additional progress with delete

src/line_gap.sml

@@ -1609,17 +1609,61 @@ struct
                   )
               end
             else if prevIdx < start then
-              (* Equivalent in gap_buffer.sml is line 350. *)
-              (* We want to delete in the middle of lefStringsHd. *)
+              (* We want to delete in the middle of leftStringsHd. 
+               * We also have to delete in the middle of leftLinesHd in order to
+               * do this. *)
               let
                 val sub1Length = start - prevIdx
                 val sub1 = String.substring (leftStringsHd, 0, sub1Length)
                 val sub2Start = finish - prevIdx
-                val sub2 = String.substring (leftStringsHd, sub2Start,
-                String.size leftStringsHd - sub2Start)
+                val sub2 = String.substring
+                  ( leftStringsHd
+                  , sub2Start
+                  , String.size leftStringsHd - sub2Start
+                  )
+
+                val sub1Lines =
+                  let
+                    val midpoint = binSearch (String.size sub1 - 1, leftLinesHd)
+                  in
+                    if midpoint >= 0 then
+                      let
+                        val slice = VectorSlice.slice
+                          (leftLinesHd, 0, SOME (midpoint + 1))
+                      in
+                        VectorSlice.vector slice
+                      end
+                    else
+                      Vector.fromList []
+                  end
+
+                val sub2Lines =
+                  let
+                    val midpoint = forwardBinSearch (sub2Start, leftLinesHd)
+                  in
+                    if midpoint < Vector.length leftLinesHd then
+                      Vector.tabulate
+                        ( Vector.length leftLinesHd - midpoint
+                        , fn idx =>
+                            Vector.sub (leftLinesHd, idx + midpoint) - sub2Start
+                        )
+                    else
+                      Vector.fromList []
+                  end
               in
-              0
+                { idx = prevIdx + String.size sub1
+                , line =
+                    (curLine - Vector.length leftLinesHd)
+                    + Vector.length sub1Lines
+                , leftStrings = sub1 :: leftStringsTl
+                , leftLines = sub1Lines :: leftLinesTl
+                , rightStrings = sub2 :: rightStrings
+                , rightLines = sub2Lines :: rightLines
+                }
               end
+            else
+              (* Equivalent in gap_buffer.sml is line 365. *)
+              0
           else
             0
         end