lib: add ASCII fast path to getStringWidth()

A lot of strings that are going to be passed to `getStringWidth()`
are ASCII strings, for which the calculation is rather easy and
calling into C++ can be skipped.

                                                       confidence improvement accuracy (*)    (**)   (***)
     misc/getstringwidth.js n=100000 type='ascii'            ***    328.99 %      ±21.73% ±29.25% ±38.77%
     misc/getstringwidth.js n=100000 type='emojiseq'                  2.94 %       ±7.66% ±10.19% ±13.26%
     misc/getstringwidth.js n=100000 type='fullwidth'                 4.70 %       ±5.64%  ±7.50%  ±9.76%

PR-URL: https://github.com/nodejs/node/pull/29301
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

1 files changed, 26 insertions, 7 deletions

diff --git a/lib/internal/readline/utils.js b/lib/internal/readline/utils.js
index c6cd13a6bd..f72a03bb39 100644
--- a/lib/internal/readline/utils.js
+++ b/lib/internal/readline/utils.js
@@ -34,13 +34,32 @@ if (internalBinding('config').hasIntl) {
   const icu = internalBinding('icu');
   getStringWidth = function getStringWidth(str, options) {
     options = options || {};
-    if (!Number.isInteger(str))
-      str = stripVTControlCharacters(String(str));
-    return icu.getStringWidth(
-      str,
-      Boolean(options.ambiguousAsFullWidth),
-      Boolean(options.expandEmojiSequence)
-    );
+    if (Number.isInteger(str)) {
+      // Provide information about the character with code point 'str'.
+      return icu.getStringWidth(
+        str,
+        Boolean(options.ambiguousAsFullWidth),
+        false
+      );
+    }
+    str = stripVTControlCharacters(String(str));
+    let width = 0;
+    for (let i = 0; i < str.length; i++) {
+      // Try to avoid calling into C++ by first handling the ASCII portion of
+      // the string. If it is fully ASCII, we skip the C++ part.
+      const code = str.charCodeAt(i);
+      if (code < 127) {
+        width += code >= 32;
+        continue;
+      }
+      width += icu.getStringWidth(
+        str.slice(i),
+        Boolean(options.ambiguousAsFullWidth),
+        Boolean(options.expandEmojiSequence)
+      );
+      break;
+    }
+    return width;
   };
   isFullWidthCodePoint =
     function isFullWidthCodePoint(code, options) {