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

struct Node { 
	int element;
	struct Node *next;
};

struct List { 
	struct Node *head;
	int count;
};

// Initialize a new linked list:  set the count to zero and the head pointer to NULL.

void init( struct List *list ) {
	list->head = NULL;
	list->count = 0;
}

// Return the first element in the linked list.  If the list is empty, return 0.

int front( struct List *list ) {
	if ( list->count == 0 ) {
		return 0;
	}

	return list->head->element;
}

// Allocate memory for a new node storing the argument 'value' and setting its next
// pointer to the address of the current head of the linked list,
// then setting the head of the linked list to this new node and updating the count.

void push( struct List *list, int value ) {
	struct Node *nd = malloc( sizeof( struct Node ) );
	printf( "Allocating a new node at %p\n", nd );
	nd->element = value;
	nd->next = list->head;

	list->head = nd;
	++(list->count);  // The parenthesis are not necessary, but it's much clearer
}

// If the linked list is empty, do nothing.
// Otherwise, temporarily store a pointer to the current head, updating the
// head of the linked list, decrement the count, and then free the memory
// for the stored node.

void pop( struct List *list ) {
	if ( list->count == 0 ) {
		return;
	}

	struct Node *nd = list->head;
	list->head = list->head->next;
	--(list->count);  // The parenthesis are not necessary, but it's much clearer

	free( nd );
}

// Clear the linked list by repeatedly calling 'pop' until the list is empty.

void clear( struct List *list ) {
	while ( list->count != 0 ) {
		pop( list );
	}
}

int main() {
	struct List *ls = malloc( sizeof( struct List ) );
	printf( "Allocating a new linked list at %p\n", ls );
	init( ls );

	push( ls, 5 );
	push( ls, 7 );
	printf( "Current head:  %d\n", front( ls ) );
	printf( "Current count:  %d\n", ls->count );
	pop( ls );
	printf( "Current head:  %d\n", front( ls ) );
	printf( "Current count:  %d\n", ls->count );

	clear( ls );
	printf( "Current count:  %d\n", ls->count );
	free( ls );

	return 0;
}