lib: add ASCII fast path to getStringWidth()

benchmark/misc/getstringwidth.js

@@ -0,0 +1,25 @@
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  type: ['ascii', 'emojiseq', 'fullwidth'],
+  n: [10e4]
+}, {
+  flags: ['--expose-internals']
+});
+
+function main({ n, type }) {
+  const { getStringWidth } = require('internal/readline/utils');
+
+  const str = ({
+    ascii: 'foobar'.repeat(100),
+    emojiseq: '👨‍👨‍👧‍👦👨‍👩‍👦‍👦👨‍👩‍👧‍👧👩‍👩‍👧‍👦'.repeat(10),
+    fullwidth: '你好'.repeat(150)
+  })[type];
+
+  bench.start();
+  for (let j = 0; j < n; j += 1)
+    getStringWidth(str);
+  bench.end(n);
+}

lib/internal/readline/utils.js

@@ -34,13 +34,31 @@ 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)) {
+      return icu.getStringWidth(
+        str,
+        Boolean(options.ambiguousAsFullWidth),
+        Boolean(options.expandEmojiSequence)
+      );
+    }
+    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) {

test/parallel/test-icu-stringwidth.js

@@ -69,3 +69,17 @@ assert.strictEqual(
 
 // Control chars and combining chars are zero
 assert.strictEqual(readline.getStringWidth('\u200E\n\u220A\u20D2'), 1);
+
+// Test that the fast path for ASCII characters yields results consistent
+// with the 'slow' path.
+for (const ambiguousAsFullWidth of [ false, true ]) {
+  for (let i = 0; i < 256; i++) {
+    const char = String.fromCharCode(i);
+    assert.strictEqual(
+      readline.getStringWidth(i, { ambiguousAsFullWidth }),
+      readline.getStringWidth(char, { ambiguousAsFullWidth }));
+    assert.strictEqual(
+      readline.getStringWidth(char + '🎉', { ambiguousAsFullWidth }),
+      readline.getStringWidth(char, { ambiguousAsFullWidth }) + 2);
+  }
+}