fix(SquirrelInputController): boundary check to prevent crash

- make sure candidatePreview is trimmed only when it is legal
- add clamped() util function to clamp out-of-bounds values
- prevent index out-of-bounds when candidatePreview is empty

Author: ecstasoy

sources/SquirrelInputController.swift

@@ -466,12 +466,18 @@ private extension SquirrelInputController {
                caretPos: candidatePreview.utf16.count - max(0, preedit.utf16.distance(from: caretPos, to: preedit.endIndex)))
         } else {
           if end < caretPos && start < caretPos {
-            candidatePreview = String(candidatePreview[..<candidatePreview.index(candidatePreview.endIndex, offsetBy: -max(0, preedit.distance(from: end, to: caretPos)))])
+            let trimCount = preedit.distance(from: end, to: caretPos).clamped(to: 0..<candidatePreview.count)
+            candidatePreview = String(candidatePreview[..<candidatePreview.index(candidatePreview.endIndex, offsetBy: -trimCount)])
           } else if end < preedit.endIndex && caretPos <= start {
-            candidatePreview = String(candidatePreview[..<candidatePreview.index(candidatePreview.endIndex, offsetBy: -max(0, preedit.distance(from: end, to: preedit.endIndex)))])
+            let trimCount = preedit.distance(from: end, to: preedit.endIndex).clamped(to: 0..<candidatePreview.count)
+            candidatePreview = String(candidatePreview[..<candidatePreview.index(candidatePreview.endIndex, offsetBy: -trimCount)])
           }
+          
+          let selStart = candidatePreview.isEmpty ? 0 : start.utf16Offset(in: candidatePreview)
+          let selLength = candidatePreview.utf16.count - selStart
+          
           show(preedit: candidatePreview,
-               selRange: NSRange(location: start.utf16Offset(in: candidatePreview), length: candidatePreview.utf16.distance(from: start, to: candidatePreview.endIndex)),
+               selRange: NSRange(location: selStart, length: selLength),
                caretPos: candidatePreview.utf16.count)
         }
       } else {
@@ -565,3 +571,11 @@ private extension SquirrelInputController {
     }
   }
 }
+
+// clamp value into half open interval
+extension BinaryInteger {
+    func clamped(to limits: Range<Self>) -> Self {
+      guard !limits.isEmpty else { return limits.lowerBound }
+        return min(max(self, limits.lowerBound), limits.upperBound - 1)
+    }
+}