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

/**
 * Stack
 */
typedef struct Stack {
    int maxSize;
    void **values;
    int top;
} Stack;

Stack *stack_create(int maxSize) {
    Stack *stack = malloc(sizeof(Stack));

    stack->maxSize = maxSize;

    void **values = malloc(sizeof(void*) * maxSize);
    stack->values = values;

    stack->top = 0;

    return stack;
}

bool stack_is_empty(Stack *stack) {
    return stack->top == 0;
}

bool stack_is_full(Stack *stack) {
    return stack->top == stack->maxSize;
}

void *stack_push(Stack *stack, void *value) {
    if (stack_is_full(stack)) {
        return NULL;
    }

    stack->values[stack->top++] = value;
    return value;
}

void *stack_pop(Stack *stack) {
    if (stack_is_empty(stack)) {
        return NULL;
    }

    return stack->values[--stack->top];
}

int main(int argc, char** argv) {
    Stack *stack = stack_create(2);

    int *x1 = malloc(sizeof(int));
    *x1 = 10;
    stack_push(stack, x1);

    int *x2 = malloc(sizeof(int));
    *x2 = 20;
    stack_push(stack, x2);

    int *x3 = malloc(sizeof(int));
    *x3 = 30;

    bool itWorks = stack_push(stack, x3) == NULL
            && *(int*) stack_pop(stack) == 20
            && *(int*) stack_pop(stack) == 10
            &&  stack_pop(stack) == NULL;
    printf("%s", itWorks ? "IT WORKS! :)" : "IT DOESN'T WORK :(");

    return 0;
}