#include <iostream>
#include <string>

using std::string;
using std::cout;
using std::endl;

string makeString(int n);
int countX(const string& s);
int countXTail(const string& s, std::string::size_type n, int tot);
void lengthen(string& s);

int main()
{
  string s1 = makeString(10000);
  cout << "done" << endl;
  cout << countX(s1) << endl;

  auto name = std::string{"Jim"};
  lengthen(name);
  cout << name << endl;
}

/**
 * Creates a string containing n x's.
 */
string makeString(int n)
{
  string s;
  for (int i = 0; i < n; i++)
    {
      s += 'x'; // C++ strings are mutable; build char-by-char using append
    }
  return s;
}

/**
 * Counts the number of 'x' characters in s.
 */
int countX(const string& s)
{
  // note use of reference parameter const string& to avoid copie
  // required for call-by-value
  return countXTail(s, 0, 0); // call recursive helper function
}

int countXTail(const string& s, std::string::size_type n, int tot)
{
  // tail recursion not optimized to a loop by default; this code
  // will crash b/c it runs out of stack space for long strings
  // (g++ has option to turn on tail-recursion optimization)
  if (n == s.length())
    {
      return tot;
    }
  else if (s[n] == 'x')
    {
      return countXTail(s, n + 1, tot + 1);
    }
  else
    {
      return countXTail(s, n + 1, tot);
    }
}

void lengthen(string& s)
{
  // here we use the (non-const) reference parameter b/c the function
  // is supposed to change the string that is passed to it
  s += 'x';
}