in BinSearch.equalOrMore and BinSearch.equalOrLess, return ~1 if no item is found, for the sake of less ambiguity in usage. Functions that call these have also been adapted.

fcore/bin-search.sml

@@ -3,7 +3,7 @@ struct
   local
     fun reverseLinearSearch (findNum, idx, vec) =
       if idx < 0 then
-        idx
+        ~1
       else
         let
           val curVal = Vector.sub (vec, idx)
@@ -38,7 +38,7 @@ struct
   local
     fun forwardLinearSearch (findNum, idx, vec) =
       if idx = Vector.length vec then
-        idx
+        ~1
       else
         let
           val curVal = Vector.sub (vec, idx)

fcore/cursor.sml

@@ -29,7 +29,13 @@ struct
           val linePos = BinSearch.equalOrLess (strPos - 1, lhd)
           val lineIdx = Vector.sub (lhd, linePos)
         in
-          absIdx - strPos + lineIdx + 1
+          if linePos = ~1 then
+            (* no previous line in lhd *)
+            helpVi0 (absIdx - strPos, stl, ltl)
+          else
+            let val lineIdx = Vector.sub (lhd, linePos)
+            in absIdx - strPos + lineIdx + 1
+            end
         end
       else
         helpVi0 (absIdx - strPos, stl, ltl)
@@ -224,6 +230,7 @@ struct
            * because we know lnHd definitely contains
            * a lineIdx less or equal to strIdx *)
           let
+            (* todo: what if BinSearch doesn't find anything? *)
             val lnIdx = BinSearch.equalOrLess (strIdx, lnHd)
             val lnIdx = Vector.sub (lnHd, lnIdx)
           in

fcore/search-list.sml

@@ -262,7 +262,7 @@ struct
     else
       let
         val pos = BinSearch.equalOrMore (cursorIdx + 1, searchList)
-        val pos = if pos < Vector.length searchList then pos else 0
+        val pos = if pos = ~1 then 0 else pos
         val count = count - 1
       in
         loopNextMatch (pos, searchList, count)
@@ -286,7 +286,7 @@ struct
     else
       let
         val pos = BinSearch.equalOrLess (cursorIdx - 1, searchList)
-        val pos = if pos < 0 then Vector.length searchList - 1 else pos
+        val pos = if pos = ~1 then Vector.length searchList - 1 else pos
         val count = count - 1
       in
         loopPrevMatch (pos, searchList, count)

fcore/text-builder/normal-mode-text-builder.sml

@@ -29,7 +29,10 @@ struct
               Utils.getRelativeLineStartFromRightHead (startLine, curLine, lhd)
             (* get absolute idx of line *)
             val absIdx = curIdx + strPos
+
             val searchPos = BinSearch.equalOrMore (absIdx, searchList)
+            val searchPos =
+              if searchPos = ~1 then Vector.length searchList else searchPos
 
             val env = Utils.initEnv
               ( windowWidth

fcore/text-builder/text-builder-with-cursor.sml

@@ -60,7 +60,7 @@ struct
       let
         val searchPos = BinSearch.equalOrMore (pos + 1, #searchList env)
       in
-        if searchPos = Vector.length line then
+        if searchPos = ~1 then
           (* next line is not in this node *)
           let
             val absIdx = absIdx - pos

fcore/text-builder/text-builder-with-highlight.sml

@@ -75,7 +75,7 @@ struct
       let
         val searchPos = BinSearch.equalOrMore (pos + 1, #searchList env)
       in
-        if searchPos = Vector.length line then
+        if searchPos = ~1 then
           (* next line is not in this node *)
           let
             val absIdx = absIdx - pos