From 4d3daea33f7f2750303a62a97c342ea880f75644 Mon Sep 17 00:00:00 2001
From: Allan Sandfeld Jensen <allan.jensen@qt.io>
Date: Thu, 5 Jan 2023 15:23:25 +0100
Subject: [PATCH] Avoid unneccessarily long text scans

Limit the scans to the processed length.

Fixes: QTBUG-109838
Change-Id: If4b19bf6f97d788d0a227af3a80962bef26016fd
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 7c84550f3bfdf93cb8a071e6bb23d54d57109e84)
---
 src/gui/text/qfontmetrics.cpp | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/src/gui/text/qfontmetrics.cpp b/src/gui/text/qfontmetrics.cpp
index 42c17ced584..e8b447955ed 100644
--- a/src/gui/text/qfontmetrics.cpp
+++ b/src/gui/text/qfontmetrics.cpp
@@ -509,6 +509,8 @@ int QFontMetrics::rightBearing(QChar ch) const
     return qRound(rb);
 }
 
+static constexpr QLatin1Char s_variableLengthStringSeparator('\x9c');
+
 /*!
     Returns the horizontal advance in pixels of the first \a len characters of \a
     text. If \a len is negative (the default), the entire string is
@@ -523,9 +525,11 @@ int QFontMetrics::rightBearing(QChar ch) const
 */
 int QFontMetrics::horizontalAdvance(const QString &text, int len) const
 {
-    int pos = text.indexOf(QLatin1Char('\x9c'));
+    int pos = (len >= 0)
+            ? QStringView(text).left(len).indexOf(s_variableLengthStringSeparator)
+            : text.indexOf(s_variableLengthStringSeparator);
     if (pos != -1) {
-        len = (len < 0) ? pos : qMin(pos, len);
+        len = pos;
     } else if (len < 0) {
         len = text.length();
     }
@@ -819,13 +823,13 @@ QString QFontMetrics::elidedText(const QString &text, Qt::TextElideMode mode, in
     QString _text = text;
     if (!(flags & Qt::TextLongestVariant)) {
         int posA = 0;
-        int posB = _text.indexOf(QLatin1Char('\x9c'));
+        int posB = _text.indexOf(s_variableLengthStringSeparator);
         while (posB >= 0) {
             QString portion = _text.mid(posA, posB - posA);
             if (size(flags, portion).width() <= width)
                 return portion;
             posA = posB + 1;
-            posB = _text.indexOf(QLatin1Char('\x9c'), posA);
+            posB = _text.indexOf(s_variableLengthStringSeparator, posA);
         }
         _text = _text.mid(posA);
     }
@@ -1341,9 +1345,11 @@ qreal QFontMetricsF::rightBearing(QChar ch) const
 */
 qreal QFontMetricsF::horizontalAdvance(const QString &text, int length) const
 {
-    int pos = text.indexOf(QLatin1Char('\x9c'));
+    int pos = (length >= 0)
+            ? QStringView(text).left(length).indexOf(s_variableLengthStringSeparator)
+            : text.indexOf(s_variableLengthStringSeparator);
     if (pos != -1)
-        length = (length < 0) ? pos : qMin(pos, length);
+        length = pos;
     else if (length < 0)
         length = text.length();
 
@@ -1641,13 +1647,13 @@ QString QFontMetricsF::elidedText(const QString &text, Qt::TextElideMode mode, q
     QString _text = text;
     if (!(flags & Qt::TextLongestVariant)) {
         int posA = 0;
-        int posB = _text.indexOf(QLatin1Char('\x9c'));
+        int posB = _text.indexOf(s_variableLengthStringSeparator);
         while (posB >= 0) {
             QString portion = _text.mid(posA, posB - posA);
             if (size(flags, portion).width() <= width)
                 return portion;
             posA = posB + 1;
-            posB = _text.indexOf(QLatin1Char('\x9c'), posA);
+            posB = _text.indexOf(s_variableLengthStringSeparator, posA);
         }
         _text = _text.mid(posA);
     }