Support placing cursor in ligature with mouse or touch

We need to find out the closest element in the ligature to
the point we clicked (or tapped), currently we do this by
dividing the width of that ligature glyph evenly by the number
of characters it covered. We only support Common and Greek script
at this point, ligatures in other scripts are still handled as a
whole.

Task-number: QTBUG-19260
Reviewed-by: Eskil

4 files changed, 102 insertions, 7 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index ff27bc6..0df543b 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -2825,6 +2825,72 @@ QFixed QTextEngine::offsetInLigature(const QScriptItem *si, int pos, int max, in
     return 0;
 }
 
+// Scan in logClusters[from..to-1] for glyph_pos
+int QTextEngine::getClusterLength(unsigned short *logClusters,
+                                  const HB_CharAttributes *attributes,
+                                  int from, int to, int glyph_pos, int *start)
+{
+    int clusterLength = 0;
+    for (int i = from; i < to; i++) {
+        if (logClusters[i] == glyph_pos && attributes[i].charStop) {
+            if (*start < 0)
+                *start = i;
+            clusterLength++;
+        }
+        else if (clusterLength)
+            break;
+    }
+    return clusterLength;
+}
+
+int QTextEngine::positionInLigature(const QScriptItem *si, int end,
+                                    QFixed x, QFixed edge, int glyph_pos)
+{
+    unsigned short *logClusters = this->logClusters(si);
+    int clusterStart = -1;
+    int clusterLength = 0;
+
+    if (si->analysis.script != QUnicodeTables::Common &&
+        si->analysis.script != QUnicodeTables::Greek) {
+        if (glyph_pos == -1)
+            return si->position + end;
+        else {
+            int i;
+            for (i = 0; i < end; i++)
+                if (logClusters[i] == glyph_pos)
+                    break;
+            return si->position + i;
+        }
+    }
+
+    if (glyph_pos == -1 && end > 0)
+        glyph_pos = logClusters[end - 1];
+    else {
+        if (x < edge)
+            glyph_pos--;
+    }
+
+    const HB_CharAttributes *attrs = attributes();
+    clusterLength = getClusterLength(logClusters, attrs, 0, end, glyph_pos, &clusterStart);
+
+    if (clusterLength) {
+        const QGlyphLayout &glyphs = shapedGlyphs(si);
+        QFixed glyphWidth = glyphs.effectiveAdvance(glyph_pos);
+        // the approximate width of each individual element of the ligature
+        QFixed perItemWidth = glyphWidth / clusterLength;
+        QFixed left = x > edge ? edge : edge - glyphWidth;
+        int n = ((x - left) / perItemWidth).floor().toInt();
+        QFixed dist = x - left - n * perItemWidth;
+        int closestItem = dist > (perItemWidth / 2) ? n + 1 : n;
+        int pos = si->position + clusterStart + closestItem;
+        // Jump to the next charStop
+        while (!attrs[pos].charStop && pos < end)
+            pos++;
+        return pos;
+    }
+    return si->position + end;
+}
+
 int QTextEngine::previousLogicalPosition(int oldPos) const
 {
     const HB_CharAttributes *attrs = attributes();
diff --git a/src/gui/text/qtextengine_p.h b/src/gui/text/qtextengine_p.h
index e06ca1c..8e1d796 100644
--- a/src/gui/text/qtextengine_p.h
+++ b/src/gui/text/qtextengine_p.h
@@ -620,6 +620,7 @@ public:
     QFixed leadingSpaceWidth(const QScriptLine &line);
 
     QFixed offsetInLigature(const QScriptItem *si, int pos, int max, int glyph_pos);
+    int positionInLigature(const QScriptItem *si, int end, QFixed x, QFixed edge, int glyph_pos);
     int previousLogicalPosition(int oldPos) const;
     int nextLogicalPosition(int oldPos) const;
     int lineNumberForTextPosition(int pos);
@@ -642,6 +643,7 @@ private:
     void resolveAdditionalFormats() const;
     int endOfLine(int lineNum);
     int beginningOfLine(int lineNum);
+    int getClusterLength(unsigned short *logClusters, const HB_CharAttributes *attributes, int from, int to, int glyph_pos, int *start);
 };
 
 class QStackTextEngine : public QTextEngine {
diff --git a/src/gui/text/qtextlayout.cpp b/src/gui/text/qtextlayout.cpp
index 9501c0b..20edb87 100644
--- a/src/gui/text/qtextlayout.cpp
+++ b/src/gui/text/qtextlayout.cpp
@@ -2691,6 +2691,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
             }
 
             int glyph_pos = -1;
+            QFixed edge;
             // has to be inside run
             if (cpos == QTextLine::CursorOnCharacter) {
                 if (si.analysis.bidiLevel % 2) {
@@ -2701,6 +2702,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                             if (pos < x)
                                 break;
                             glyph_pos = gs;
+                            edge = pos;
                             break;
                         }
                         pos -= glyphs.effectiveAdvance(gs);
@@ -2713,6 +2715,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                             if (pos > x)
                                 break;
                             glyph_pos = gs;
+                            edge = pos;
                         }
                         pos += glyphs.effectiveAdvance(gs);
                         ++gs;
@@ -2726,6 +2729,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                         while (gs <= ge) {
                             if (glyphs.attributes[gs].clusterStart && qAbs(x-pos) < dist) {
                                 glyph_pos = gs;
+                                edge = pos;
                                 dist = qAbs(x-pos);
                             }
                             pos -= glyphs.effectiveAdvance(gs);
@@ -2735,6 +2739,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                         while (ge >= gs) {
                             if (glyphs.attributes[ge].clusterStart && qAbs(x-pos) < dist) {
                                 glyph_pos = ge;
+                                edge = pos;
                                 dist = qAbs(x-pos);
                             }
                             pos += glyphs.effectiveAdvance(ge);
@@ -2746,6 +2751,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                         while (gs <= ge) {
                             if (glyphs.attributes[gs].clusterStart && qAbs(x-pos) < dist) {
                                 glyph_pos = gs;
+                                edge = pos;
                                 dist = qAbs(x-pos);
                             }
                             pos += glyphs.effectiveAdvance(gs);
@@ -2757,6 +2763,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                             pos += glyphs.effectiveAdvance(gs);
                             if (glyphs.attributes[gs].clusterStart && qAbs(x-pos) < dist) {
                                 glyph_pos = gs;
+                                edge = pos;
                                 dist = qAbs(x-pos);
                             }
                             ++gs;
@@ -2773,16 +2780,11 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                         if (rtl && nchars > 0)
                             return insertionPoints[lastLine ? nchars : nchars - 1];
                     }
-                    return si.position + end;
+                    return eng->positionInLigature(&si, end, x, pos, -1);
                 }
             }
             Q_ASSERT(glyph_pos != -1);
-            int j;
-            for (j = 0; j < eng->length(item); ++j)
-                if (logClusters[j] == glyph_pos)
-                    break;
-//             qDebug("at pos %d (in run: %d)", si.position + j, j);
-            return si.position + j;
+            return eng->positionInLigature(&si, end, x, edge, glyph_pos);
         }
     }
     // right of last item
diff --git a/tests/auto/qtextlayout/tst_qtextlayout.cpp b/tests/auto/qtextlayout/tst_qtextlayout.cpp
index b5712fb..e68a65e 100644
--- a/tests/auto/qtextlayout/tst_qtextlayout.cpp
+++ b/tests/auto/qtextlayout/tst_qtextlayout.cpp
@@ -128,6 +128,7 @@ private slots:
     void textWidthWithStackedTextEngine();
     void textWidthWithLineSeparator();
     void cursorInLigatureWithMultipleLines();
+    void xToCursorForLigatures();
 
 private:
     QFont testFont;
@@ -1477,5 +1478,29 @@ void tst_QTextLayout::cursorInLigatureWithMultipleLines()
     QVERIFY(line.cursorToX(0) != line.cursorToX(1));
 }
 
+void tst_QTextLayout::xToCursorForLigatures()
+{
+#if !defined(Q_WS_MAC)
+    QSKIP("This test can not be run on Mac", SkipAll);
+#endif
+    QTextLayout layout("fi", QFont("Times", 20));
+    layout.beginLayout();
+    QTextLine line = layout.createLine();
+    layout.endLayout();
+
+    QVERIFY(line.xToCursor(0) != line.xToCursor(line.naturalTextWidth() / 2));
+
+    // U+0061 U+0308
+    QTextLayout layout2(QString::fromUtf8("\x61\xCC\x88"), QFont("Times", 20));
+
+    layout2.beginLayout();
+    line = layout2.createLine();
+    layout2.endLayout();
+
+    qreal width = line.naturalTextWidth();
+    QVERIFY(line.xToCursor(0) == line.xToCursor(width / 2) ||
+            line.xToCursor(width) == line.xToCursor(width / 2));
+}
+
 QTEST_MAIN(tst_QTextLayout)
 #include "tst_qtextlayout.moc"