Enhance comparing arbitrary selections

1. Implement `is_enabled` to to grey out the "Compare selection" menu
entry in the context menu when it is not applicable.
2. Reset the stored selections when the user opens "Compare selection".
3. Make it easier to use:  After the user makes one selection and
chooses "Mark selection...", it enough to make a next selection and
choose "Compare selection" directly.  (I.e. it is not necessary anymore
to store both(!) selections using the "Mark..." command.)

Context.sublime-menu

@@ -2,7 +2,7 @@
 	{ "caption": "-" },
 	{ "caption": "Compare with...", "command": "sbs_compare" },
 	{ "caption": "Mark selection for comparison", "command": "sbs_mark_sel" },
-	{ "caption": "Compare selection", "command": "sbs_compare", "args": { "compare_selections": true } },
+	{ "caption": "Compare selections", "command": "sbs_compare", "args": { "compare_selections": true } },
 	{ "caption": "Previous modification", "command": "sbs_prev_diff" },
 	{ "caption": "Next modification", "command": "sbs_next_diff" },
 	{ "caption": "-" }

README.md

@@ -28,7 +28,7 @@ Usage Options
   - Right click somewhere in the active view and select "Compare with..."
   - Right click on a tab and select "Compare with active tab"
   - Highlight text, right click -> "Mark selection for comparison"
-   - Mark a second selection, then right click -> "Compare selections"
+    Mark a second selection, then right click -> "Compare selections"
   - Create two selections by holding CTRL, then "Compare selections"
   - From the command line: [see README_COMMANDS.md](README_COMMANDS.md)
   - Jump to next: `alt+n`, jump to previous: `alt+p`

sbs_compare.py

@@ -335,7 +335,18 @@ def compare_views( self, view1, view2 ):
 			sublime.message_dialog( intraDiff + str( len( highlightA ) ) + ' lines removed, ' + str( len( highlightB ) ) + ' lines added\n' + str( numDiffs ) + ' line differences total' )
 
 
-	def run( self, edit, with_active = False, group = -1, index = -1, compare_selections = False ):		
+	def is_enabled( self, with_active = False, group = -1, index = -1, compare_selections = False ):
+		if compare_selections:
+			if len(self.view.sel()) == 2:
+				return all(map(bool, self.view.sel()))
+			if all(map(bool, sbs_markedSelection)):
+				return True
+			if sbs_markedSelection[1] and len(self.view.sel()) == 1 and bool(self.view.sel()[0]):
+				return True
+			return False
+		return True
+
+	def run( self, edit, with_active = False, group = -1, index = -1, compare_selections = False ):
 		global sbs_markedSelection, sbs_files
 
 		active_view = self.view
@@ -518,18 +529,17 @@ def compare_from_views( view1, view2 ):
 			compare_from_views( view1, view2 )			
 			del sbs_files[:]
 		elif compare_selections == True:
-			selA = sbs_markedSelection[0]
-			selB = sbs_markedSelection[1]
-
 			sel = active_view.sel()
 
-			selNum = 0
-			for selection in sel:
-				selNum += 1
-
-			if selNum == 2:
+			if len(sel) == 2:
 				selA = active_view.substr( sel[0] )
 				selB = active_view.substr( sel[1] )
+			else:
+				selA = sbs_markedSelection[0]
+				selB = sbs_markedSelection[1]
+				if not selA:
+					selA, selB = selB, active_view.substr( sel[0] )
+				sbs_markedSelection = [ '', '' ]
 
 			syntax = active_view.settings().get( 'syntax' )
 			create_comparison( selA, selB, syntax, 'selection A', 'selection B' )