Contract Programming

The unittest block can be implemented trivially by copying the checks that are already written in main(). The only addition below is the test for the case when the second team wins:

int addPoints(int goals1, int goals2,
              ref int points1, ref int points2)
in {
    assert(goals1 >= 0);
    assert(goals2 >= 0);
    assert(points1 >= 0);
    assert(points2 >= 0);

} out (result) {
    assert((result >= 0) && (result <= 2));

} do {
    int winner;

    if (goals1 > goals2) {
        points1 += 3;
        winner = 1;

    } else if (goals1 < goals2) {
        points2 += 3;
        winner = 2;

    } else {
        ++points1;
        ++points2;
        winner = 0;
    }

    return winner;
}

unittest {
    int points1 = 10;
    int points2 = 7;
    int winner;

    // First team wins
    winner = addPoints(3, 1, points1, points2);
    assert(points1 == 13);
    assert(points2 == 7);
    assert(winner == 1);

    // Draw
    winner = addPoints(2, 2, points1, points2);
    assert(points1 == 14);
    assert(points2 == 8);
    assert(winner == 0);

    // Second team wins
    winner = addPoints(0, 1, points1, points2);
    assert(points1 == 14);
    assert(points2 == 11);
    assert(winner == 2);
}

void main() {
    // ...
}

Expression-based contracts can be useful for this function:

int addPoints(int goals1, int goals2,
              ref int points1, ref int points2)
in (goals1 >= 0)
in (goals2 >= 0)
in (points1 >= 0)
in (points2 >= 0)
out (result; (result >= 0) && (result <= 2)) {
    // ...
}