sys: dlist: Add sys_dlist_peek_prev_no_check and sys_dlist_peek_prev

Added function for peeking into previous item in the list.

Signed-off-by: Krzysztof Chruscinski <[email protected]>

Author: nordic-krch

include/misc/dlist.h

@@ -310,6 +310,41 @@ static inline sys_dnode_t *sys_dlist_peek_next(sys_dlist_t *list,
 	return node ? sys_dlist_peek_next_no_check(list, node) : NULL;
 }
 
+/**
+ * @brief get a reference to the previous item in the list, node is not NULL
+ *
+ * Faster than sys_dlist_peek_prev() if node is known not to be NULL.
+ *
+ * @param list the doubly-linked list to operate on
+ * @param node the node from which to get the previous element in the list
+ *
+ * @return a pointer to the previous element from a node, NULL if node is the
+ *	   tail
+ */
+
+static inline sys_dnode_t *sys_dlist_peek_prev_no_check(sys_dlist_t *list,
+							sys_dnode_t *node)
+{
+	return (node == list->head) ? NULL : node->prev;
+}
+
+/**
+ * @brief get a reference to the previous item in the list
+ *
+ * @param list the doubly-linked list to operate on
+ * @param node the node from which to get the previous element in the list
+ *
+ * @return a pointer to the previous element from a node, NULL if node is the
+ * 	   tail or NULL (when node comes from reading the head of an empty
+ * 	   list).
+ */
+
+static inline sys_dnode_t *sys_dlist_peek_prev(sys_dlist_t *list,
+					       sys_dnode_t *node)
+{
+	return node ? sys_dlist_peek_prev_no_check(list, node) : NULL;
+}
+
 /**
  * @brief get a reference to the tail item in the list
  *