[LyX/master] Cache the value of GuiFontMetrics::lbearing().

[Jean-Marc Lasgouttes]

commit d3979e798cd9c85041bb5e6158002384a8cddb8c
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Wed Mar 20 10:56:16 2019 +0100

    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 |   25 +++++++++++++++++--------
 src/frontends/qt4/GuiFontMetrics.h   |    2 ++
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index b57f514..5f70c8d 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -148,20 +148,29 @@ 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 6d50d35..44897c4 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_;