/*
 * Copyright 2011 Hao Nguyen
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package gwt.g2d.client.graphics;

import com.google.gwt.canvas.dom.client.Context2d;

/**
 * Helper class for drawing dashed line.
 * Reference: http://davidowens.wordpress.com/2010/09/07/html-5-canvas-and-dashed-lines/
 * 
 * @author hao1300@gmail.com
 */
public class DashedLineRenderer {	
	
	/**
	 * Draw a dashed line from (fromX, fromY) to (toX, toY).
	 * 
	 * @param context
	 * @param fromX x-coordinate of the starting point
	 * @param fromY y-coordinate of the starting point
	 * @param toX x-coordinate of the ending point
	 * @param toY y-coordinate of the ending point
	 * @param dashLength length of the dash
	 * @param gapLength length of the gap in between dashes
	 */
	public static void drawDashedLine(Context2d context, double fromX, double fromY, 
			double toX, double toY, double dashLength, double gapLength) {
	  DashedLineHelper checkX = GreaterThanHelper.instance;
	  DashedLineHelper checkY = GreaterThanHelper.instance;

	  if (fromY - toY > 0) {
	    checkY = LessThanHelper.instance;
	  }
	  if (fromX - toX > 0) {
	    checkX = LessThanHelper.instance;
	  }

	  context.moveTo(fromX, fromY);
	  double offsetX = fromX;
	  double offsetY = fromY;
	  boolean dash = true;
	  double ang = Math.atan2(toY - fromY, toX - fromX);
	  while (!(checkX.isThereYet(offsetX, toX) && checkY.isThereYet(offsetY, toY))) {
	  	double len = (dash) ? dashLength : gapLength;
	  	
	    offsetX = checkX.getCap(toX, offsetX + (Math.cos(ang) * len));
	    offsetY = checkY.getCap(toY, offsetY + (Math.sin(ang) * len));

	    if (dash) {
	    	context.lineTo(offsetX, offsetY);
	    } else {
	    	context.moveTo(offsetX, offsetY);
	    }
	    dash = !dash;
	  }
	}
	
	/**
	 * Helper class for checking the dash line.
	 */
	private interface DashedLineHelper {
		
		/**
		 * Checks whether the point has been reached yet.
		 * 
		 * @param from
		 * @param to
		 * @return
		 */
		boolean isThereYet(double from, double to);
		
		/**
		 * Gets the cap of the two values. If the line is increasing, this will
		 * return max(v1, v2). If the line is decreasing, this will return
		 * min(v1, v2).
		 * 
		 * @param v1
		 * @param v2
		 * @return
		 */
		double getCap(double v1, double v2);
	}
	
	/**
	 * Helper for a decreasing line.
	 */
	private static class LessThanHelper implements DashedLineHelper {
		private static DashedLineHelper instance = new LessThanHelper();
		
		@Override
		public double getCap(double v1, double v2) {
			return Math.max(v1, v2);
		}

		@Override
		public boolean isThereYet(double from, double to) {
			return from <= to;
		}		
	}
	
	/**
	 * Helper for an increasing line.
	 */
	private static class GreaterThanHelper implements DashedLineHelper {
		private static DashedLineHelper instance = new GreaterThanHelper();
		
		@Override
		public double getCap(double v1, double v2) {
			return Math.min(v1, v2);
		}

		@Override
		public boolean isThereYet(double from, double to) {
			return from >= to;
		}		
	}
}