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

/*typedef struct {

  void *data;
  struct StackNode *next;

} StackNode;*/

// TODO: grundlegende Stackfunktionen implementieren:
/* `push`: legt ein Element oben auf den Stack,
 * `pop`: entfernt das oberste Element,
 * `top`: liefert das oberste Element zurück,
 * `clearStack`: gibt den gesamten Speicher frei. */

// [A] -> [B] -> [C] -> NULL
// head -> stack.next

StackNode *createNode(void *data) {

  StackNode *node = malloc(sizeof(StackNode));

  if (node == NULL)
    return NULL;

  node->data = data;
  node->next = NULL;
  node->prev = NULL;

  return node;
}

// Pushes data as pointer onto the stack.
StackNode *push(StackNode *stack, void *data) {

  StackNode *newNode = createNode(data);

  if (newNode == NULL) {
    return stack;
  }

  newNode->next = stack;
  if (stack != NULL) {
    stack->prev = newNode;
  }

  return newNode;
}

// Deletes the top element of the stack (latest added element) and releases its
// memory. (Pointer to data has to be freed by caller.)
StackNode *pop(StackNode *stack) {

  if (stack == NULL)
    return NULL;

  StackNode *nextNode = stack->next;
  if (nextNode != NULL) {
    nextNode->prev = NULL;
  }

  free(stack);

  stack = NULL;

  return nextNode;
}

// Returns the data of the top element.
void *top(StackNode *stack) { return stack ? stack->data : NULL; }

// Clears stack and releases all memory.
void clearStack(StackNode **stack) {

  while (*stack != NULL) {
    StackNode *next = (*stack)->next;
    free(*stack);
    *stack = next;
  }
}