#include <stdlib.h>
#include "stack.h"

typedef struct Stack_node {
    void *data;
    struct Stack_node *next;
} StackNode;

// Pushes data onto the stack.
// Returns the new top of the stack.
StackNode *push(StackNode *stack, void *data) {
    StackNode *node = malloc(sizeof(StackNode));
    if (!node) return stack;  // allocation failed → leave unchanged

    node->data = data;
    node->next = stack;
    return node; // new top element
}

// Deletes the top element and returns the new stack top.
// Caller must free the *data* itself.
StackNode *pop(StackNode *stack) {
    if (!stack) return NULL;

    StackNode *next = stack->next;
    free(stack);   // free only the node, NOT the data
    return next;
}

// Returns the data at the top of the stack.
void *top(StackNode *stack) {
    if (!stack) return NULL;
    return stack->data;
}

// Clears entire stack (but does NOT free the data).
void clearStack(StackNode *stack) {
    while (stack) {
        StackNode *next = stack->next;
        free(stack);
        stack = next;
    }
}