SCUMM: Document resource usage count / expiry a little bit

engines/scumm/resource.cpp

@@ -762,12 +762,13 @@ byte *ScummEngine::getStringAddressVar(int i) {
 }
 
 void ResourceManager::increaseExpireCounter() {
-	if (!(++_expireCounter)) {
-		increaseResourceCounter();
+	++_expireCounter;
+	if (_expireCounter == 0) {	// overflow?
+		increaseResourceCounters();
 	}
 }
 
-void ResourceManager::increaseResourceCounter() {
+void ResourceManager::increaseResourceCounters() {
 	int i, j;
 	byte counter;
 
@@ -781,9 +782,9 @@ void ResourceManager::increaseResourceCounter() {
 	}
 }
 
-void ResourceManager::setResourceCounter(int type, int idx, byte flag) {
-	_types[type].flags[idx] &= ~RF_USAGE;
-	_types[type].flags[idx] |= flag;
+void ResourceManager::setResourceCounter(int type, int idx, byte counter) {
+	_types[type].flags[idx] &= RF_LOCK;	// Clear lower 7 bits, preserve the lock bit.
+	_types[type].flags[idx] |= counter;	// Update the usage counter
 }
 
 /* 2 bytes safety area to make "precaching" of bytes in the gdi drawer easier */
@@ -969,7 +970,7 @@ void ResourceManager::expireResources(uint32 size) {
 
 	if (_expireCounter != 0xFF) {
 		_expireCounter = 0xFF;
-		increaseResourceCounter();
+		increaseResourceCounters();
 	}
 
 	if (size + _allocatedSize < _maxHeapThreshold)
@@ -1000,7 +1001,7 @@ void ResourceManager::expireResources(uint32 size) {
 		nukeResource(best_type, best_res);
 	} while (size + _allocatedSize > _minHeapThreshold);
 
-	increaseResourceCounter();
+	increaseResourceCounters();
 
 	debugC(DEBUG_RESOURCE, "Expired resources, mem %d -> %d", oldAllocatedSize, _allocatedSize);
 }

engines/scumm/resource.h

@@ -106,9 +106,19 @@ public:
 		 * Array of size _num containing the sizes of each resource of this type.
 		 */
 		uint32 *_size;
+
 	protected:
 		/**
-		 * Array of size _num containing TODO of each resource of this type.
+		 * Array of size _num containing some information on each resource of
+		 * this type.
+		 * First off, the uppermost bit indicates whether the resources is
+		 * locked into memory.
+		 * Secondly, the lower 7 bits contain a counter. This counter measures
+		 * roughly how old it is; a resource starts out with a count of 1 and
+		 * can go as high as 127. When memory falls low resp. when the engine
+		 * decides that it should throw out some unused stuff, then it begins
+		 * by removing the resources with the highest counter (excluding locked
+		 * resources and resources that are known to be in use).
 		 */
 		byte *flags;
 
@@ -118,6 +128,7 @@ public:
 		 * whether the resource is modified.
 		 */
 		byte *_status;
+
 	public:
 		/**
 		 * Array of size _num containing for each resource of this type the
@@ -161,26 +172,42 @@ public:
 	void allocResTypeData(int id, uint32 tag, int num, ResTypeMode mode);
 	void freeResources();
 
-	byte *createResource(int type, int index, uint32 size);
-	void nukeResource(int type, int i);
+	byte *createResource(int type, int idx, uint32 size);
+	void nukeResource(int type, int idx);
 
-	bool isResourceLoaded(int type, int index) const;
+	bool isResourceLoaded(int type, int idx) const;
 
-	void lock(int type, int i);
-	void unlock(int type, int i);
-	bool isLocked(int type, int i) const;
+	void lock(int type, int idx);
+	void unlock(int type, int idx);
+	bool isLocked(int type, int idx) const;
 
-	void setModified(int type, int i);
-	bool isModified(int type, int i) const;
+	void setModified(int type, int idx);
+	bool isModified(int type, int idx) const;
 
+	/**
+	 * This method increments the _expireCounter, and if it overflows (which happens
+	 * after at most 256 calls), it calls increaseResourceCounter.
+	 * It is invoked in the engine's main loop ScummEngine::scummLoop().
+	 */
 	void increaseExpireCounter();
-	void setResourceCounter(int type, int index, byte flag);
-	void increaseResourceCounter();
+
+	/**
+	 * Update the specified resource's counter.
+	 */
+	void setResourceCounter(int type, int idx, byte counter);
+
+	/**
+	 * Increment the counter of all unlocked loaded resources.
+	 * The maximal count is 255.
+	 * This is called by increaseExpireCounter and expireResources,
+	 * but also by ScummEngine::startScene.
+	 */
+	void increaseResourceCounters();
 
 	void resourceStats();
 
 //protected:
-	bool validateResource(const char *str, int type, int index) const;
+	bool validateResource(const char *str, int type, int idx) const;
 protected:
 	void expireResources(uint32 size);
 };

engines/scumm/room.cpp

@@ -115,7 +115,7 @@ void ScummEngine::startScene(int room, Actor *a, int objectNr) {
 	VAR(VAR_ROOM) = room;
 	_fullRedraw = true;
 
-	_res->increaseResourceCounter();
+	_res->increaseResourceCounters();
 
 	_currentRoom = room;
 	VAR(VAR_ROOM) = room;