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engines/ags/engine/ac/dynobj/cc_static_array.h

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+/* ScummVM - Graphic Adventure Engine
+ *
+ * ScummVM is the legal property of its developers, whose names
+ * are too numerous to list here. Please refer to the COPYRIGHT
+ * file distributed with this source distribution.
+ *
+ * This program 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, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+//=============================================================================
+//
+// CCStaticArray manages access to an array of script objects,
+// where an element's size counted by script's bytecode may differ from the
+// real element size in the engine's memory.
+// The purpose of this is to remove size restriction from the engine's structs
+// exposed to scripts.
+//
+// FIXME: [ivan-mogilko] the above was meant to work, but in reality it doesn't
+// and won't, at least not without some extra workarounds.
+// The problem that I missed here is following:
+//   when the script compiler is told to get an Nth element of a global struct
+//   array, such as character[n], it calculates the memory address as
+//   array address + sizeof(Character) * n.
+//   If this address is used for the read/write operations, these ops can be
+//   intercepted by interpreter and remapped into the real fields
+//      (see IScriptObject::ReadN, WriteN interface)
+//   But if this address is used IN POINTER COMPARISON, then we cannot do
+//   anything. And if our real struct in the engine is stored on a different
+//   relative memory offset than one expected by compiler, then this pointer
+//   comparison will fail, e.g. script expression like
+//      if (player == character[n])
+//
+// NOTE: on the other hand, similar effect could be achieved by separating
+// object data into two or more structs, where "base" structs are stored in
+// the exposed arrays (part of API), while extending structs are stored
+// separately. This is more an issue of engine data design.
+//
+//=============================================================================
+
+#ifndef AGS_ENGINE_AC_DYNOBJ_STATIC_ARRAY_H
+#define AGS_ENGINE_AC_DYNOBJ_STATIC_ARRAY_H
+
+#include "ags/engine/ac/dynobj/cc_ags_dynamic_object.h"
+
+namespace AGS3 {
+
+
+struct CCStaticArray : public AGSCCStaticObject {
+public:
+	~CCStaticArray() override {}
+
+	void Create(IScriptObject *mgr, size_t elem_script_size, size_t elem_mem_size, size_t elem_count = SIZE_MAX /*unknown*/);
+
+	inline IScriptObject *GetObjectManager() const {
+		return _mgr;
+	}
+
+	// Legacy support for reading and writing object values by their relative offset
+	inline void *GetElementPtr(void *address, intptr_t legacy_offset) {
+		return static_cast<uint8_t *>(address) + (legacy_offset / _elemScriptSize) * _elemMemSize;
+	}
+
+	void	*GetFieldPtr(void *address, intptr_t offset) override;
+	void 	Read(void *address, intptr_t offset, uint8_t *dest, size_t size) override;
+	uint8_t ReadInt8(void *address, intptr_t offset) override;
+	int16_t ReadInt16(void *address, intptr_t offset) override;
+	int32_t ReadInt32(void *address, intptr_t offset) override;
+	float 	ReadFloat(void *address, intptr_t offset) override;
+	void 	Write(void *address, intptr_t offset, const uint8_t *src, size_t size) override;
+	void 	WriteInt8(void *address, intptr_t offset, uint8_t val) override;
+	void 	WriteInt16(void *address, intptr_t offset, int16_t val) override;
+	void 	WriteInt32(void *address, intptr_t offset, int32_t val) override;
+	void 	WriteFloat(void *address, intptr_t offset, float val) override;
+
+private:
+	IScriptObject	 *_mgr = nullptr;
+	size_t			 _elemScriptSize = 0u;
+	size_t			 _elemMemSize = 0u;
+	size_t			 _elemCount = 0u;
+};
+
+} // namespace AGS3
+
+#endif