }
                else {
                    answer -= numbers[i + 1];
                }
                break;
            case '*':
                if (i == 0) {
                    answer = numbers[i] * numbers[i + 1];
                }
                else {
                    answer *= numbers[i + 1];
                }
                break;
            case '/':
                if (i == 0) {
                    if (numbers[i + 1] == 0) {
                        error = true;
                        cout << "Error! Can't divide by zero!" << endl;
                    }
                    else {
                        answer = numbers[i] / numbers[i + 1];
                    }
                }
                else {
                    if (numbers[i + 1] == 0) {
                        error = true;
                        cout << "Error! Can't divide by zero!" << endl;
                    }
                    else {
                        answer /= numbers[i + 1];
                    }
                }
                break;
            }
        }
        if (error != true) {
            cout << "Answer: " << answer << endl;
        }
    }
}

int main() {
    char input[maxInputLenght];
    int numbers[maxLenghtOfArray];
    char operators[maxLenghtOfArray];
    bool stopTheCalculator = false;
    do {
        int numbersTyped = INT_MAX;
        do {
            cout << "Enter an expression to calculate or type \"exit\": " << endl;
            cin.getline(input, maxInputLenght);
            if (exit(input) == true) {
                stopTheCalculator = true;
                break;
            }
            if (inputValidation(input) == false) {
                cout << "This expression is invalid!" << endl;
            }
        } while (inputValidation(input) == false);
        if (stopTheCalculator != true) {
            for (int i = 0;i < numbersTyped;i++) {
                numbers[i] = convertInputNumbersFromCharToInt(input, numbersTyped)[i];
            }
            for (int i = 0;operatorsFunction(input)[i] != '\0';i++) {
                operators[i] = operatorsFunction(input)[i];
            }
            calculationOfInput(numbers, operators, numbersTyped);
        }
    } while (stopTheCalculator == false);
}