position.cpp

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    Node* lastNode = startNode;
    for (; !currentPos.atStart(); currentPos.decrement()) {
        Node* currentNode = currentPos.node();
        
        // Don't check for an editability change if we haven't moved to a different node,
        // to avoid the expense of computing isContentEditable().
        if (currentNode != lastNode) {
            // Don't change editability.
            bool currentEditable = currentNode->isContentEditable();
            if (startEditable != currentEditable)
                break;
            lastNode = currentNode;
        }

        // If we've moved to a position that is visually disinct, return the last saved position. There 
        // is code below that terminates early if we're *about* to move to a visually distinct position.
        if (endsOfNodeAreVisuallyDistinctPositions(currentNode) && currentNode != boundary)
            return lastVisible;

        // skip position in unrendered or invisible node
        RenderObject* renderer = currentNode->renderer();
        if (!renderer || renderer->style()->visibility() != VISIBLE)
            continue;
                 
        // track last visible streamer position
        if (isStreamer(currentPos))
            lastVisible = currentPos;
        
        // Don't move past a position that is visually distinct.  We could rely on code above to terminate and 
        // return lastVisible on the next iteration, but we terminate early to avoid doing a nodeIndex() call.
        if (endsOfNodeAreVisuallyDistinctPositions(currentNode) && currentPos.atStartOfNode())
            return lastVisible;

        // Return position after tables and nodes which have content that can be ignored.
        if (editingIgnoresContent(currentNode) || isTableElement(currentNode)) {
            if (currentPos.atEndOfNode())
                return Position(currentNode, maxDeepOffset(currentNode));
            continue;
        }

        // return current position if it is in rendered text
        if (renderer->isText() && toRenderText(renderer)->firstTextBox()) {
            if (currentNode != startNode) {
                // This assertion fires in layout tests in the case-transform.html test because
                // of a mix-up between offsets in the text in the DOM tree with text in the
                // render tree which can have a different length due to case transformation.
                // Until we resolve that, disable this so we can run the layout tests!
                //ASSERT(currentOffset >= renderer->caretMaxOffset());
                return Position(currentNode, renderer->caretMaxOffset());
            }

            unsigned textOffset = currentPos.offsetInLeafNode();
            RenderText* textRenderer = toRenderText(renderer);
            InlineTextBox* lastTextBox = textRenderer->lastTextBox();
            for (InlineTextBox* box = textRenderer->firstTextBox(); box; box = box->nextTextBox()) {
                if (textOffset <= box->start() + box->len()) {
                    if (textOffset > box->start())
                        return currentPos;
                    continue;
                }

                if (box == lastTextBox || textOffset != box->start() + box->len() + 1)
                    continue;

                // The text continues on the next line only if the last text box is not on this line and
                // none of the boxes on this line have a larger start offset.

                bool continuesOnNextLine = true;
                InlineBox* otherBox = box;
                while (continuesOnNextLine) {
                    otherBox = otherBox->nextLeafChild();
                    if (!otherBox)
                        break;
                    if (otherBox == lastTextBox || otherBox->renderer() == textRenderer && static_cast<InlineTextBox*>(otherBox)->start() > textOffset)
                        continuesOnNextLine = false;
                }

                otherBox = box;
                while (continuesOnNextLine) {
                    otherBox = otherBox->prevLeafChild();
                    if (!otherBox)
                        break;
                    if (otherBox == lastTextBox || otherBox->renderer() == textRenderer && static_cast<InlineTextBox*>(otherBox)->start() > textOffset)
                        continuesOnNextLine = false;
                }

                if (continuesOnNextLine)
                    return currentPos;
            }
        }
    }

    return lastVisible;
}

// This function and upstream() are used for moving back and forth between visually equivalent candidates.
// For example, for the text node "foo     bar" where whitespace is collapsible, there are two candidates 
// that map to the VisiblePosition between 'b' and the space.  This function will return the right candidate 
// and upstream() will return the left one.
// Also, downstream() will return the last position in the last atomic node in boundary for all of the positions
// in boundary after the last candidate, where endsOfNodeAreVisuallyDistinctPositions(boundary).
Position Position::downstream() const
{
    Node* startNode = node();
    if (!startNode)
        return Position();

    // iterate forward from there, looking for a qualified position
    Node* boundary = enclosingVisualBoundary(startNode);
    PositionIterator lastVisible = *this;
    PositionIterator currentPos = lastVisible;
    bool startEditable = startNode->isContentEditable();
    Node* lastNode = startNode;
    for (; !currentPos.atEnd(); currentPos.increment()) {   
        Node* currentNode = currentPos.node();
        
        // Don't check for an editability change if we haven't moved to a different node,
        // to avoid the expense of computing isContentEditable().
        if (currentNode != lastNode) {
            // Don't change editability.
            bool currentEditable = currentNode->isContentEditable();
            if (startEditable != currentEditable)
                break;
            lastNode = currentNode;
        }

        // stop before going above the body, up into the head
        // return the last visible streamer position
        if (currentNode->hasTagName(bodyTag) && currentPos.atEndOfNode())
            break;
            
        // Do not move to a visually distinct position.
        if (endsOfNodeAreVisuallyDistinctPositions(currentNode) && currentNode != boundary)
            return lastVisible;
        // Do not move past a visually disinct position.
        // Note: The first position after the last in a node whose ends are visually distinct
        // positions will be [boundary->parentNode(), originalBlock->nodeIndex() + 1].
        if (boundary && boundary->parentNode() == currentNode)
            return lastVisible;

        // skip position in unrendered or invisible node
        RenderObject* renderer = currentNode->renderer();
        if (!renderer || renderer->style()->visibility() != VISIBLE)
            continue;
            
        // track last visible streamer position
        if (isStreamer(currentPos))
            lastVisible = currentPos;

        // Return position before tables and nodes which have content that can be ignored.
        if (editingIgnoresContent(currentNode) || isTableElement(currentNode)) {
            if (currentPos.offsetInLeafNode() <= renderer->caretMinOffset())
                return Position(currentNode, renderer->caretMinOffset());
            continue;
        }

        // return current position if it is in rendered text
        if (renderer->isText() && toRenderText(renderer)->firstTextBox()) {
            if (currentNode != startNode) {
                ASSERT(currentPos.atStartOfNode());
                return Position(currentNode, renderer->caretMinOffset());
            }

            unsigned textOffset = currentPos.offsetInLeafNode();
            RenderText* textRenderer = toRenderText(renderer);
            InlineTextBox* lastTextBox = textRenderer->lastTextBox();
            for (InlineTextBox* box = textRenderer->firstTextBox(); box; box = box->nextTextBox()) {
                if (textOffset <= box->end()) {
                    if (textOffset >= box->start())
                        return currentPos;
                    continue;
                }

                if (box == lastTextBox || textOffset != box->start() + box->len())
                    continue;

                // The text continues on the next line only if the last text box is not on this line and
                // none of the boxes on this line have a larger start offset.

                bool continuesOnNextLine = true;
                InlineBox* otherBox = box;
                while (continuesOnNextLine) {
                    otherBox = otherBox->nextLeafChild();
                    if (!otherBox)
                        break;
                    if (otherBox == lastTextBox || otherBox->renderer() == textRenderer && static_cast<InlineTextBox*>(otherBox)->start() >= textOffset)
                        continuesOnNextLine = false;
                }

                otherBox = box;
                while (continuesOnNextLine) {
                    otherBox = otherBox->prevLeafChild();
                    if (!otherBox)
                        break;
                    if (otherBox == lastTextBox || otherBox->renderer() == textRenderer && static_cast<InlineTextBox*>(otherBox)->start() >= textOffset)
                        continuesOnNextLine = false;
                }

                if (continuesOnNextLine)
                    return currentPos;
            }
        }
    }
    
    return lastVisible;
}

bool Position::hasRenderedNonAnonymousDescendantsWithHeight(RenderObject* renderer)
{
    RenderObject* stop = renderer->nextInPreOrderAfterChildren();
    for (RenderObject *o = renderer->firstChild(); o && o != stop; o = o->nextInPreOrder())
        if (o->node()) {
            if ((o->isText() && toRenderText(o)->linesBoundingBox().height()) ||
                (o->isBox() && toRenderBox(o)->borderBoundingBox().height()))
                return true;
        }
    return false;
}

bool Position::nodeIsUserSelectNone(Node* node)
{
    return node && node->renderer() && node->renderer()->style()->userSelect() == SELECT_NONE;
}

bool Position::isCandidate() const
{
    if (isNull())
        return false;
        
    RenderObject *renderer = node()->renderer();
    if (!renderer)
        return false;
    
    if (renderer->style()->visibility() != VISIBLE)
        return false;

    if (renderer->isBR())
        return offset() == 0 && !nodeIsUserSelectNone(node()->parent());

    if (renderer->isText())
        return inRenderedText() && !nodeIsUserSelectNone(node());

    if (isTableElement(node()) || editingIgnoresContent(node()))
        return (offset() == 0 || offset() == maxDeepOffset(node())) && !nodeIsUserSelectNone(node()->parent());

    if (!node()->hasTagName(htmlTag) && renderer->isBlockFlow() && !hasRenderedNonAnonymousDescendantsWithHeight(renderer) &&
       (toRenderBox(renderer)->height() || node()->hasTagName(bodyTag)))
        return offset() == 0 && !nodeIsUserSelectNone(node());
    
    return false;
}

bool Position::inRenderedText() const
{
    if (isNull() || !node()->isTextNode())
        return false;
        
    RenderObject *renderer = node()->renderer();
    if (!renderer)
        return false;
    
    RenderText *textRenderer = toRenderText(renderer);
    for (InlineTextBox *box = textRenderer->firstTextBox(); box; box = box->nextTextBox()) {
        if (offset() < static_cast<int>(box->start()) && !textRenderer->containsReversedText()) {
            // The offset we're looking for is before this node
            // this means the offset must be in content that is
            // not rendered. Return false.
            return false;
        }
        if (box->containsCaretOffset(offset()))
            // Return false for offsets inside composed characters.
            return offset() == 0 || offset() == textRenderer->nextOffset(textRenderer->previousOffset(offset()));
    }
    
    return false;
}

static unsigned caretMaxRenderedOffset(const Node* n)
{
    RenderObject* r = n->renderer();
    if (r)
        return r->caretMaxRenderedOffset();
    
    if (n->isCharacterDataNode())
        return static_cast<const CharacterData*>(n)->length();
    return 1;
}

bool Position::isRenderedCharacter() const
{
    if (isNull() || !node()->isTextNode())
        return false;
        
    RenderObject* renderer = node()->renderer();
    if (!renderer)
        return false;
    
    RenderText* textRenderer = toRenderText(renderer);
    for (InlineTextBox* box = textRenderer->firstTextBox(); box; box = box->nextTextBox()) {
        if (offset() < static_cast<int>(box->start()) && !textRenderer->containsReversedText()) {
            // The offset we're looking for is before this node
            // this means the offset must be in content that is
            // not rendered. Return false.
            return false;
        }
        if (offset() >= static_cast<int>(box->start()) && offset() < static_cast<int>(box->start() + box->len()))
            return true;
    }
    
    return false;
}

bool Position::rendersInDifferentPosition(const Position &pos) const
{
    if (isNull() || pos.isNull())
        return false;

    RenderObject *renderer = node()->renderer();
    if (!renderer)
        return false;
    
    RenderObject *posRenderer = pos.node()->renderer();
    if (!posRenderer)
        return false;

    if (renderer->style()->visibility() != VISIBLE ||
        posRenderer->style()->visibility() != VISIBLE)
        return false;
    
    if (node() == pos.node()) {
        if (node()->hasTagName(brTag))
            return false;

        if (offset() == pos.offset())
            return false;
            
        if (!node()->isTextNode() && !pos.node()->isTextNode()) {
            if (offset() != pos.offset())
                return true;