Initial stubs for extending the save/load version checking/upgrade code,

with support for tests on individually versioned features as well as
savegame versions.
This is mainly for improved handling of features added to patchpacks.
The current status-quo of picking a random number for the savegame
version when releasing a patch breaks loading of savegames from newer
trunk savegame versions, with or without the same patch applied, as
savegame upgrade code is not run, which can also lead to crashes on load.
Instead don't change the savegame version (but set the high bit to
keep trunk versions away), but instead include the versions of individual
features using a separate mechanism.

src/saveload/extended_ver_sl.h

@@ -0,0 +1,79 @@
+/* $Id$ */
+
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file extended_ver_sl.h Functions/types related to handling save/load extended version info. */
+
+#ifndef EXTENDED_VER_SL_H
+#define EXTENDED_VER_SL_H
+
+#include "../core/bitmath_func.hpp"
+
+#include <vector>
+
+/**
+ * List of extended features, each feature has its own (16 bit) version
+ */
+enum ExtendedSaveLoadFeatureIndex {
+	XSLFI_NULL                          = 0,      ///< Unused value, to indicate that no extended feature test is in use
+
+	XSLFI_SIZE,                                   ///< Total count of features, including null feature
+};
+
+extern uint16 _sl_xv_feature_versions[XSLFI_SIZE];
+
+/**
+ * Operator to use when combining traditional savegame number test with an extended feature version test
+ */
+enum ExtendedSaveLoadFeatureTestOperator {
+	XSLFTO_OR                           = 0,      ///< Test if traditional savegame version is in bounds OR extended feature is in version bounds
+	XSLFTO_AND                                    ///< Test if traditional savegame version is in bounds AND extended feature is in version bounds
+};
+
+/**
+ * Structure to describe an extended feature version test, and how it combines with a traditional savegame version test
+ */
+struct ExtendedSaveLoadFeatureTest {
+	private:
+	uint64 value;
+
+	public:
+	ExtendedSaveLoadFeatureTest()
+			: value(0) { }
+
+	ExtendedSaveLoadFeatureTest(ExtendedSaveLoadFeatureTestOperator op, ExtendedSaveLoadFeatureIndex feature, uint16 min_version = 1, uint16 max_version = 0xFFFF)
+	{
+		this->value = 0;
+		SB(this->value, 0, 16, feature);
+		SB(this->value, 16, 16, min_version);
+		SB(this->value, 32, 16, max_version);
+		SB(this->value, 48, 16, op);
+	}
+
+	bool IsFeaturePresent(uint16 savegame_version, uint16 savegame_version_from, uint16 savegame_version_to) const;
+};
+
+bool SlXvIsFeaturePresent(ExtendedSaveLoadFeatureIndex feature, uint16 min_version = 1, uint16 max_version = 0xFFFF);
+
+/**
+ * Returns true if @p feature is missing (i.e. has a version of 0)
+ */
+inline bool SlXvIsFeatureMissing(ExtendedSaveLoadFeatureIndex feature)
+{
+	return !SlXvIsFeaturePresent(feature);
+}
+
+void SlXvResetState();
+
+void SlXvSetCurrentState();
+
+void SlXvCheckSpecialSavegameVersions();
+
+bool SlXvIsChunkDiscardable(uint32 id);
+
+#endif /* EXTENDED_VER_SL_H */