TITANIC: Add logic to prevent removing locked/marked stars via skymap

If you want to remove locked stars you can do so using the D key,
but previously, you could also deselect a currently locked star
via the skymap and it would crash.

It previously crashed if:

1. You had 2 stars locked on and you tried to remove the 1st
2. Had 1 locked and 1 unlocked and you tried to unlock the 1st
3. Had 2 locked and 1 unlocked and you tried to unlock any of
the other two locked stars.

Refactoring would allow quicker comprehension of the logic of
this section of code.

Fixes #10126.

1 files changed, 19 insertions, 2 deletions

diff --git a/engines/titanic/star_control/star_crosshairs.cpp b/engines/titanic/star_control/star_crosshairs.cpp
index 7227a18d03..61d11bf950 100644
--- a/engines/titanic/star_control/star_crosshairs.cpp
+++ b/engines/titanic/star_control/star_crosshairs.cpp
@@ -39,9 +39,15 @@ void CStarCrosshairs::selectStar(int index, CVideoSurface *surface,
 			// All the stars selected so far have been matched. Only allow
 			// a selection addition if not all three stars have been found
 			if (!isSolved()) {
-				// Don't allow the most recent match to be re-selected
+				// Don't allow the most recent match or the one before
+                            // it to be re-selected (while they are locked/matched)
 				if (_positions[index] != _entries[_entryIndex]) {
-					surface->lock();
+					if (_entryIndex == 1) {//2 stars are matched
+                                          if (_positions[index] == _entries[_entryIndex-1]) {
+                                                 return;
+                                          }
+                                   }
+                                   surface->lock();
 
 					// Draw crosshairs around the selected star
 					CSurfaceArea surfaceArea(surface);
@@ -63,6 +69,7 @@ void CStarCrosshairs::selectStar(int index, CVideoSurface *surface,
 			// So we allow the user to reselect it to remove the selection, or shift
 			// the selection to some other star
 			if (_positions[index] == _entries[_entryIndex]) {
+                            // Player has selected the most recent star
 				// Remove the crosshairs for the previously selected star
 				surface->lock();
 				CSurfaceArea surfaceArea(surface);
@@ -76,6 +83,16 @@ void CStarCrosshairs::selectStar(int index, CVideoSurface *surface,
 				const CBaseStarEntry *starP = starField->getDataPtr(_positions[index]._index1);
 				markers->addStar(starP);
 			} else {
+                            // Player has selected some other star other than the most recent
+                            // Remove/Add it if it is not one of the other star(s) already matched
+
+                            // Check that it is not a previously star and don't remove it if it is
+                            for (int i=0;i<_entryIndex;i++) {
+                                   if (_positions[index] == _entries[i]) {
+                                          return;
+                                   }
+                            }
+                            
 				// Erase the prior selection and draw the new one
 				surface->lock();
 				CSurfaceArea surfaceArea(surface);