// Message Encryption program
// Kristophe F. Dizon
// Febraury 22, 2020
// Program takes user's input for desired "key" to be used for encrypting message from
//     an input file. Program will then return a file with containing the encrypted
//     message.


#include <iostream>
#include <fstream>

using namespace std;

// Encrypt a string of words
// Pre: encryptionKey contains a alphabetic char and stringToEncrypt contains a string.
// Post: Return an encrypted version of the original string passed in.
string StringEncryptorDecryptor(/*IN*/char encryptionKey, /*IN*/string stringToEncrypt);

// Prompt user for file name and return ifstream
// Pre: inData declared
// Post: inData contains valid ifstream for data file
void GetFileInfo(/* IN/OUT */string& fileName, /* IN/OUT */ifstream& inData);

// Creates a new file with a changed file extension
// Pre: fileName contains a valid string name of a given file
// Post: Returns a string that changes the extension of a file
string ChangeFileExtension(/* IN */string fileName);

// Prompt user for a valid character to be used for encyption key
// Pre: encryptionKey is a valid char variable
// Post: intializes encryptionKey with a valid char.
void GetCharacter(/* IN/OUT */char& encryptionKey);

int main()
{
    string secretMessage{}; // Place to store inData from file stream
    string decryptMessage{}; // Place to store user input for decrypting a file
    char encryptionKey{}; // Place to store user selected encryption key
    string fileName{}; // Holds user input of file name
    ifstream inData{}; // Place to hold a valid input file stream
    ofstream outData{}; // Place to hold a valid output file stream

    // Takes users input for intializing encryptionKey and loops while input is not alphabetic.
    GetCharacter(encryptionKey);

    // Loops until a valid file and inData contains valid input stream
    GetFileInfo(fileName, inData);

    // Open an output file with original the file name but with the extension changed
    outData.open(ChangeFileExtension(fileName));

    // Loop until fail state (end of file)
    while(!inData.eof()){
        // Take data from inData stream and store in secretMessage
        getline(inData,secretMessage);
        // Encrypt secretMessage and store into outData (output file)
        outData << StringEncryptorDecryptor(encryptionKey, secretMessage);
    }

    // Alerting user file has been ecrypted and a new file has been made
    cout << "Your file has now been encrypted." << endl;
    cout << "The Encrypted filename is: " << ChangeFileExtension(fileName) << endl;

    // Closing ifstream/ofstream
    outData.close();
    inData.close();

    // Asking users to decrypt file and storing choice into decryptMessage
    cout <<"\nWould you like to decrypt a file?(yes/no)" << endl;
    cin >> decryptMessage;
    // Handle cases for decryptMessage
    if(decryptMessage == "Yes" ||decryptMessage == "yes"||decryptMessage == "y" ){
        // Loops until a valid file and inData contains valid input stream
        GetFileInfo(fileName, inData);
        cout <<"Your file has been decrypted and is as follows : " << endl;
        // Loop until fail state (end of file)
        while(!inData.eof()){
            // Take data from inData stream and store in secretMessage
            getline(inData,secretMessage);
            // Decrypt secretMessage and print out to screen
            cout << StringEncryptorDecryptor(encryptionKey, secretMessage);
        }
    }
    // if decryptMessage is no, end program
    else{
        cout <<"Goodbye." << endl;
    }

  return 0;
}

string StringEncryptorDecryptor(/*IN*/char encryptionKey, /*IN*/string stringToEncrypt){
    string word = stringToEncrypt;
    for (int i{0};i < stringToEncrypt.size() ; i++){
        word[i] = stringToEncrypt[i] ^ encryptionKey;
    }
    return word;
};

void GetFileInfo(/* IN/OUT */string& fileName, /* IN/OUT */ifstream& inData){
    do {
        // Prompt user for file name and open file
        cout << "Enter file containing input values: ";

        cin >> fileName;
        inData.open(fileName.c_str());
        if (!inData){
            cout << "Invalid file!\n\n";
            }
    } while (!inData); // Loop until valid file name provided
};

string ChangeFileExtension(/* IN */string fileName){
    string changedExtension{};
    changedExtension = fileName.substr(0, fileName.find_last_of('.'))+"11.txt";

    return changedExtension;
};

void GetCharacter(/* IN/OUT */char& encryptionKey){
    do{
        cout << "Choose a single character to encrypt your file. " << endl;
        cin.get(encryptionKey);
        cin.ignore(INT_MAX,'\n');
        if(!isalpha(encryptionKey))
            cout << "Your input was not a character (alphabetic)."<<endl;
    }while(!isalpha(encryptionKey));
    encryptionKey = toupper(encryptionKey);
};