From 2753924fcbeb29fa5b34782bd6ad6ed4fe4b51f1 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Thu, 13 Feb 2025 16:09:20 +0100
Subject: [PATCH] QByteArray: optimize replace(char, char)

Write every byte only once.

Amends 83c2c5055405f09043c35b93cd5ba9b9969f3174 (as well as the start
of the public history).

Pick-to: 6.9 6.8
Task-number: QTBUG-106185
Change-Id: I883b2f00c754806882131a09dc3cbc5613420151
Reviewed-by: Anton Kudryavtsev <antkudr@mail.ru>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
---
 src/corelib/text/qbytearray.cpp | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 2cc55ef0138..382f08200cc 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -2651,8 +2651,18 @@ QByteArray &QByteArray::replace(char before, char after)
 {
     if (before != after) {
         if (const auto pos = indexOf(before); pos >= 0) {
-            const auto detachedData = data();
-            std::replace(detachedData + pos, detachedData + size(), before, after);
+            if (d.needsDetach()) {
+                QByteArray tmp(size(), Qt::Uninitialized);
+                auto dst = tmp.d.data();
+                dst = std::copy(d.data(), d.data() + pos, dst);
+                *dst++ = after;
+                std::replace_copy(d.data() + pos + 1, d.end(), dst, before, after);
+                swap(tmp);
+            } else {
+                // in-place
+                d.data()[pos] = after;
+                std::replace(d.data() + pos + 1, d.end(), before, after);
+            }
         }
     }
     return *this;