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

ds.h

@@ -22,6 +22,11 @@ typedef struct {
 	void *val;
 } _ds_hmp_kv_t;
 
+typedef struct {
+  unsigned  n;
+  ds_sll_t *items;
+} ds_stack_t;
+
 #define ds_ll_foreach(t, ll) for (t *cur = ll; cur; cur = cur->next)
 
 /**
@@ -168,4 +173,37 @@ void *ds_hmp_get(ds_hmp_t *hmp, char *key);
  */
 void *ds_hmp_remove(ds_hmp_t *hmp, char *key);
 
+/**
+ * @brief initialize a new stack
+ *
+ * @return the stack
+ */
+ds_stack_t *ds_stack_init(void);
+
+/**
+ * @brief push new data onto the stack
+ *
+ * @param stack the stack
+ * @param data the data
+ * @return 0 on success
+ */
+int ds_stack_push(ds_stack_t *stack, void *data);
+
+/**
+ * @brief pop the data off the top of the stack
+ *
+ * @param stack the stack
+ * @return the data
+ */
+void *ds_stack_pop(ds_stack_t *stack);
+
+/**
+ * @brief peek forward x items
+ *
+ * @param stack the stack
+ * @param x how many items to look ahead
+ * @return the data or NULL if out of range
+ */
+void *ds_stack_peek(ds_stack_t *stack, unsigned x);
+
 #endif