#define MAX_STACK_SIZE 100
#define I_AM_NO_A_VALID_RESULT 9999
typedef int StackElement;


struct Stack {
    StackElement arr[MAX_STACK_SIZE];
    int top;
};

void stack_init(struct Stack* stack) {
    if (!stack)
        exit(-1);
    stack->top = 0;
}

size_t stack_size(struct Stack* stack) {
    if (!stack)
        exit(-1);
    return stack->top;
}

StackElement stack_peek(struct Stack* stack) {
    if (!stack)
        exit(-1);
    if (stack_size(stack) != 0) {
        return stack->arr[stack->top - 1];
    }
}

void stack_push(struct Stack* stack, StackElement element) {
    if (!stack)
        exit(-1);
    if (stack->top < MAX_STACK_SIZE) {
        stack->arr[stack->top] = element;
        stack->top++;
    }
}

StackElement stack_pop(struct Stack* stack) {
    if (!stack)
        exit(-1);
    if (stack_size(stack) > 0) {
        stack->top--;
        return stack->arr[stack->top];
    }
}

//********* Тесты *******************************************
#define check_expr(expr, expected) {\
    int result = (expr); \
    if (result != (expected)) {\
        printf("Ожидал:\t '" #expr "'\t to == '%d', получил '%d' \t(функция %s в строке %d).\n", \
               (expected), result, __FUNCTION__, __LINE__);\
        error_count++;\
    }\
}