添加函数注释

src/list.c

@@ -189,6 +189,21 @@ static void list_print(struct _list* self)
     }
 }
 
+/**
+ * @brief list slice
+ *  if index < 0, from the end of list. for example:
+ *  list[-1] is the last element in list.
+ * 
+ * @param self 
+ * @param start start index
+ * @param end end index 
+ * @param step step, if step < 0, return a reverse list.
+ * @return struct _list* 
+ *  a copy of the list, from start to end, step by step.
+ *      if step < 0, return a reverse list.
+ *      if step > 0, return a forward list.
+ *      if step == 0, return NULL.
+ */
 struct _list* list_slice(struct _list *self, int start, int end, int step)
 {
     assert(self != NULL);

test/test_list.c

@@ -440,14 +440,20 @@ static void test_list_slice(void)
     }
 
     // python: list[0:] -> []
-    list2 = list->slice(list, 0, 0, 1); // if start == end
-    TEST_ASSERT_NULL(list2);
-
     list2 = list->slice(list, 1, 5, 0); // if step == 0
     TEST_ASSERT_NULL(list2);
 
+    list2 = list->slice(list, 0, 0, 1); // if start == end
+    TEST_ASSERT_NOT_NULL(list2);
+    TEST_ASSERT_TRUE(list2->empty(list2));
+
     list2 = list->slice(list, 1, 5, -1); // if start < end && step < 0
     TEST_ASSERT_NULL(list2);
+    TEST_ASSERT_TRUE(list2->empty(list2));
+
+    list2 = list->slice(list, 5, 1, 1); // if start > end && step > 0
+    TEST_ASSERT_NULL(list2);
+    TEST_ASSERT_TRUE(list2->empty(list2));
 
     // python: list[0:]
     list2 = list->slice(list, 0, len, 1);