Add basic control of cursor position in text input mode

Using the arrow keys, the cursor can be moved left or right.

src/core.nim

@@ -12,10 +12,13 @@ type
 
   ModeInfo* = object
     case mode*: Mode
-    of MdNormal, MdSearch: discard
+    of MdNormal: discard
+    of MdSearch:
+      searchCursorPos*: int = 0
     of MdInputText:
       promptText*: string
       input*: string = ""
+      textCursorPos*: int = 0
       callbackText*: proc (input: string)
     of MdInputBool:
       promptBool*: string

src/draw.nim

@@ -168,13 +168,15 @@ proc drawFooter(index: int, lenEntries: int, lenSelected: int, hidden: bool,
     nb.print(offsetErrMsg, y, " " & errMsg, c8(clrRed), c8(clrBlack))
   nb.cursor = (TB_HIDE_CURSOR, TB_HIDE_CURSOR)
 
-proc drawInputFooter(prompt: string, query: string, nb: var Nimbox) =
+proc drawInputFooter(prompt: string, query: string, cursorPos: int,
+    nb: var Nimbox) =
   let
     y = nb.height() - 1
-    offset = prompt.len + 1
+    offset = prompt.wcswidth + 1
+    cursorPos = offset + query[0..cursorPos - 1].wcswidth
   nb.print(0, y, prompt, c8(clrYellow), c8(clrBlack))
   nb.print(offset, y, query, c8(clrYellow), c8(clrBlack))
-  nb.cursor = (offset + query.wcswidth, y)
+  nb.cursor = (cursorPos, y)
 
 proc errMsg(err: ErrorKind): string =
   case err
@@ -215,10 +217,12 @@ proc redraw*(s: State, nb: var Nimbox) =
                  s.showHidden, s.currentSearchQuery != "",
                  errMsg, nb)
     of MdSearch:
-      drawInputFooter("search:", s.currentSearchQuery, nb)
+      drawInputFooter("search:", s.currentSearchQuery,
+          s.modeInfo.searchCursorPos, nb)
     of MdInputText:
-      drawInputFooter(s.modeInfo.promptText, s.modeInfo.input, nb)
+      drawInputFooter(s.modeInfo.promptText, s.modeInfo.input,
+          s.modeInfo.textCursorPos, nb)
     of MdInputBool:
-      drawInputFooter(s.modeInfo.promptBool, "", nb)
+      drawInputFooter(s.modeInfo.promptBool, "", 0, nb)
 
   nb.present()

src/nimmm.nim

@@ -128,22 +128,37 @@ type
     PrComplete,
 
 proc processInputTextMode(event: nimbox.Event,
-    input: var string): ProcessInputTextModeResult =
+                          input: var string,
+                          cursorPos: var int): ProcessInputTextModeResult =
   ## common input processing for MdInputText and MdSearch
   case event.kind
   of EventType.Key:
     case event.sym
     of Symbol.Escape:
       return PrCanceled
     of Symbol.Backspace:
-      if input.len > 0:
-        input.setLen(input.high)
+      if cursorPos > 0:
+        let (_, runeLen) = lastRune(input, cursorPos - 1)
+        input = input[0..cursorPos - 1 - runeLen] & input.substr(cursorPos)
+        cursorPos -= runeLen
     of Symbol.Enter:
       return PrComplete
     of Symbol.Space:
-      input.add(" ")
+      let inserted = " "
+      input.insert(inserted, cursorPos)
+      cursorPos += inserted.len
     of Symbol.Character:
-      input.add($event.ch.Rune)
+      let inserted = $event.ch.Rune
+      input.insert(inserted, cursorPos)
+      cursorPos += inserted.len
+    of Symbol.Left:
+      if cursorPos > 0:
+        let (_, runeLen) = lastRune(input, cursorPos - 1)
+        cursorPos -= runeLen
+    of Symbol.Right:
+      if cursorPos < input.len:
+        let runeLen = runeLenAt(input, cursorPos)
+        cursorPos += runeLen
     else:
       discard
   of EventType.Mouse, EventType.Resize, EventType.None:
@@ -208,7 +223,8 @@ proc mainLoop(nb: var Nimbox, enable256Colors: bool) =
           discard
       # Input text mode: Ignore keymap
       of MdInputText:
-        case processInputTextMode(event, s.modeInfo.input)
+        case processInputTextMode(event, s.modeInfo.input,
+            s.modeInfo.textCursorPos)
         of PrCanceled:
           s.modeInfo = ModeInfo(mode: MdNormal)
         of PrComplete:
@@ -220,7 +236,8 @@ proc mainLoop(nb: var Nimbox, enable256Colors: bool) =
           discard
       # Incremental search mode: Ignore keymap
       of MdSearch:
-        case processInputTextMode(event, s.currentSearchQuery)
+        case processInputTextMode(event, s.currentSearchQuery,
+            s.modeInfo.searchCursorPos)
         of PrCanceled:
           s.resetTab()
         of PrComplete: