sys/util.h: Add IS_EMPTY and LIST_DROP_EMPTY macros

IS_EMPTY macro allows to check if defined name is empty, i.e.
does not contain replacement list.
LIST_DROP_EMPTY macro may be used to process __VA_ARGS__ type lists,
e.g. a,b,,c , and remove empty elements.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

include/sys/util.h

@@ -345,6 +345,81 @@ u8_t u8_to_dec(char *buf, u8_t buflen, u8_t value);
  */
 #define IF_ENABLED(_flag, _code) \
 	COND_CODE_1(_flag, _code, ())
+
+
+/**
+ * @brief Check if defined name does have replacement string.
+ *
+ * If defined macro has value this will return true, otherwise
+ * it will return false. It only works with defined macros, so additional
+ * test (if defined) may be needed in some cases.
+ *
+ * This macro may be used, with COND_CODE_* macros, while processing
+ * __VA_ARG__ to avoid processing empty arguments.
+ *
+ * Note that this macro is indented to check macro names that evaluate
+ * to replacement lists being empty or containing numbers or macro name
+ * like tokens.
+ *
+ * Example:
+ *
+ *	#define EMPTY
+ *	#define NON_EMPTY	1
+ *	#undef  UNDEFINED
+ *	IS_EMPTY(EMPTY)
+ *	IS_EMPTY(NON_EMPTY)
+ *	IS_EMPTY(UNDEFINED)
+ *	#if defined(EMPTY) && IS_EMPTY(EMPTY) == true
+ *	...
+ *	#endif
+ *
+ * In above examples, the invocations of IS_EMPTY(...) will return: true,
+ * false, true and conditional code will be included.
+ *
+ * @param a		Makro to check
+ */
+#define IS_EMPTY(a) Z_IS_EMPTY_(a, true, false,)
+#define Z_IS_EMPTY_(...) Z_IS_EMPTY__(__VA_ARGS__)
+#define Z_IS_EMPTY__(a, ...) Z_IS_EMPTY___(_ZZ##a##ZZ0, __VA_ARGS__)
+#define Z_IS_EMPTY___(...) Z_IS_EMPTY____(GET_ARGS_LESS_1(__VA_ARGS__))
+#define Z_IS_EMPTY____(...) GET_ARG2(__VA_ARGS__)
+
+/**
+ * @brief Remove empty arguments from list.
+ *
+ * Due to evaluation, __VA_ARGS__ and other preprocessor generated lists
+ * may contain empty elements, e.g.:
+ *
+ *	#define LIST ,a,b,,d,
+ *
+ * In above example the first, the element between b and d, and the last
+ * are empty.
+ * When processing such lists, by for-each type loops, all empty elements
+ * will be processed, and may require filtering out within a loop.
+ * To make that process easier, it is enough to invoke LIST_DROP_EMPTY
+ * which will remove all empty elements from list.
+ *
+ * Example:
+ *	LIST_DROP_EMPTY(list)
+ * will return:
+ *	a,b,d
+ * Notice that ',' are preceded by space.
+ *
+ * @param ... 		list to be processed
+ */
+#define LIST_DROP_EMPTY(...) \
+	Z_LIST_DROP_FIRST(FOR_EACH(Z_LIST_NO_EMPTIES, __VA_ARGS__))
+
+/* Adding ',' after each element would add empty element at the end of
+ * list, which is hard to remove, so instead precede each element with ',',
+ * this way first element is empty, and this one is easy to drop.
+ */
+#define Z_LIST_ADD_ELEM(e) EMPTY, e
+#define Z_LIST_DROP_FIRST(...) GET_ARGS_LESS_1(__VA_ARGS__)
+#define Z_LIST_NO_EMPTIES(e) \
+	COND_CODE_1(IS_EMPTY(e), (), (Z_LIST_ADD_ELEM(e)))
+
+
 /**
  * @brief Insert code depending on result of flag evaluation.
  *
@@ -376,6 +451,25 @@ u8_t u8_to_dec(char *buf, u8_t buflen, u8_t value);
  */
 #define __GET_ARG2_DEBRACKET(ignore_this, val, ...) __DEBRACKET val
 
+/**
+ * @brief Macro with empty replacement list
+ *
+ * This trivial definition is provided to use where macro is expected
+ * to evaluate to empty replacement string or when it is needed to
+ * cheat checkpatch.
+ *
+ * Examples
+ *
+ *	#define LIST_ITEM(n) , item##n
+ *
+ * would cause error with checkpatch, but:
+ *
+ *	#define LIST_TIEM(n) EMPTY, item##m
+ *
+ * would not.
+ */
+#define EMPTY
+
 /**
  * @brief Get first argument from variable list of arguments
  */

tests/unit/util/main.c

@@ -288,6 +288,37 @@ static void test_FOR_EACH_IDX_FIXED_ARG(void)
 	zassert_equal(a2, 3, "Unexpected value %d", a2);
 }
 
+static void test_IS_EMPTY(void)
+{
+	#define test_IS_EMPTY_REAL_EMPTY
+	#define test_IS_EMPTY_NOT_EMPTY XXX_DO_NOT_REPLACE_XXX
+	zassert_true(IS_EMPTY(test_IS_EMPTY_REAL_EMPTY),
+		     "Expected to be empty");
+	zassert_false(IS_EMPTY(test_IS_EMPTY_NOT_EMPTY),
+		      "Expected to be non-empty");
+}
+
+static void test_LIST_DROP_EMPTY(void)
+{
+	/*
+	 * The real definition should be:
+	 *  #define TEST_BROKEN_LIST ,Henry,,Dorsett,Case,
+	 * but checkpatch complains, so below equivalent is defined.
+	 */
+	#define TEST_BROKEN_LIST EMPTY, Henry, EMPTY, Dorsett, Case,
+	#define TEST_FIXED_LIST LIST_DROP_EMPTY(TEST_BROKEN_LIST)
+	#define TEST_MKSTR(a) #a,
+	static const char *const arr[] = {
+		FOR_EACH(TEST_MKSTR, TEST_FIXED_LIST)
+	};
+
+	zassert_equal(sizeof(arr) / sizeof(char *), 3,
+		      "Failed to cleanup list");
+	zassert_equal(strcmp(arr[0], "Henry"), 0, "Failed at 0");
+	zassert_equal(strcmp(arr[1], "Dorsett"), 0, "Failed at 1");
+	zassert_equal(strcmp(arr[2], "Case"), 0, "Failed at 0");
+}
+
 void test_main(void)
 {
 	ztest_test_suite(test_lib_sys_util_tests,
@@ -303,7 +334,9 @@ void test_main(void)
 			 ztest_unit_test(test_FOR_EACH),
 			 ztest_unit_test(test_FOR_EACH_FIXED_ARG),
 			 ztest_unit_test(test_FOR_EACH_IDX),
-			 ztest_unit_test(test_FOR_EACH_IDX_FIXED_ARG)
+			 ztest_unit_test(test_FOR_EACH_IDX_FIXED_ARG),
+			 ztest_unit_test(test_IS_EMPTY),
+			 ztest_unit_test(test_LIST_DROP_EMPTY)
 	);
 
 	ztest_run_test_suite(test_lib_sys_util_tests);