SaveState: Keep an undo for each slot by default.

Core/Config.cpp

@@ -354,6 +354,7 @@ static ConfigSetting generalSettings[] = {
 	ConfigSetting("DumpAudio", &g_Config.bDumpAudio, false),
 	ConfigSetting("SaveLoadResetsAVdumping", &g_Config.bSaveLoadResetsAVdumping, false),
 	ConfigSetting("StateSlot", &g_Config.iCurrentStateSlot, 0, true, true),
+	ConfigSetting("EnableStateUndo", &g_Config.bEnableStateUndo, true, true, true),
 	ConfigSetting("RewindFlipFrequency", &g_Config.iRewindFlipFrequency, 0, true, true),
 
 	ConfigSetting("GridView1", &g_Config.bGridView1, true),

Core/Config.h

@@ -199,6 +199,7 @@ public:
 	int iMaxRecent;
 	int iCurrentStateSlot;
 	int iRewindFlipFrequency;
+	bool bEnableStateUndo;
 	bool bEnableAutoLoad;
 	bool bEnableCheats;
 	bool bReloadCheats;

Core/SaveState.cpp

@@ -411,11 +411,16 @@ namespace SaveState
 	{
 		std::string fn = GenerateSaveSlotFilename(gameFilename, slot, STATE_EXTENSION);
 		std::string shot = GenerateSaveSlotFilename(gameFilename, slot, SCREENSHOT_EXTENSION);
+		std::string fnUndo = GenerateSaveSlotFilename(gameFilename, slot, UNDO_STATE_EXTENSION);
+		std::string shotUndo = GenerateSaveSlotFilename(gameFilename, slot, UNDO_SCREENSHOT_EXTENSION);
 		if (!fn.empty()) {
 			auto renameCallback = [=](bool status, const std::string &message, void *data) {
 				if (status) {
-					if (File::Exists(fn)) {
+					if (File::Exists(fnUndo) && g_Config.bEnableStateUndo) {
-						File::Delete(fn);
+						File::Delete(fnUndo);
+					}
+					if (File::Exists(fn) && g_Config.bEnableStateUndo) {
+						File::Rename(fn, fnUndo);
 					}
 					File::Rename(fn + ".tmp", fn);
 				}
@@ -424,6 +429,9 @@ namespace SaveState
 				}
 			};
 			// Let's also create a screenshot.
+			if (File::Exists(shot) && g_Config.bEnableStateUndo) {
+				File::Rename(shot, shotUndo);
+			}
 			SaveScreenshot(shot, Callback(), 0);
 			Save(fn + ".tmp", renameCallback, cbUserData);
 		} else {
@@ -433,12 +441,43 @@ namespace SaveState
 		}
 	}
 
+	bool UndoSaveSlot(const std::string &gameFilename, int slot) {
+		std::string fn = GenerateSaveSlotFilename(gameFilename, slot, STATE_EXTENSION);
+		std::string shot = GenerateSaveSlotFilename(gameFilename, slot, SCREENSHOT_EXTENSION);
+		std::string fnUndo = GenerateSaveSlotFilename(gameFilename, slot, UNDO_STATE_EXTENSION);
+		std::string shotUndo = GenerateSaveSlotFilename(gameFilename, slot, UNDO_SCREENSHOT_EXTENSION);
+
+		// Do nothing if there's no undo.
+		if (File::Exists(fnUndo)) {
+			// Swap them so they can undo again to redo.  Mistakes happen.
+			if (File::Exists(shotUndo)) {
+				File::Rename(shot, shot + ".tmp");
+				File::Rename(shotUndo, shot);
+				File::Rename(shot + ".tmp", shotUndo);
+			}
+
+			File::Rename(fn, fn + ".tmp");
+			File::Rename(fnUndo, fn);
+			File::Rename(fn + ".tmp", fnUndo);
+
+			return true;
+		}
+
+		return false;
+	}
+
 	bool HasSaveInSlot(const std::string &gameFilename, int slot)
 	{
 		std::string fn = GenerateSaveSlotFilename(gameFilename, slot, STATE_EXTENSION);
 		return File::Exists(fn);
 	}
 
+	bool HasUndoSaveInSlot(const std::string &gameFilename, int slot)
+	{
+		std::string fn = GenerateSaveSlotFilename(gameFilename, slot, STATE_EXTENSION);
+		return File::Exists(fn + ".undo");
+	}
+
 	bool HasScreenshotInSlot(const std::string &gameFilename, int slot)
 	{
 		std::string fn = GenerateSaveSlotFilename(gameFilename, slot, SCREENSHOT_EXTENSION);

Core/SaveState.h

@@ -28,6 +28,8 @@ namespace SaveState
 	static const int NUM_SLOTS = 5;
 	static const char *STATE_EXTENSION = "ppst";
 	static const char *SCREENSHOT_EXTENSION = "jpg";
+	static const char *UNDO_STATE_EXTENSION = "undo.ppst";
+	static const char *UNDO_SCREENSHOT_EXTENSION = "undo.jpg";
 
 	void Init();
 	void Shutdown();
@@ -36,8 +38,10 @@ namespace SaveState
 	void NextSlot();
 	void SaveSlot(const std::string &gameFilename, int slot, Callback callback, void *cbUserData = 0);
 	void LoadSlot(const std::string &gameFilename, int slot, Callback callback, void *cbUserData = 0);
+	bool UndoSaveSlot(const std::string &gameFilename, int slot);
 	// Checks whether there's an existing save in the specified slot.
 	bool HasSaveInSlot(const std::string &gameFilename, int slot);
+	bool HasUndoSaveInSlot(const std::string &gameFilename, int slot);
 	bool HasScreenshotInSlot(const std::string &gameFilename, int slot);
 
 	int GetCurrentSlot();