tdf#30731: Use ligature caret positions from the font

When ligature caret positions from the font are available, use them for
more accurate caret positions instead of evenly distributing the glyph
width over grapheme clusters.

Change-Id: I0ecfa35e1fff2b264b105182a4b29b2ebd033093
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/138955
Tested-by: Jenkins
Reviewed-by: خالد حسني <khaled@aliftype.com>

1 files changed, 46 insertions, 8 deletions

diff --git a/vcl/source/gdi/CommonSalLayout.cxx b/vcl/source/gdi/CommonSalLayout.cxx
index f75dc12dedd0..014d4f738d9f 100644
--- a/vcl/source/gdi/CommonSalLayout.cxx
+++ b/vcl/source/gdi/CommonSalLayout.cxx
@@ -673,18 +673,56 @@ void GenericSalLayout::GetCharWidths(std::vector<DeviceCoordinate>& rCharWidths,
                 nGraphemeCount++;
             }
 
+            std::vector<DeviceCoordinate> aWidths(nGraphemeCount);
+
+            // Check if the glyph has ligature caret positions.
+            unsigned int nCarets = nGraphemeCount;
+            std::vector<hb_position_t> aCarets(nGraphemeCount);
+            hb_ot_layout_get_ligature_carets(GetFont().GetHbFont(),
+                aGlyphItem.IsRTLGlyph() ? HB_DIRECTION_RTL : HB_DIRECTION_LTR,
+                aGlyphItem.glyphId(), 0, &nCarets, aCarets.data());
+
+            // Carets are 1-less than the grapheme count (since the last
+            // position is defined by glyph width), if the count does not
+            // match, ignore it.
+            if (nCarets == nGraphemeCount - 1)
+            {
+                // Scale the carets and apply glyph offset to them since they
+                // are based on the default glyph metrics.
+                double fScale = 0;
+                GetFont().GetScale(&fScale, nullptr);
+                for (size_t i = 0; i < nCarets; i++)
+                    aCarets[i] = (aCarets[i] * fScale) + aGlyphItem.xOffset();
+
+                // Use the glyph width for the last caret.
+                aCarets[nCarets] = aGlyphItem.newWidth();
+
+                // Carets are absolute from the X origin of the glyph, turn
+                // them to relative widths that we need below.
+                for (size_t i = 0; i < nGraphemeCount; i++)
+                    aWidths[i] = aCarets[i] - (i == 0 ? 0 : aCarets[i - 1]);
+
+                // Carets are in visual order, but we want widths in logical
+                // order.
+                if (aGlyphItem.IsRTLGlyph())
+                    std::reverse(aWidths.begin(), aWidths.end());
+            }
+            else
+            {
+                // The glyph has no carets, distribute the width evenly.
+                auto nWidth = aGlyphItem.newWidth() / nGraphemeCount;
+                std::fill(aWidths.begin(), aWidths.end(), nWidth);
+
+                // Add rounding difference to the last component to maintain
+                // ligature width.
+                aWidths[nGraphemeCount - 1] += aGlyphItem.newWidth() - (nWidth * nGraphemeCount);
+            }
+
             // Set the width of each grapheme cluster.
             nPos = aGlyphItem.charPos();
-            auto nWidth = aGlyphItem.newWidth() / nGraphemeCount;
-            // rounding difference
-            auto nDiff = aGlyphItem.newWidth() - (nWidth * nGraphemeCount);
-            for (unsigned int i = 0; i < nGraphemeCount; i++)
+            for (auto nWidth : aWidths)
             {
                 rCharWidths[nPos - mnMinCharPos] += nWidth;
-                // add rounding difference to last component to maintain
-                // ligature width.
-                if (i == nGraphemeCount - 1)
-                    rCharWidths[nPos - mnMinCharPos] += nDiff;
                 nPos = xBreak->nextCharacters(rStr, nPos, aLocale,
                     css::i18n::CharacterIteratorMode::SKIPCELL, 1, nDone);
             }