main.cpp

#include<string>
#include<iostream>

using namespace std;

const char SEPARATOR = ',';
const string UC_MODE = "uppercase";
const string LC_MODE = "lowercase";
const string PT_MODE = "puntuaction";
const char NULL_VALUE = 0;
string mode = UC_MODE;

bool isDigitOrSeparator(char c){
  int n = c - '0';

  return (n >= 0 && n <=9) || (n == -4);
}

string nextMode(){

  if(mode == UC_MODE){
    return LC_MODE;
  }

  if(mode == LC_MODE){
    return PT_MODE;
  }

  return UC_MODE;

}

char decode_uppercase(int num){
  int code = num % 27;
  
  if (code == 0){
    mode = nextMode();
    return NULL_VALUE;
  }

  char modifier = '@';

  return code + modifier;
}

char decode_lowercase(int num){
  int code = num % 27;

  if (code == 0){
    mode = nextMode();
    return NULL_VALUE;
  }

  char modifier = '`';

  return code + modifier;
}

char decode_punctuaction(int num){

  int code = num % 9;

  if (code == 0){
    mode = nextMode();
    return NULL_VALUE;
  }

  switch (code)
  {
  case 1:
    return '!';
  case 2:
    return '?';
  case 3:
    return ',';
  case 4:
    return '.';
  case 5:
    return ' ';
  case 6:
    return ';';
  case 7:
    return '"';
  case 8:
    return '\'';
  default:
    return NULL_VALUE;
  }
}

char decode(int num){

  if (mode == LC_MODE){
    return decode_lowercase(num);
  }

  if (mode == PT_MODE){
    return decode_punctuaction(num);
  }

  return decode_uppercase(num);

}

void read(string msg){

  string num;
  const char SEPARATOR = ',';

  msg.append({SEPARATOR});

  for (int i = 0; i < msg.length(); i++){

    char c = msg.at(i);

    if(!isDigitOrSeparator(c)){

      cout << endl <<  "The " << c << " character was found, which is not valid for a secret message";
      break;
    }

    if (c == SEPARATOR){
      int n = stoi(num);
      char decoded = decode(n);
      if(decoded != NULL_VALUE){
        cout << decoded;
      }
      num.clear();
    } else {
      num.append({c});
    }
  }

  cout << endl;
}



int main(int argc, char* argv[]){

  if (argc != 2){
    cout << "The secret message was not provided" << endl;
    return -1;
  }

  string msg = argv[1];

  read(msg);

  return 0;
}