From d400b17bee6120e2042bb71eeadf10e64983005d Mon Sep 17 00:00:00 2001
From: Peter Hutterer <peter.hutterer@who-t.net>
Date: Wed, 19 Feb 2025 13:44:00 +1000
Subject: [PATCH] util: add the macros required for magic vararg expansion

A set of macros that expand to different things depending on the
number of arguments passed into the macro. Can be used for anything
but in the test case we use it to differ between stringifying the single
argument or taking a custom string for that same argument.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1188>
---
 src/util-macros.h | 51 +++++++++++++++++++++++++++++++++++++++++++++++
 test/test-utils.c | 19 ++++++++++++++++++
 2 files changed, 70 insertions(+)

diff --git a/src/util-macros.h b/src/util-macros.h
index 2083bfc9..f420e426 100644
--- a/src/util-macros.h
+++ b/src/util-macros.h
@@ -77,3 +77,54 @@
 
 #define _unused_ __attribute__((unused))
 #define _fallthrough_ __attribute__((fallthrough))
+
+/* Returns the number of macro arguments, this expands
+ * _VARIABLE_MACRO_NARGS(a, b, c) to NTH_ARG(a, b, c, 15, 14, 13, .... 4, 3, 2, 1).
+ * _VARIABLE_MACRO_NTH_ARG always returns the 16th argument which in our case is 3.
+ *
+ * If we want more than 16 values _VARIABLE_MACRO_COUNTDOWN and
+ * _VARIABLE_MACRO_NTH_ARG both need to be updated.
+ */
+#define _VARIABLE_MACRO_NARGS(...)  _VARIABLE_MACRO_NARGS1(__VA_ARGS__, _VARIABLE_MACRO_COUNTDOWN)
+#define _VARIABLE_MACRO_NARGS1(...) _VARIABLE_MACRO_NTH_ARG(__VA_ARGS__)
+
+/* Add to this if we need more than 16 args */
+#define _VARIABLE_MACRO_COUNTDOWN \
+	15, 14, 13, 12, 11, 10, 9, 8, 7,  6,  5,  4,  3,  2, 1, 0
+
+/* Return the 16th argument passed in. See _VARIABLE_MACRO_NARGS above for usage.
+ * Note this is 1-indexed.
+ */
+#define _VARIABLE_MACRO_NTH_ARG( \
+	_1,  _2,  _3,  _4,  _5,  _6,  _7, _8, \
+	_9, _10, _11, _12, _13, _14, _15,\
+	 N, ...) N
+
+/* Defines a different expansion of macros depending on the
+ * number of arguments, e.g. it turns
+ * VARIABLE_MACRO(_ARG, a, b, c) into _ARG3(a, b, c)
+ *
+ * This can be used to have custom macros that expand to different things
+ * depending on the number of arguments. This example converts a
+ * single macro argument into value + stringify, two arguments into
+ * first and second argument.
+ *
+ *     #define _ARG1(_1)     _1, #_1,
+ *     #define _ARG2(_1, _2) _1, _2,
+ *
+ *     #define MYMACRO(...) _VARIABLE_MACRO(_ARG, __VA_ARGS__)
+ *
+ *     static void foo(int value, char *name) { printf("%d: %s\n", value, name); }
+ *
+ *     int main(void) {
+ *         foo(MYMACRO(0));           // prints "0: 0"
+ *         foo(MYMACRO(0, "zero"));   // prints "0: zero"
+ *         return 0;
+ *     }
+ *
+ * The first argument to VARIABLE_MACRO defines the prefix of the
+ * expander macros (here _ARG -> _ARG0, _ARG1, ...). These need to be defined
+ * in the caller for the number of arguments accepted
+ * (up to _VARIABLE_MACRO_COUNTDOWN args).
+ */
+#define _VARIABLE_MACRO(func, ...) CONCAT(func, _VARIABLE_MACRO_NARGS(__VA_ARGS__)) (__VA_ARGS__)
diff --git a/test/test-utils.c b/test/test-utils.c
index ecbe0bf4..ca806990 100644
--- a/test/test-utils.c
+++ b/test/test-utils.c
@@ -2225,6 +2225,24 @@ START_TEST(attribute_cleanup)
 }
 END_TEST
 
+START_TEST(macros_expand)
+{
+#define _A1(_1) _1, #_1
+#define _A2(_1, _2) _1, _2
+#define A(...) _VARIABLE_MACRO(_A, __VA_ARGS__)
+	char buf[64];
+	snprintf(buf, sizeof(buf), "%d:%s", A(0));
+	litest_assert_str_eq(buf, "0:0");
+	snprintf(buf, sizeof(buf), "%d:%s", A(100));
+	litest_assert_str_eq(buf, "100:100");
+	snprintf(buf, sizeof(buf), "%d:%s", A(100, "hundred"));
+	litest_assert_str_eq(buf, "100:hundred");
+#undef _A1
+#undef _A2
+#undef A
+}
+END_TEST
+
 int main(void)
 {
 	struct litest_runner *runner = litest_runner_new();
@@ -2297,6 +2315,7 @@ int main(void)
 
 	ADD_TEST(newtype_test);
 	ADD_TEST(attribute_cleanup);
+	ADD_TEST(macros_expand);
 
 	enum litest_runner_result result = litest_runner_run_tests(runner);
 	litest_runner_destroy(runner);