util: add a tristate helper

Yet another crazy macro that allows to define a logical tristate.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

src/util-tristate.h

@@ -0,0 +1,131 @@
+/*
+ * Copyright © 2020 Red Hat, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+/**
+ * A type-safe tristate implementation. A tristate value has three options,
+ * usually a logical 'on' and 'off' plus the 'unset' value.
+ *
+ * Usage:
+ *
+ *   DEFINE_TRISTATE(yes, no, unset);
+ *   DEFINE_TRISTATE(on, off, neither);
+ *
+ *   tristate t = tristate_unset;
+ *   if (something)
+ *	t = tristate_yes;
+ *   else if (something_else)
+ *      t = tristate_no;
+ *
+ *  if (tristate_is_yes(t))
+ *      printf("yep");
+ *
+ *  switch (tristate_value(t)) {
+ *  case tristate_val_yes:
+ *  case tristate_val_no:
+ *  case tristate_val_unset:
+ *  }
+ *
+ *  Basic type safety is provided - mixing tristates types causes an
+ *  abort(). For example:
+ *
+ *  DEFINE_TRISTATE(yes, no, unset);
+ *  DEFINE_TRISTATE(on, off, neither);
+ *
+ *  tristate t1 = tristate_unset;
+ *  tristate t2 = tristate_off;
+ *  tristate t3 = tristate_neither;
+ *
+ *  t2 and t3 have the same "type". t1 is a different "type".
+ *
+ *  tristate_is_neither(t1) // this will abort
+ *  tristate_is_yes(t2) // this will abort
+ */
+#pragma once
+
+#include <stdbool.h>
+
+typedef struct {
+	unsigned val;
+} tristate;
+
+/* Implementation detail:
+ * Tristate value is type_mask | val
+ *   where val are the 2 LSB with
+ *      11 ... logical true state
+ *      10 ... logical false state
+ *      00 ... unset state
+ * All other bits are the type mask. This type mask is used to check that
+ * two different tristate definitions cannot be intermixed.
+ */
+static const unsigned _TRISTATE_TYPE_MASK = ~0x3;
+
+/* implementation detail, ignore */
+static inline void _tristate_check_type(const tristate *t1, unsigned type) {
+	assert((t1->val & _TRISTATE_TYPE_MASK) == type || !"Invalid tristate type comparison");
+}
+
+/**
+ * For the three given arguments on, off and none, define:
+ * - tristate_on, tristate_off and tristate_none as constant values to
+ *   assign. For example: tristate t = tristate_on;
+ * - tristate_is_on(), tristate_is_off(), tristate_is_none() as functions to check
+ *   a tristate. This function will abort if different tristate types are
+ *   mixed. For example:
+ *      tristate t = tristate_on;
+ *      if (tristate_is_none(t)) { .... }
+ * - tristate_onoff_value() to retrieve the value from a tristate to be used
+ *   in e.g. a switch statement. The values are tristate_val_on,
+ *   tristate_val_off, trisate_val_none. For example:
+ *   switch(tristate_onoff_value(t)) {
+ *      case tristate_val_on: break;
+ *      case tristate_val_off: break;
+ *      case tristate_val_none: break;
+ *    }
+ */
+#define DEFINE_TRISTATE(_on, _off, _none)						\
+	static const unsigned _TRISTATE_TYPE_##_on##_off = (__LINE__ << 2);		\
+	static const unsigned tristate_val_##_on   = _TRISTATE_TYPE_##_on##_off | 3;	\
+	static const unsigned tristate_val_##_off  = _TRISTATE_TYPE_##_on##_off | 2;	\
+	static const unsigned tristate_val_##_none = _TRISTATE_TYPE_##_on##_off | 0;	\
+	static const tristate tristate_##_on =   { .val = tristate_val_##_on };		\
+	static const tristate tristate_##_off =  { .val = tristate_val_##_off };	\
+	static const tristate tristate_##_none = { .val = tristate_val_##_none };	\
+	static inline bool tristate_is_##_on(tristate t) {				\
+		_tristate_check_type(&t, _TRISTATE_TYPE_##_on##_off);			\
+		return t.val == tristate_##_on.val;					\
+	}										\
+	static inline bool tristate_is_##_off(tristate t) {				\
+		_tristate_check_type(&t, _TRISTATE_TYPE_##_on##_off);			\
+		return t.val == tristate_##_off.val;					\
+	}										\
+	static inline bool tristate_is_##_none(tristate t) {				\
+		_tristate_check_type(&t, _TRISTATE_TYPE_##_on##_off);			\
+		return t.val == tristate_##_none.val;					\
+	}										\
+	static inline signed char tristate_##_on##_off_value(tristate t) {		\
+		_tristate_check_type(&t, _TRISTATE_TYPE_##_on##_off);			\
+		return t.val;								\
+	}										\
+	struct __useless_struct_to_allow_trailing_semicolon__
+
+