Finding bounding box of a single character in QTextEdit of QT 3.1

by Jarno Elonen, 2003-06-20

The following C++ function finds the location and size (bounding box) of given character in given paragraph of a component derived from QT 3.1's QTextEdit, taking word wrapping into account. It could also pretty easily be modified to work on an QTextEdit pointer (i.e. without subclassing).

QRect MyMultiLineEdit::findCharacterBounds( int para, int index )
{
    if( para < 0 || para > paragraphs() ||
        index < 0 || index > paragraphLength(para) )
            return QRect(); //invalid rectangle

    const QFontMetrics& fm = fontMetrics();
    const QString& paratext = text(para);

    // Find index of the first character on the same line as parameter
    // 'index' using binary search. Very fast, even for long texts.
    int linestart = 0;
    int indexline = lineOfChar( para, index );
    if ( indexline > 0 )
    {
        int min = 0, max = index;
        int i = (min + max)/2;
        int iline = lineOfChar( para, i );
        while ( iline != indexline-1 ||
                lineOfChar( para, i+1 ) != indexline )
        {
            Q_ASSERT( min != max && min != i && max != i );
            if ( iline < indexline )
                min = i;
            else
                max = i;
            i = (min + max)/2;
            iline = lineOfChar( para, i );
        }
        linestart = i+1;
    }
    Q_ASSERT( linestart >= 0 );
    
    // FIXME as soon as it's possible to ask real margins from QTextEdit:
    const int left_margin = 4;
    const int top_margin = 4;
    
    const QRect& linerect = paragraphRect(para);
    return QRect(
        contentsToViewport( QPoint(
            left_margin + linerect.left() + fm.width( paratext.mid( linestart, index-linestart )), 
            top_margin + linerect.top() + indexline * fm.lineSpacing() + fm.leading())),
        QSize(
            fm.charWidth( paratext, index ),
            fm.height()
        ));
}

The code is hereby released into the Public Domain.

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