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/* File: stack.c 
 * Desc: implementation of the Stack interface using opaque types. 
 * Cristina Cifuentes 
 * 13 Aug 1997 
 */ 
 
#include  
#include  
#include "stack.h" 
 
struct _Stack { 
    int count;			/* # elements */ 
    struct elem { 
	void        *x;		/* actual element */ 
	struct elem *link;	/* next element of the stack */ 
    } *head; 
}; 
 
 
Stack stackNew (void) 
{ Stack stk; 
 
	stk = (Stack)malloc(sizeof(struct _Stack)); 
	stk->count = 0; 
	stk->head = NULL; 
	return stk; 
} 
 
 
int stackEmpty (Stack stk) 
{ 
	assert (stk); 
	return (stk->count == 0); 
} 
 
 
void stackPush (Stack stk, void *x) 
{ struct elem *t; 
 
	assert (stk); 
	t = (struct elem *)malloc(sizeof(struct elem)); 
	t->x = x; 
	t->link = stk->head; 
	stk->head = t; 
	stk->count++; 
} 
 
 
void *stackPop (Stack stk) 
{ void *x; 
  struct elem *t; 
 
	assert (stk); 
	assert (stk->count > 0); 
	t = stk->head; 
	stk->head = t->link; 
	stk->count--; 
	x = t->x; 
	free (t); 
	t = NULL; 
	return x; 
} 
 
 
void stackFree (Stack *stk) 
{ struct elem *t, *u; 
 
	assert (stk && *stk); 
	for (t = (*stk)->head; t; t = u) 
	{ 
		u = t->link; 
		free (t); 
		t = NULL; 
	} 
	free (*stk); 
	*stk = NULL; 
}