feat(stack): introduce a stack ds, internally this uses a sll

ds.c

@@ -367,3 +367,56 @@ void
 
   return NULL;
 }
+
+ds_stack_t
+*ds_stack_init(void)
+{
+  ds_stack_t *stack = malloc(sizeof(ds_stack_t));
+  stack->n = 0;
+
+  return stack;
+}
+
+int
+ds_stack_push(ds_stack_t *stack, void *data)
+{
+  if (stack->n == 0) {
+    stack->items = ds_sll_new_node(data);
+    stack->n++;
+    return 0;
+  }
+  int r = ds_sll_add(&stack->items, data, 0);
+  if (!r) {
+    stack->n++;
+  }
+  return r;
+}
+
+void
+*ds_stack_pop(ds_stack_t *stack)
+{
+  void *r = ds_sll_remove(&stack->items, 0);
+  if (r != NULL) {
+    stack->n--;
+  }
+  return r;
+}
+
+void
+*ds_stack_peek(ds_stack_t *stack, unsigned x)
+{
+  int i;
+  ds_sll_t *cur;
+
+  if (x > stack->n) {
+    return NULL;
+  }
+
+  for (cur = stack->items, i = 0; cur; cur = cur->next, i++) {
+    if (i == x) {
+      return cur->data;
+    }
+  }
+
+  return NULL;
+}