SCI32: Fix loading autosaves (slot 0)

Fixes bugs in trac #11029:

- Slot 1 loading when user selects slot 0 in ScummVM UI
- Slot 1 loading when slot 0 specified on command line
- QFG4 slot 0 not appearing in game's original Restore UI

4 files changed, 27 insertions, 14 deletions

diff --git a/engines/sci/engine/file.cpp b/engines/sci/engine/file.cpp
index c311810997..8d93269604 100644
--- a/engines/sci/engine/file.cpp
+++ b/engines/sci/engine/file.cpp
@@ -349,9 +349,13 @@ void listSavegames(Common::Array<SavegameDesc> &saves) {
 		const Common::String &filename = *iter;
 
 #ifdef ENABLE_SCI32
-		const int id = strtol(filename.end() - 3, NULL, 10);
-		if (id == kNewGameId || id == kAutoSaveId) {
-			continue;
+		// exclude new game and autosave slots, except for QFG4,
+		//  whose autosave should appear as a normal saved game
+		if (g_sci->getGameId() != GID_QFG4) {
+			const int id = strtol(filename.end() - 3, NULL, 10);
+			if (id == kNewGameId || id == kAutoSaveId) {
+				continue;
+			}
 		}
 #endif
 
diff --git a/engines/sci/engine/file.h b/engines/sci/engine/file.h
index b94caa039f..aafa881245 100644
--- a/engines/sci/engine/file.h
+++ b/engines/sci/engine/file.h
@@ -47,8 +47,11 @@ enum {
 
 	// SCI engine expects game IDs to start at 0, but slot 0 in ScummVM is
 	// reserved for autosave, so non-autosave games get their IDs shifted up
-	// when saving or restoring, and shifted down when enumerating save games
-	kSaveIdShift = 1
+	// when saving or restoring, and shifted down when enumerating save games.
+	// ScummVM slot 0 can't be shifted down as -1 is an illegal SCI save ID
+	// so it is instead wrapped around to 99 and then back to 0 when shifting up.
+	kSaveIdShift = 1,
+	kMaxShiftedSaveId = 99
 };
 #endif
 
diff --git a/engines/sci/engine/guest_additions.cpp b/engines/sci/engine/guest_additions.cpp
index f07fb4ce5f..7f5a11cb63 100644
--- a/engines/sci/engine/guest_additions.cpp
+++ b/engines/sci/engine/guest_additions.cpp
@@ -698,13 +698,15 @@ int GuestAdditions::runSaveRestore(const bool isSave, reg_t outDescription, cons
 		description.fromString(descriptionString);
 	}
 
+	// The autosave slot in ScummVM takes up slot 0, but in SCI the first
+	// non-autosave save game number needs to be 0, so reduce the save
+	// number here to match what would come from the normal SCI save/restore
+	// dialog. Wrap slot 0 around to kMaxShiftedSaveId so that it remains
+	// a legal SCI value.
 	if (saveNo > 0) {
-		// The autosave slot in ScummVM takes up slot 0, but in SCI the first
-		// non-autosave save game number needs to be 0, so reduce the save
-		// number here to match what would come from the normal SCI save/restore
-		// dialog. There is additional special code for handling the autosave
-		// game inside of kRestoreGame32.
 		saveNo -= kSaveIdShift;
+	} else if (saveNo == 0) {
+		saveNo = kMaxShiftedSaveId;
 	}
 
 	return saveNo;
diff --git a/engines/sci/engine/kfile.cpp b/engines/sci/engine/kfile.cpp
index 5edac18bc0..e3d4fb2afd 100644
--- a/engines/sci/engine/kfile.cpp
+++ b/engines/sci/engine/kfile.cpp
@@ -1333,6 +1333,8 @@ reg_t kSaveGame32(EngineState *s, int argc, reg_t *argv) {
 			// Autosave slot 1 is a "new game" save
 			saveNo = kNewGameId;
 		}
+	} else if (saveNo == kMaxShiftedSaveId) {
+		saveNo = 0;
 	} else {
 		saveNo += kSaveIdShift;
 	}
@@ -1394,6 +1396,8 @@ reg_t kRestoreGame32(EngineState *s, int argc, reg_t *argv) {
 			// Autosave slot 1 is a "new game" save
 			saveNo = kNewGameId;
 		}
+	} else if (saveNo == kMaxShiftedSaveId) {
+		saveNo = 0;
 	} else {
 		saveNo += kSaveIdShift;
 	}
@@ -1419,13 +1423,12 @@ reg_t kCheckSaveGame32(EngineState *s, int argc, reg_t *argv) {
 	int16 saveNo = argv[1].toSint16();
 	const Common::String gameVersion = argv[2].isNull() ? "" : s->_segMan->getString(argv[2]);
 
-	Common::Array<SavegameDesc> saves;
-	listSavegames(saves);
-
 	if (gameName == "Autosave" || gameName == "Autosv") {
 		if (saveNo == 1) {
 			saveNo = kNewGameId;
 		}
+	} else if (saveNo == kMaxShiftedSaveId) {
+		saveNo = 0;
 	} else {
 		saveNo += kSaveIdShift;
 	}
@@ -1488,7 +1491,8 @@ reg_t kGetSaveFiles32(EngineState *s, int argc, reg_t *argv) {
 		// At least Phant2 requires use of strncpy, since it creates save game
 		// names of exactly kMaxSaveNameLength
 		strncpy(target, save.name, kMaxSaveNameLength);
-		saveIds.setFromInt16(i, save.id - kSaveIdShift);
+		int16 sciSaveId = (save.id == 0) ? kMaxShiftedSaveId : (save.id - kSaveIdShift);
+		saveIds.setFromInt16(i, sciSaveId);
 	}
 
 	descriptions.charAt(kMaxSaveNameLength * saves.size()) = '\0';