in cursor.sml's 'helpTillPrevChr' function, properly handle edge case where graphical chr is followed by multiple line breaks

fcore/app-update.sml

@@ -426,13 +426,13 @@ struct
       buildTextAndClear (app, buffer, low)
     end
 
-  fun helpDeleteToChr (app: app_type, buffer, cursorIdx, count, fMove, chr) =
+  fun helpDeleteToChr (app: app_type, buffer, cursorIdx, count, fMove, fInc, chr) =
     if count = 0 then
       buildTextAndClearAfterChr (app, buffer, cursorIdx)
     else
       let
         val buffer = LineGap.goToIdx (cursorIdx, buffer)
-        val otherIdx = fMove (buffer, cursorIdx, chr) + 1
+        val otherIdx = fInc (fMove (buffer, cursorIdx, chr), 1)
 
         val low = Int.min (cursorIdx, otherIdx)
         val high = Int.max (cursorIdx, otherIdx)
@@ -440,11 +440,11 @@ struct
 
         val buffer = LineGap.delete (low, length, buffer)
       in
-        helpDeleteToChr (app, buffer, low, count - 1, fMove, chr)
+        helpDeleteToChr (app, buffer, low, count - 1, fMove, fInc, chr)
       end
 
-  fun deleteToChr (app: app_type, count, fMove, chr) =
-    helpDeleteToChr (app, #buffer app, #cursorIdx app, count, fMove, chr)
+  fun deleteToChr (app: app_type, count, fMove, fInc, chr) =
+    helpDeleteToChr (app, #buffer app, #cursorIdx app, count, fMove, fInc, chr)
 
   (* command-parsing functions *)
   (** number of characters which are integers *)
@@ -573,17 +573,23 @@ struct
       (* have to continue parsing string *)
       case String.sub (str, strPos + 1) of
         #"t" => 
-          (* todo: delete till chr, forwards *)
+          (* delete till chr, forwards *)
           (case newCmd of
             CHAR_EVENT chr =>
-              deleteToChr (app, 1, Cursor.tillNextChr, chr)
+              deleteToChr (app, 1, Cursor.tillNextChr, op+, chr)
           | KEY_ESC =>
               clearMode app
           | RESIZE_EVENT (width, height) => 
               resizeText (app, width, height))
       | #"T" => 
-          (* todo: delete till chr, backwards *)
+          (* delete till chr, backwards *)
+          (case newCmd of
+            CHAR_EVENT chr =>
+              deleteToChr (app, 1, Cursor.tillPrevChr, op-, chr)
+          | KEY_ESC =>
               clearMode app
+          | RESIZE_EVENT (width, height) => 
+              resizeText (app, width, height))
       | #"d" => 
           (* todo: delete whole line *)
           clearMode app

fcore/cursor.sml

@@ -1384,13 +1384,18 @@ struct
           (* tl is empty; return 0 for lineGap start *) 
           0
 
-  fun helpTillPrevChr (strPos, str, absIdx, stl, ltl, origIdx, findChr, lastNonLine) =
+  fun helpTillPrevChr 
+    ( strPos, str, absIdx, stl, ltl
+    , origIdx, findChr
+    , lastNonLine, lastLine, lastValid
+    ) =
       if strPos < 0 then
         case (stl, ltl) of
           (shd :: stl, lhd :: ltl) => 
             helpTillPrevChr 
               ( String.size shd - 1, shd, absIdx, stl, ltl
-            , origIdx, findChr, lastNonLine
+              , origIdx, findChr
+              , lastNonLine, lastLine, lastValid
               )
         | (_, _) => 
             origIdx
@@ -1399,18 +1404,49 @@ struct
           val chr = String.sub (str, strPos)
         in
           if chr = findChr then
+            if lastLine = lastNonLine then
               lastNonLine
+            else if absIdx + 1 = lastLine then
+              lastValid + 1
+            else
+              Int.min (lastLine, lastNonLine)
           else
             let
+              val lastLine = 
+                if chr = #"\n" then
+                  absIdx
+                else
+                  lastLine
               val lastNonLine =
-                if chr = #"\n" orelse chr = #"\r" then
+                if chr = #"\n" then
                   lastNonLine
                 else
                   absIdx
+
+              (* There is a slightly tricky edge case 
+               * which is the reason the lastValid variable.
+               * Say we have a string "a\n\n\nbcd"
+               * and we type "Ta" with the cursor at the end.
+               * We want the cursor to go to the second line break
+               * because (graphical-chr -> \n) should not be selectable.
+               * However, with only lastLine and lastNonLine variables,
+               * we only have information about the most recent \n 
+               * and the most recent graphical-chr.
+               * This means we don't have information about the case 
+               * where a graphical-chr is followed by multiple '\n's.
+               * The lastValid variable keeps track of this information
+               * so we can use it to provide the expected behaviour.
+               * *)
+              val lastValid =
+                if lastLine = lastNonLine + 1 then
+                  lastLine + 1
+                else
+                  Int.min (lastLine, lastNonLine)
             in
               helpTillPrevChr
                 ( strPos - 1, str, absIdx - 1
-              , stl, ltl, origIdx, findChr, lastNonLine
+                , stl, ltl, origIdx, findChr
+                , lastNonLine, lastLine, lastValid
                 )
             end
         end
@@ -1419,13 +1455,17 @@ struct
     (* we want to start iterating from Prev char after strIdx *)
     if strIdx > 0 then
       helpTillPrevChr 
-        (strIdx - 1, shd, absIdx - 1, stl, ltl, absIdx, findChr, absIdx)
+        ( strIdx - 1, shd, absIdx - 1
+        , stl, ltl, absIdx, findChr
+        , absIdx, absIdx, absIdx
+        )
     else
       case (stl, ltl) of
         (stlhd :: stltl, ltlhd :: ltltl) =>
           helpTillPrevChr
             ( String.size stlhd - 1, stlhd, absIdx - 1
-            , stltl, ltltl, absIdx, findChr, absIdx
+            , stltl, ltltl, absIdx, findChr
+            , absIdx, absIdx, absIdx
             )
       | (_, _) => 
           (* tl is empty; return 0 for lineGap start *)