#include <iostream>
#include <string>

using namespace std;

bool GetContinue();
int GetShift();
string GetPlainText();

string Encrypt(string text, int shift);

void  PrintText(string text);

int main() {
    bool keepGoing{false};
    string plainText;
    string cryptText;
    int shift;

    keepGoing = GetContinue();
    while (keepGoing) {

         shift = GetShift();
         plainText = GetPlainText();

         cryptText = Encrypt(plainText, shift);
         PrintText(cryptText);

         keepGoing = GetContinue();
    }

    return 0;
}

string ToLower(string s) {
    size_t i;

    for( i = 0; i < s.size(); ++i ) {
       s[i] = tolower(s[i]);
    }

    return s;
}

bool GetContinue(){
    string answer;
 
    while (answer!= "yes" and answer != "no") {
         cout << "Do you wish to encrypt a message (yes/no)? ";
         getline(cin, answer);
         cout << endl;

         answer= ToLower(answer);
     } while (answer!= "yes" and answer != "no");

    return answer == "yes";
}

int FixShift(int value) {
    while (value < 0) {
       value += 26;
    }

    return value % 26;
}


int GetShift(){
    int value;

    cout << "Enter the shift amount => ";
    cin >> value;
    cout << endl;

    cin.ignore(100,'\n');

    return FixShift(value);
}

string GetPlainText(){
    string plainText;
    cout << "Enter the message to encrypt on one line: ";
    getline(cin, plainText);
    cout << endl;

    return plainText;
}

char RealShift(char letter, int shift) {
    int offset;

    offset = letter - 'a';
    offset += shift;

    offset = FixShift(offset);

    return static_cast<char>('a' + offset);
}

char DoShift(char letter, int shift) {
   bool isUpper{false};
   char shifted{letter};

   if (isalpha(letter) ) {
       isUpper = isupper(letter);
       letter = tolower(letter);

       shifted = RealShift(letter, shift);

       if (isUpper) {
          shifted = toupper(shifted);
       }
   }

   return shifted;
}

string Encrypt(string text, int shift ){

    size_t i;

    for(i =0; i < text.size(); ++i) {
       text[i] = DoShift(text[i], shift);
    }

    return text;
}

void  PrintText(string text){
    cout << text << endl;
}