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
(cherry picked from commit 5338d78aa9d80ddd2bcb21e6b22cd2cf1522a7d3)

Change-Id: Ic747e9458d561aca0848dcd1e8b94e0a23fd8189
Reviewed-on: http://codereview.qt.nokia.com/196
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>

4 files changed, 107 insertions, 7 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index b43c8f448b..ad4f6d3fc6 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -2825,6 +2825,75 @@ 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,
+                                    bool cursorOnCharacter)
+{
+    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;
+        if (cursorOnCharacter && closestItem > 0)
+            closestItem--;
+        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 ed24d5967b..055974a4ad 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, bool cursorOnCharacter);
     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 2e2a6234ae..add25cdad9 100644
--- a/src/gui/text/qtextlayout.cpp
+++ b/src/gui/text/qtextlayout.cpp
@@ -2741,6 +2741,7 @@ 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) {
@@ -2751,6 +2752,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                             if (pos < x)
                                 break;
                             glyph_pos = gs;
+                            edge = pos;
                             break;
                         }
                         pos -= glyphs.effectiveAdvance(gs);
@@ -2763,6 +2765,7 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
                             if (pos > x)
                                 break;
                             glyph_pos = gs;
+                            edge = pos;
                         }
                         pos += glyphs.effectiveAdvance(gs);
                         ++gs;
@@ -2776,6 +2779,7 @@ 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);
@@ -2785,6 +2789,7 @@ 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);
@@ -2796,6 +2801,7 @@ 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);
@@ -2807,6 +2813,7 @@ 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;
@@ -2823,16 +2830,13 @@ 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,
+                                                   cpos == QTextLine::CursorOnCharacter);
                 }
             }
             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,
+                                           cpos == QTextLine::CursorOnCharacter);
         }
     }
     // right of last item
diff --git a/tests/auto/qtextlayout/tst_qtextlayout.cpp b/tests/auto/qtextlayout/tst_qtextlayout.cpp
index be0a83fe38..2414ab3e37 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"