From e0879b14602c0aff25ba951f6aa1277e4a94f988 Mon Sep 17 00:00:00 2001
From: Jean-Marc Lasgouttes <lasgouttes@lyx.org>
Date: Wed, 20 Mar 2019 10:56:16 +0100
Subject: [PATCH] Cache the value of GuiFontMetrics::lbearing().

This seems to be necessary on windows, where math editing can get very
slow. Note that other methods like rbearing already use a cache.

In the future all these caches for single characters shall be unified.

(cherry picked from commit d3979e798cd9c85041bb5e6158002384a8cddb8c)
---
 src/frontends/qt4/GuiFontMetrics.cpp | 25 +++++++++++++++++--------
 src/frontends/qt4/GuiFontMetrics.h   |  2 ++
 status.23x                           |  2 ++
 3 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp b/src/frontends/qt4/GuiFontMetrics.cpp
index 8ed82280c0..63f61f2e3f 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -135,20 +135,29 @@ int GuiFontMetrics::strikeoutPos() const
 }
 
 
+namespace {
+int const outOfLimitMetric = -10000;
+}
+
+
 int GuiFontMetrics::lbearing(char_type c) const
 {
-	if (!is_utf16(c))
+	int value = lbearing_cache_.value(c, outOfLimitMetric);
+	if (value != outOfLimitMetric)
+		return value;
+
+	if (is_utf16(c))
+		value = metrics_.leftBearing(ucs4_to_qchar(c));
+	else {
 		// FIXME: QFontMetrics::leftBearing does not support the
 		//        full unicode range. Once it does, we could use:
-		//return metrics_.leftBearing(toqstr(docstring(1, c)));
-		return 0;
-
-	return metrics_.leftBearing(ucs4_to_qchar(c));
-}
+		// metrics_.leftBearing(toqstr(docstring(1, c)));
+		value = 0;
+	}
 
+	lbearing_cache_.insert(c, value);
 
-namespace {
-int const outOfLimitMetric = -10000;
+	return value;
 }
 
 
diff --git a/src/frontends/qt4/GuiFontMetrics.h b/src/frontends/qt4/GuiFontMetrics.h
index 3f50dfe5d2..6cea40f3ec 100644
--- a/src/frontends/qt4/GuiFontMetrics.h
+++ b/src/frontends/qt4/GuiFontMetrics.h
@@ -101,6 +101,8 @@ private:
 	/// fill in \c metrics_cache_ at specified value.
 	AscendDescend const fillMetricsCache(char_type) const;
 
+	/// Cache of char left bearings
+	mutable QHash<char_type, int> lbearing_cache_;
 	/// Cache of char right bearings
 	mutable QHash<char_type, int> rbearing_cache_;
 
diff --git a/status.23x b/status.23x
index d7afa3592b..29f9d20add 100644
--- a/status.23x
+++ b/status.23x
@@ -51,6 +51,8 @@ What's new
 
 - Fix bad error message (bug 11486).
 
+- Improve performance on Windows with lots of math insets.
+
 
 * DOCUMENTATION AND LOCALIZATION
 
-- 
2.39.5