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

//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. */

// Pushes data as pointer onto the stack.
StackNode *push(StackNode *stack, void *data)
{
    StackNode *newNode = (StackNode *)malloc(sizeof(StackNode));
    
    if(newNode == NULL)
    {
        fprintf(stderr,"Fehler! Der Speicher für den neuen StackNode konnte nicht allokiert werden!\n");
        return stack;
    }
    
    newNode->data = data;   //saves data in newNode
    newNode->next = stack;  //pushes the newNode on top of stack (newNode->next points to previous Top-Node)

    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)
    {
        fprintf(stderr,"Fehler! Stack darf zum poppen nicht leer sein!\n");
        return stack;
    }
    
    StackNode *old = stack;  //temporary pointer to old top of stack

    stack= stack->next;  //update stack pointer to point to next element

    free(old);  //free the memory of the old StackNode (old->data is not beeing freed)
    
    return stack;
}

// Returns the data of the top element.
void *top(StackNode *stack)
{
    if (stack == NULL)
    {
        fprintf(stderr,"Fehler! Kann von einem leeren Stack nicht das oberste Element lesen!\n");
        return NULL;
    }
    void *data = stack->data;

    return data;
}

// Clears stack and releases all memory.
void clearStack(StackNode *stack)
{
    StackNode *current = stack;
    StackNode *next;

    while (current != NULL)
    {
        next = current->next;

        if (current->data != NULL) 
        {
            free(current->data);
            current->data = NULL;
        }

        free(current);
        current = NULL;
        
        current = next;
    }
}