NIM GAME

[CuteComputerXx]

I began taking C++, most of the assignments that I had been given so far were easy for me except this one. This program is a NIM game, but the user plays against the computer and the computer always has to win. So far I understood that what ever the user inputs 4 must be subtracted from it for the computer to make the choice and in the end win. there are 13 stones. This is what I got so far. NOTE: I know how to validate the function. The problem is I don't know where to even start. A guide will be helpful. Thanks.

  #include <iostream>
  using namespace std;

  // function protypes
  int Player1Pick(int stones_in_pile, int last_taken_p2);
  int Player2Pick(int stones_in_pile);
  bool ValidPick(int num_of_stones);
  
 int main()
{
	int stones_left = 13, P1Taken, P2Taken;
	
	cout << "Welcome, you have been choosen to play a game with 13 stones,  otherwise known as the NIM game\n."
	     << " You'll be playing against I, the MIGHTY COMPUTER. The object of this game is to take 1,2 or 3\n"
	     << " stones from the said pile. The player that manages to remove the last stone wins this game.\n "
	     << " You're going to need all the luck from the world to win. Because I'll never lose!!!>:)";
	
   do{
       logic to play the game goes here

	
	
	
	
      }while (stones_left>0);
	
	return 0;
}
///////////////////////////////////////////////////////////////////
int Player1Pick(int stones_in_pile, int last_taken_p2)
{
      // the computer picks a stone here and returns to that number	
	
}
///////////////////////////////////////////////////////////////////
int Player2Pick(int stones_in_pile)
{
     //prompt user to pick his stones, call valid pick function, force valid input from the user and return the number of stones the player picked	
	
	
}
/////////////////////////////////////////////////////////////////////
bool ValidPick(int num_of_stones)
{
	// validate that the user inputed 1, 2 , or 3
	// return true if ok, false if not
}

[d1ff1cul1010]

So do you understand the logic behind how the computer would win every time with perfect play? basically whoever picks the 5th stone can win the game (edit: well actually the 1st because then you control who can get to 5 and subsequently 9 and then 13 for the win.) not sure if that was already explained or if you already figured that part out.
an example
make it so:
1. when stonesleft is 13, the computer plays 1. (12 left then)
2. user plays 1, 2 or 3 (stonesleft is between 11 and 9)
3. computer plays stonesleft - 8 (stonesleft is now 8)
4. user plays 1,2 or 3 (stonesleft is now between 7 and 5)
5. computer plays stonesleft - 4 (stonesleft is now 4)
6. user plays 1,2 or 3 again (stonesleft is now between 3 and 1)
7. computer wins with a final stonesleft - 0 aka just stones left

didn't know how much help you wanted with the coding itself, if you wanted to make it more general you could use do a program with 1 + (4 mod (last pick)) and loop that. let us know what you're thinking

[CuteComputerXx]

I might have it figured it out, though the problem I'm having is with the validation the last part. Can I use if statements or do I have to use something like bool check to make it work.