/*
    * Copyright 2008 Google Inc.
    * 
    * 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 org.vectomatic.dom.svg.impl;
  
  import org.vectomatic.dom.svg.OMSVGSVGElement;
  import org.vectomatic.dom.svg.ui.ExternalSVGResource;
  import org.vectomatic.dom.svg.ui.SVGResource;
  import org.vectomatic.dom.svg.utils.OMSVGParser;
  
  import com.google.gwt.core.client.JavaScriptObject;
  import com.google.gwt.http.client.Request;
  import com.google.gwt.http.client.RequestBuilder;
  import com.google.gwt.http.client.RequestCallback;
  import com.google.gwt.http.client.RequestException;
  import com.google.gwt.http.client.Response;
  import com.google.gwt.resources.client.ResourceCallback;
  import com.google.gwt.resources.client.ResourceException;
  
  /**
   * Implementation of ExternalSVGResource derived from Google's original
   * ExternalTextResourcePrototype implementation
   * @author laaglu
   */
  public class ExternalSVGResourcePrototype implements ExternalSVGResource {
  
  	/**
  	 * Maps the HTTP callback onto the ResourceCallback.
  	 */
  	private class ESRCallback implements RequestCallback {
  		final ResourceCallback<SVGResource> callback;
  
  		public ESRCallback(ResourceCallback<SVGResource> callback) {
  			this.callback = callback;
  		}
  
  		public void onError(Request request, Throwable exception) {
  			callback.onError(new ResourceException(
  					ExternalSVGResourcePrototype.this,
  					"Unable to retrieve external resource", exception));
  		}
  
  		public void onResponseReceived(Request request, final Response response) {
  			// Get the contents of the JSON bundle
  			String responseText = response.getText();
  
  			// Call eval() on the object.
  			JavaScriptObject jso = evalObject(responseText);
  			if (jso == null) {
  				callback.onError(new ResourceException(
  						ExternalSVGResourcePrototype.this,
  						"eval() returned null"));
  				return;
  			}
  
  			// Populate the TextResponse cache array
  			for (int i = 0; i < cache.length; i++) {
  				final String resourceText = extractString(jso, i);
  				cache[i] = new SVGResource() {
  
  					public String getName() {
  						return name;
  					}
  
  					public OMSVGSVGElement getSvg() {
  						return OMSVGParser.parse(resourceText);
  					}
  
  				};
  			}
  
  			// Finish by invoking the callback
  			callback.onSuccess(cache[index]);
  		}
  	}
  
  	/**
  	 * Evaluate the JSON payload. The regular expression to validate the safety
  	 * of the payload is taken from RFC 4627 (D. Crockford).
  	 * 
  	 * @param data
  	 *            the raw JSON-encapsulated string bundle
  	 * @return the evaluated JSON object, or <code>null</code> if there is an
  	 *         error.
  	 */
  	private static native JavaScriptObject evalObject(String data) /*-{
  	    var safe = !(/[^,:{}\[\]0-9.\-+Eaeflnr-u \n\r\t]/.test(
 	      data.replace(/"(\\.|[^"\\])*"/g, '')));
 
 	    if (!safe) {
 	      return null;
 	    }
 
 	    return eval('(' + data + ')') || null;
 	  }-*/;
 
 	/**
 	 * Extract the specified String from a JavaScriptObject that is array-like.
 	 * @param jso
 	 * the JavaScriptObject returned from {@link #evalObject(String)}
 	 * @param index
 	 * the index of the string to extract
 	 * @return the requested string, or <code>null</code> if it does not exist.
 	 */
 	private static native String extractString(JavaScriptObject jso, int index) /*-{
 	    return (jso.length > index) && jso[index] || null;
 	  }-*/;
 
 	/**
 	 * This is a reference to an array nominally created in the IRB that
 	 * contains the ExternalSVGResource. It is intended to be shared between
 	 * all instances of the ETR that have a common parent IRB.
 	 */
 	private final SVGResource[] cache;
 	private final int index;
 	private final String name;
 	private final String url;
 
 	/**
 	 * Constructor
 	 * @param name The resource name
 	 * @param url The resource URL
 	 * @param cache A cache of loaded resource
 	 * @param index The index for this resource in the cache
 	 */
 	public ExternalSVGResourcePrototype(String name, String url,
 			SVGResource[] cache, int index) {
 		this.name = name;
 		this.url = url;
 		this.cache = cache;
 		this.index = index;
 	}
 
 	/**
 	 * Returns the SVG resource name
 	 * @return the SVG resource name
 	 */
 	public String getName() {
 		return name;
 	}
 
 	/**
 	 * Possibly fire off an HTTPRequest for the SVG resource.
 	 * @param callback The request callback
 	 */
 	public void getSvg(ResourceCallback<SVGResource> callback)
 			throws ResourceException {
 
 		// If we've already parsed the JSON bundle, short-circuit.
 		if (cache[index] != null) {
 			callback.onSuccess(cache[index]);
 			return;
 		}
 
 		// Otherwise, fire an HTTP request.
 		RequestBuilder rb = new RequestBuilder(RequestBuilder.GET, url);
 		try {
 			rb.sendRequest("", new ESRCallback(callback));
 		} catch (RequestException e) {
 			throw new ResourceException(this,
 					"Unable to initiate request for external resource", e);
 		}
 	}
 }