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Flying Saucer is a CSS 2.1 renderer written in Java. This artifact contains the core rendering and layout code as well as Java2D output.
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/*
 * {{{ header & license
 * Copyright (c) 2007 Christophe Marchand
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 2.1
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 * }}}
 */

package org.xhtmlrenderer.swing;

import java.awt.*;
import java.awt.geom.AffineTransform;
import java.awt.print.PrinterGraphics;
import java.io.InputStream;
import java.util.ArrayList;

import org.w3c.dom.Document;
import org.xhtmlrenderer.extend.UserAgentCallback;
import org.xhtmlrenderer.layout.Layer;
import org.xhtmlrenderer.layout.PaintingInfo;
import org.xhtmlrenderer.render.Box;
import org.xhtmlrenderer.render.RenderingContext;
import org.xhtmlrenderer.simple.XHTMLPanel;

/**
 * ScalableXHTMLPanel extends {@see XHTMLPanel} to allow zoom on output.
 *
 * @author chm
 */
public class ScalableXHTMLPanel extends XHTMLPanel {

	public static final int SCALE_POLICY_NONE = 0;
	public static final int SCALE_POLICY_FIT_WIDTH = 0x01;
	public static final int SCALE_POLICY_FIT_HEIGHT = 0x02;
	public static final int SCALE_POLICY_FIT_WHOLE = SCALE_POLICY_FIT_WIDTH + SCALE_POLICY_FIT_HEIGHT;

	private static final long serialVersionUID = 1L;

	private int scalePolicy = SCALE_POLICY_NONE;
	private double scale = -1.0d;
	private ArrayList scListeners = null;
	/**
	 * The lastly calculated layout size
	 */
	private Dimension lastLayoutSize = null;

	/**
	 * Instantiates an XHTMLPanel with no {@link Document} loaded by default.
	 */
	public ScalableXHTMLPanel() {
		super();
		scListeners = new ArrayList();
	}

	/**
	 * Instantiates a panel with a custom {@link org.xhtmlrenderer.extend.UserAgentCallback}
	 * implementation.
	 *
	 * @param uac The custom UserAgentCallback implementation.
	 */
	public ScalableXHTMLPanel(UserAgentCallback uac) {
		super(uac);
		scListeners = new ArrayList();
	}

	/**
	 * Renders a Document using a URL as a base URL for relative
	 * paths.
	 *
	 * @param doc The new document value
	 * @param url The new document value
	 */
	public void setDocument(Document doc, String url) {
		resetScaleAccordingToPolicy();
		lastLayoutSize = null;
		super.setDocument(doc, url);
	}

	/**
	 * Renders a Document read from an InputStream using a URL
	 * as a base URL for relative paths.
	 *
	 * @param stream The stream to read the Document from.
	 * @param url	The URL used to resolve relative path references.
	 */
	public void setDocument(InputStream stream, String url) throws Exception {
		resetScaleAccordingToPolicy();
		lastLayoutSize = null;
		super.setDocument(stream, url);
	}

	private void resetScaleAccordingToPolicy() {
		if (getScalePolicy() != SCALE_POLICY_NONE) scale = -1.0d;
	}

	/**
	 * Search Box according to scale factor
	 *
	 * @param x The displayed x position
	 * @param y the displayed y position
	 */
	public Box find(int x, int y) {
		Point p = convertFromScaled(x, y);
		Layer l = getRootLayer();
		if (l != null) {
			return l.find(getLayoutContext(), p.x, p.y, false);
		}
		return null;
	}

	/**
	 * Force scale to use
	 *
	 * @param newScale The scale to use
	 * @throws IllegalArgumentException If newScale <= 0.0d.
	 */
	public void setScale(double newScale) throws IllegalArgumentException {
		if (newScale <= 0.0d) throw new IllegalArgumentException("Only positive scales are allowed.");
		this.scale = newScale;
		scalePolicy = SCALE_POLICY_NONE;
		lastLayoutSize = null;
		repaint(getFixedRectangle());
		scaleChanged();
	}

	public double getScale() {
		return scale;
	}

	public void addScaleChangeListener(ScaleChangeListener scl) {
		scListeners.add(scl);
	}

	public void removeScaleChangeListener(ScaleChangeListener scl) {
		scListeners.remove(scl);
	}

	private void scaleChanged() {
		ScaleChangeEvent evt = new ScaleChangeEvent(this, scale);
		for (int i = 0; i < scListeners.size(); i++) {
			ScaleChangeListener scl = (ScaleChangeListener) scListeners.get(i);
			scl.scaleChanged(evt);
		}
	}

	/**
	 * Renders according to scale factor
	 *
	 * @param c	the RenderingContext to use
	 * @param root The Layer to render
	 */
	protected void doRender(RenderingContext c, Layer root) {
		Graphics2D g = ((Java2DOutputDevice) c.getOutputDevice()).getGraphics();
		if (!(g instanceof PrinterGraphics) && isOpaque()) {
			g.setColor(getBackground());
			g.fillRect(0, 0, getWidth(), getHeight());
		}
		AffineTransform current = g.getTransform();

		PaintingInfo pI = root.getMaster().getPaintingInfo();
		Dimension layoutSize = pI.getOuterMarginCorner();

		calculateScaleAccordingToPolicy(layoutSize);

		if (lastLayoutSize == null) {
			lastLayoutSize = layoutSize;
			setPreferredSize(new Dimension((int) (lastLayoutSize.width * scale), (int) (lastLayoutSize.height * scale)));
            revalidate();
		}

		g.transform(AffineTransform.getScaleInstance(scale, scale));
		super.doRender(c, root);
		g.setTransform(current);
	}

	protected void calculateScaleAccordingToPolicy(Dimension layoutSize) {
		Rectangle viewportBounds = getFixedRectangle();
		if (getScalePolicy() == SCALE_POLICY_NONE) {
            // FIXME: float comparison
			if (scale == -1.0d) scale = 1.0d;
			return;
		}
		double xScale, yScale;
		if (viewportBounds.width < layoutSize.width) {
			xScale = (double) viewportBounds.width / layoutSize.width;
		} else {
			xScale = 1.0d;
		}
		if (viewportBounds.height < layoutSize.height) {
			yScale = (double) viewportBounds.height / layoutSize.height;
		} else {
			yScale = 1.0d;
		}
		if (getScalePolicy() == SCALE_POLICY_FIT_WIDTH) scale = xScale;
		else if (getScalePolicy() == SCALE_POLICY_FIT_HEIGHT) scale = yScale;
		else scale = Math.min(xScale, yScale);
	}

	protected Point convertToScaled(Point origin) {
		if (scale <= 0.0d) return origin;
		return new Point((int) (origin.x * scale), (int) (origin.y * scale));
	}

	protected Point convertFromScaled(Point origin) {
		if (scale <= 0.0d) return origin;
		return new Point((int) (origin.x / scale), (int) (origin.y / scale));
	}

	protected Point convertToScaled(int x, int y) {
		if (scale <= 0.0d) return new Point(x, y);
		return new Point((int) (x * scale), (int) (y * scale));
	}

	protected Point convertFromScaled(int x, int y) {
		if (scale <= 0.0d) return new Point(x, y);
		return new Point((int) (x / scale), (int) (y / scale));
	}

	public int getScalePolicy() {
		return scalePolicy;
	}

	public void setScalePolicy(int scalePolicy) {
		this.scalePolicy = scalePolicy;
		lastLayoutSize = null;
		repaint(getFixedRectangle());
	}
}