readline: move charLengthLeft() and charLengthAt()

This moves the charLengthLeft() and charLengthAt() into the internal
readline file. This allows sharing the functions internally with
other code.

PR-URL: nodejs#31112
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/readline/utils.js

@@ -36,6 +36,29 @@ CSI.kClearToLineEnd = CSI`0K`;
 CSI.kClearLine = CSI`2K`;
 CSI.kClearScreenDown = CSI`0J`;
 
+// TODO(BridgeAR): Treat combined characters as single character, i.e,
+// 'a\u0301' and '\u0301a' (both have the same visual output).
+// Check Canonical_Combining_Class in
+// http://userguide.icu-project.org/strings/properties
+function charLengthLeft(str, i) {
+  if (i <= 0)
+    return 0;
+  if ((i > 1 && str.codePointAt(i - 2) >= kUTF16SurrogateThreshold) ||
+      str.codePointAt(i - 1) >= kUTF16SurrogateThreshold) {
+    return 2;
+  }
+  return 1;
+}
+
+function charLengthAt(str, i) {
+  if (str.length <= i) {
+    // Pretend to move to the right. This is necessary to autocomplete while
+    // moving to the right.
+    return 1;
+  }
+  return str.codePointAt(i) >= kUTF16SurrogateThreshold ? 2 : 1;
+}
+
 if (internalBinding('config').hasIntl) {
   const icu = internalBinding('icu');
   // icu.getStringWidth(string, ambiguousAsFullWidth, expandEmojiSequence)
@@ -452,6 +475,8 @@ function commonPrefix(strings) {
 }
 
 module.exports = {
+  charLengthAt,
+  charLengthLeft,
   commonPrefix,
   emitKeys,
   getStringWidth,

lib/readline.js

@@ -49,6 +49,8 @@ const { validateString } = require('internal/validators');
 const { inspect } = require('internal/util/inspect');
 const EventEmitter = require('events');
 const {
+  charLengthAt,
+  charLengthLeft,
   commonPrefix,
   CSI,
   emitKeys,
@@ -591,25 +593,6 @@ Interface.prototype._wordRight = function() {
   }
 };
 
-function charLengthLeft(str, i) {
-  if (i <= 0)
-    return 0;
-  if ((i > 1 && str.codePointAt(i - 2) >= kUTF16SurrogateThreshold) ||
-      str.codePointAt(i - 1) >= kUTF16SurrogateThreshold) {
-    return 2;
-  }
-  return 1;
-}
-
-function charLengthAt(str, i) {
-  if (str.length <= i) {
-    // Pretend to move to the right. This is necessary to autocomplete while
-    // moving to the right.
-    return 1;
-  }
-  return str.codePointAt(i) >= kUTF16SurrogateThreshold ? 2 : 1;
-}
-
 Interface.prototype._deleteLeft = function() {
   if (this.cursor > 0 && this.line.length > 0) {
     // The number of UTF-16 units comprising the character to the left