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 d3979e798c)

src/frontends/qt4/GuiFontMetrics.cpp

@@ -135,20 +135,29 @@ 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;
 }
 
 

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_;
 

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