double TiledBackingStore::tileDistance(const IntRect& viewport, const Tile::Coordinate& tileCoordinate) const
{
    if (viewport.intersects(tileRectForCoordinate(tileCoordinate)))
        return 0;

    IntPoint viewCenter = viewport.location() + IntSize(viewport.width() / 2, viewport.height() / 2);
    Tile::Coordinate centerCoordinate = tileCoordinateForPoint(viewCenter);

    return std::max(abs(centerCoordinate.y() - tileCoordinate.y()), abs(centerCoordinate.x() - tileCoordinate.x()));
}

void TiledBackingStore::createTiles()
{
    // Guard here as as these can change before the timer fires.
    if (isBackingStoreUpdatesSuspended())
        return;

    // Update our backing store geometry.
    const IntRect previousRect = m_rect;
    m_rect = mapFromContents(m_client->tiledBackingStoreContentsRect());

    const IntRect visibleRect = this->visibleRect();
    m_visibleRect = visibleRect;

    if (visibleRect.isEmpty())
        return;

    IntRect keepRect;
    IntRect coverRect;
    computeCoverAndKeepRect(visibleRect, coverRect, keepRect);

    setKeepRect(keepRect);

    // Resize tiles at the edge in case the contents size has changed, but only do so
    // after having dropped tiles outside the keep rect.
    bool didResizeTiles = false;
    if (previousRect != m_rect)
        didResizeTiles = resizeEdgeTiles();

    // Search for the tile position closest to the viewport center that does not yet contain a tile.
    // Which position is considered the closest depends on the tileDistance function.
    double shortestDistance = std::numeric_limits<double>::infinity();
    Vector<Tile::Coordinate> tilesToCreate;
    unsigned requiredTileCount = 0;

    // Cover areas (in tiles) with minimum distance from the visible rect. If the visible rect is
    // not covered already it will be covered first in one go, due to the distance being 0 for tiles
    // inside the visible rect.
    Tile::Coordinate topLeft = tileCoordinateForPoint(coverRect.location());
    Tile::Coordinate bottomRight = tileCoordinateForPoint(innerBottomRight(coverRect));
    for (unsigned yCoordinate = topLeft.y(); yCoordinate <= bottomRight.y(); ++yCoordinate) {
        for (unsigned xCoordinate = topLeft.x(); xCoordinate <= bottomRight.x(); ++xCoordinate) {
            Tile::Coordinate currentCoordinate(xCoordinate, yCoordinate);
            if (tileAt(currentCoordinate))
                continue;
            ++requiredTileCount;
            double distance = tileDistance(visibleRect, currentCoordinate);
            if (distance > shortestDistance)
                continue;
            if (distance < shortestDistance) {
                tilesToCreate.clear();
                shortestDistance = distance;
            }
            tilesToCreate.append(currentCoordinate);
        }
    }

    // Now construct the tile(s) within the shortest distance.
    unsigned tilesToCreateCount = tilesToCreate.size();
    for (unsigned n = 0; n < tilesToCreateCount; ++n) {
        Tile::Coordinate coordinate = tilesToCreate[n];
        setTile(coordinate, m_backend->createTile(this, coordinate));
    }
    requiredTileCount -= tilesToCreateCount;

    // Paint the content of the newly created tiles or resized tiles.
    if (tilesToCreateCount || didResizeTiles)
        updateTileBuffers();

    // Re-call createTiles on a timer to cover the visible area with the newest shortest distance.
    if (requiredTileCount)
        m_backingStoreUpdateTimer.startOneShot(m_tileCreationDelay);
}

void TiledBackingStore::createTiles()
{
    if (m_contentsFrozen)
        return;
    
    IntRect visibleRect = mapFromContents(m_client->tiledBackingStoreVisibleRect());
    m_previousVisibleRect = visibleRect;

    if (visibleRect.isEmpty())
        return;

    // Remove tiles that extend outside the current contents rect.
    dropOverhangingTiles();

    IntRect keepRect = visibleRect;
    // Inflates to both sides, so divide inflate delta by 2
    keepRect.inflateX(visibleRect.width() * (m_keepAreaMultiplier.width() - 1.f) / 2);
    keepRect.inflateY(visibleRect.height() * (m_keepAreaMultiplier.height() - 1.f) / 2);
    keepRect.intersect(contentsRect());
    
    dropTilesOutsideRect(keepRect);
    
    IntRect coverRect = visibleRect;
    // Inflates to both sides, so divide inflate delta by 2
    coverRect.inflateX(visibleRect.width() * (m_coverAreaMultiplier.width() - 1.f) / 2);
    coverRect.inflateY(visibleRect.height() * (m_coverAreaMultiplier.height() - 1.f) / 2);
    coverRect.intersect(contentsRect());
    
    // Search for the tile position closest to the viewport center that does not yet contain a tile. 
    // Which position is considered the closest depends on the tileDistance function.
    double shortestDistance = std::numeric_limits<double>::infinity();
    Vector<Tile::Coordinate> tilesToCreate;
    unsigned requiredTileCount = 0;
    Tile::Coordinate topLeft = tileCoordinateForPoint(coverRect.location());
    Tile::Coordinate bottomRight = tileCoordinateForPoint(IntPoint(coverRect.maxX(), coverRect.maxY()));
    for (unsigned yCoordinate = topLeft.y(); yCoordinate <= bottomRight.y(); ++yCoordinate) {
        for (unsigned xCoordinate = topLeft.x(); xCoordinate <= bottomRight.x(); ++xCoordinate) {
            Tile::Coordinate currentCoordinate(xCoordinate, yCoordinate);
            if (tileAt(currentCoordinate))
                continue;
            ++requiredTileCount;
            // Distance is 0 for all currently visible tiles.
            double distance = tileDistance(visibleRect, currentCoordinate);
            if (distance > shortestDistance)
                continue;
            if (distance < shortestDistance) {
                tilesToCreate.clear();
                shortestDistance = distance;
            }
            tilesToCreate.append(currentCoordinate);
        }
    }
    
    // Now construct the tile(s)
    unsigned tilesToCreateCount = tilesToCreate.size();
    for (unsigned n = 0; n < tilesToCreateCount; ++n) {
        Tile::Coordinate coordinate = tilesToCreate[n];
        setTile(coordinate, Tile::create(this, coordinate));
    }
    requiredTileCount -= tilesToCreateCount;
    
    // Paint the content of the newly created tiles
    if (tilesToCreateCount)
        updateTileBuffers();

    // Keep creating tiles until the whole coverRect is covered.
    if (requiredTileCount)
        m_tileCreationTimer->startOneShot(m_tileCreationDelay);
}