From effac0a4856d857f26a9eee867c6f639453d0c57 Mon Sep 17 00:00:00 2001
From: FletcherDares <52580867+FletcherDares@users.noreply.github.com>
Date: Tue, 26 May 2026 00:52:41 -0400
Subject: [PATCH] Improve performance of `String#scrub` by skipping ASCII runs
 with `search_nonascii` (#16359)

Improve performance of String#scrub by skipping ascii characters after finding a multi byte character using search_nonascii. Create benchmark for String#scrub to show performance difference made by including the new ascii skips.
---
 benchmark/string_scrub.yml | 48 ++++++++++++++++++++++++++++++++++++++
 string.c                   | 12 ++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 benchmark/string_scrub.yml

diff --git a/benchmark/string_scrub.yml b/benchmark/string_scrub.yml
new file mode 100644
index 0000000000..4b5faaad8e
--- /dev/null
+++ b/benchmark/string_scrub.yml
@@ -0,0 +1,48 @@
+prelude: |
+
+  STRING_SIZE = 1024
+  def duplicate_to_length(str, target_length)
+    return "" if target_length <= 0
+    return str[0, target_length] if str.length >= target_length
+
+    (str * ((target_length / str.length) + 1))[0, target_length]
+  end
+  base = "Hello \u{1f600} world! \u{00e9}\u{00f1}"
+  padding = duplicate_to_length(base, STRING_SIZE)
+
+  valid_utf8 = (padding.b + "OK".b).force_encoding("UTF-8")
+  valid_utf8.valid_encoding?
+  unknown_but_valid_utf8 = valid_utf8.dup.b.force_encoding("UTF-8")
+  invalid_utf8 = (padding.b + "\x80\xFF".b).force_encoding("UTF-8")
+  invalid_utf8.valid_encoding?
+  unknown_but_invalid_utf8 = (padding.b + "\x80\xFF".b).force_encoding("UTF-8")
+
+  worst_case_utf8 = duplicate_to_length("\u{1f600}\u{00e9}\u{00f1}", STRING_SIZE).b.force_encoding("UTF-8")
+
+  unknown_but_valid_utf8_worst_case = worst_case_utf8.dup.b.force_encoding("UTF-8")
+  unknown_but_invalid_utf8_worst_case = (worst_case_utf8.b + "\x80\xFF".b).force_encoding("UTF-8")
+
+benchmark:
+  scrub_known_valid: |
+    string = valid_utf8.dup
+    string.scrub!
+
+  scrub_known_invalid: |
+    string = invalid_utf8.dup
+    string.scrub!
+
+  scrub_unknown_but_valid_coderange: |
+    string = unknown_but_valid_utf8.dup
+    string.scrub!
+
+  scrub_unknown_and_invalid_coderange: |
+    string = unknown_but_invalid_utf8.dup
+    string.scrub!
+
+  scrub_unknown_but_valid_coderange_worst_case: |
+    string = unknown_but_valid_utf8_worst_case.dup
+    string.scrub!
+
+  scrub_unknown_and_invalid_coderange_worst_case: |
+    string = unknown_but_invalid_utf8_worst_case.dup
+    string.scrub!
\ No newline at end of file
diff --git a/string.c b/string.c
index a59340adfd..6865b0d8e6 100644
--- a/string.c
+++ b/string.c
@@ -11850,6 +11850,18 @@ enc_str_scrub(rb_encoding *enc, VALUE str, VALUE repl, int cr)
             else if (MBCLEN_CHARFOUND_P(ret)) {
                 cr = ENC_CODERANGE_VALID;
                 p += MBCLEN_CHARFOUND_LEN(ret);
+                /*
+                 * After a valid multibyte character, skip the following ASCII run.
+                 * If the next byte is already non-ASCII, search_nonascii would only
+                 * rediscover p after its word-at-a-time setup.
+                 */
+                if (p < e && ISASCII(*p)) {
+                    p = search_nonascii(p, e);
+                    if (!p) {
+                        p = e;
+                        break;
+                    }
+                }
             }
             else if (MBCLEN_INVALID_P(ret)) {
                 /*