Task: 21 game
C: 



#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>
#include <stdlib.h>
#include <time.h>


#if __STDC_VERSION__ >= 199901L || _MSC_VER >= 1800
#include <stdbool.h>
#else
#define bool int
#define true  1
#define false 0
#endif

#define GOAL               21
#define NUMBER_OF_PLAYERS   2
#define MIN_MOVE            1
#define MAX_MOVE            3
#define BUFFER_SIZE       256

#define _(STRING) STRING



static char DESCRIPTION[] = 
    "21 Game                                                          \n"
    "                                                                 \n"
    "21 is a two player game, the game is played by choosing a number \n"
    "(1, 2, or 3) to be added to the running total. The game is won by\n"
    "the player whose chosen number causes the running total to reach \n"
    "exactly 21. The running total starts at zero.                    \n\n";

static int total;


void update(char* player, int move)
{
    printf("%8s:  %d = %d + %d\n\n", player, total + move, total, move);
    total += move;
    if (total == GOAL)
        printf(_("The winner is %s.\n\n"), player);
}


int ai()
{

#if GOAL < 32 && MIN_MOVE == 1 && MAX_MOVE == 3
    static const int precomputed[] = { 1, 1, 3, 2, 1, 1, 3, 2, 1, 1, 3, 2, 1, 1,
        3, 2, 1, 1, 3, 2, 1, 1, 3, 2, 1, 1, 3, 2, 1, 1, 3 };
    update(_("ai"), precomputed[total]);
#elif MIN_MOVE == 1 && MAX_MOVE == 3
    static const int precomputed[] = { 1, 1, 3, 2};
    update(_("ai"), precomputed[total % (MAX_MOVE + 1)]);
#else
    int i;
    int move = 1;
    for (i = MIN_MOVE; i <= MAX_MOVE; i++)
        if ((total + i - 1) % (MAX_MOVE + 1) == 0)
            move = i;
    for (i = MIN_MOVE; i <= MAX_MOVE; i++)
        if (total + i == GOAL)
            move = i;
    update(_("ai"), move);
#endif
}


void human(void)
{
    char buffer[BUFFER_SIZE];
    int move;
 
    while ( printf(_("enter your move to play (or enter 0 to exit game): ")),
            fgets(buffer, BUFFER_SIZE, stdin), 
            sscanf(buffer, "%d", &move) != 1 ||
            (move && (move < MIN_MOVE || move > MAX_MOVE || total+move > GOAL)))
        puts(_("\nYour answer is not a valid choice.\n"));
    putchar('\n');
    if (!move) exit(EXIT_SUCCESS);
    update(_("human"), move);
}


int main(int argc, char* argv[])
{
    srand(time(NULL));
    puts(_(DESCRIPTION));
    while (true)
    {
        puts(_("\n---- NEW GAME ----\n"));
        puts(_("\nThe running total is currently zero.\n"));
        total = 0;

        if (rand() % NUMBER_OF_PLAYERS)
        {
            puts(_("The first move is AI move.\n"));
            ai();
        }
        else
            puts(_("The first move is human move.\n"));

        while (total < GOAL)
        {
            human();
            ai();
        }
    }
}

C++: 

#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iomanip>
#include <limits>

using namespace std;
#define _(STR) STR


class Model
{
public:
    static const int GOAL = 21;
    static const int NUMBER_OF_PLAYERS = 2;
    static const int MIN_MOVE = 1;
    static const int MAX_MOVE = 3;
    int bestMove();
    bool update(const char* player, int move);
    bool isGameBegin();
    bool isGameOver();
protected:
    friend class View;
    View* view = nullptr;
    const char* player = nullptr;
    int oldTotal = 0;
    int newTotal = 0;
    int lastMove = 0;
};


class View
{
protected:
    Model* model;
public:
    View(Model* model);
    void init(const char* player);
    void update();
};


class Controller
{
protected:
    Model* model;
    View* view;
public:
    Controller(Model* model, View* view);
    int input();
    void clear();
    void run();
};


int Model::bestMove()
{
    
    

    int move = MIN_MOVE;
    for (int i = MIN_MOVE; i <= MAX_MOVE; i++)
        if ((newTotal + i - 1) % (MAX_MOVE + 1) == 0)
            move = i;
    for (int i = MIN_MOVE; i <= MAX_MOVE; i++)
        if (newTotal + i == GOAL)
            move = i;
    return move;
}

bool Model::update(const char* player, int move)
{
    if (move >= MIN_MOVE && move <= MAX_MOVE && newTotal + move <= GOAL)
    {
        this->player = player;
        oldTotal = newTotal;
        newTotal = oldTotal + move;
        lastMove = move;
        view->update();
        return true;
    }
    else
        return false;
}

bool Model::isGameBegin()
{
    return oldTotal == 0;
}

bool Model::isGameOver()
{
    return newTotal == GOAL;
}


View::View(Model* model)
{
    this->model = model;
    model->view = this;
}

void View::init(const char* player)
{
    if (model->newTotal == 0)
        cout << _("----NEW GAME----\n\n")
             << _("The running total is currently zero.\n")
             << _("The first move is ") << player << _(" move.\n\n");
}

void View::update()
{
    cout << setw(8) << model->player << ": " << model->newTotal << " = "
         << model->oldTotal << " + " << model->lastMove << endl << endl;
    if (model->isGameOver())
        cout << endl << _("The winner is ") << model->player << _(".\n\n\n");
}


Controller::Controller(Model* model, View* view)
{
    this->model = model;
    this->view = view;
}

void Controller::run()
{
    if (rand() % Model::NUMBER_OF_PLAYERS == 0)
    {
        view->init("AI");
        model->update("AI", model->bestMove());
    }
    else
        view->init("human");

    while (!model->isGameOver())
    {
        while (!model->update("human", input()))
            clear();
        model->update("AI", model->bestMove());
    }
}

int Controller::input()
{
    int value;
    cout << _("enter a valid number to play (or enter 0 to exit game): ");
    cin >> value;
    cout << endl;
    if (!cin.fail())
    {
        if (value == 0)
            exit(EXIT_SUCCESS);
        else
            return value;
    }
    else
        return model->MIN_MOVE - 1;
}

void Controller::clear()
{
    cout << _("Your answer is not a valid choice.") << endl;
    cin.clear();
    cin.ignore((streamsize)numeric_limits<int>::max, '\n');
}


int main(int argc, char* argv)
{
    srand(time(NULL));

    cout << _(
        "21 Game                                                          \n"
        "                                                                 \n"
        "21 is a two player game, the game is played by choosing a number \n"
        "(1, 2, or 3) to be added to the running total. The game is won by\n"
        "the player whose chosen number causes the running total to reach \n"
        "exactly 21. The running total starts at zero.                    \n\n");

    while (true)
    {
        Model* model = new Model();
        View* view = new View(model);
        Controller* controler = new  Controller(model, view);

        controler->run();

        delete controler;
        delete model;
        delete view;
    }
    return EXIT_SUCCESS; 
}