From d3979e798cd9c85041bb5e6158002384a8cddb8c 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.
---
 src/frontends/qt4/GuiFontMetrics.cpp | 31 ++++++++++++++++++----------
 src/frontends/qt4/GuiFontMetrics.h   |  2 ++
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp b/src/frontends/qt4/GuiFontMetrics.cpp
index b57f514aa6..5f70c8d705 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -148,20 +148,29 @@ int GuiFontMetrics::strikeoutPos() const
 }
 
 
-int GuiFontMetrics::lbearing(char_type c) const
-{
-	if (!is_utf16(c))
-		// 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));
+namespace {
+int const outOfLimitMetric = -10000;
 }
 
 
-namespace {
-int const outOfLimitMetric = -10000;
+int GuiFontMetrics::lbearing(char_type c) const
+{
+	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:
+		// metrics_.leftBearing(toqstr(docstring(1, c)));
+		value = 0;
+	}
+
+	lbearing_cache_.insert(c, value);
+
+	return value;
 }
 
 
diff --git a/src/frontends/qt4/GuiFontMetrics.h b/src/frontends/qt4/GuiFontMetrics.h
index 6d50d35425..44897c459e 100644
--- a/src/frontends/qt4/GuiFontMetrics.h
+++ b/src/frontends/qt4/GuiFontMetrics.h
@@ -103,6 +103,8 @@ private:
 	/// fill in \c metrics_cache_ at specified value.
 	AscendDescend const fillMetricsCache(char_type) const;
 
+	/// Cache of char leftt bearings
+	mutable QHash<char_type, int> lbearing_cache_;
 	/// Cache of char right bearings
 	mutable QHash<char_type, int> rbearing_cache_;