package com.skatestown.invoice;

import java.io.File;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.CharacterData;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;

/**
 * Check Skatestown invoice totals using a DOM parser
 */
 
 public class InvoiceCheckerDOM implements InvoiceChecker 
   {
	   /**
	    * Check invoice totals
		* @param inoviceXML Invoice XML document
		* @exception Exception any exception returned during processing
		*/
		public void checkInvoice(File invoiceXML) throws Exception {
			// Invoice running total
			double runningTotal = 0.0;
			
			// Obtain parser instance and parse the document
			DocumentBuilderFactory factory = 
			    DocumentBuilderFactory.newInstance();
			DocumentBuilder builder = factory.newDocumentBuilder();
			Document doc = builder.parse(invoiceXML);
			
			// Calculate order subtotal
			NodeList itemList = doc.getElementsByTagName("item");
			for (int i=0; i<itemList.getLength(); i++) {
				// Extract quantity and price
				Element item = (Element) itemList.item(i);
				Integer qty = Integer.valueOf(item.getAttribute("quantity"));
				Double price = Double.valueOf(item.getAttribute("unitPrice"));
				// Add subtotal to running total
				runningTotal += qty.intValue() * price.doubleValue();
			}
			
			
			// Add tax
			Node nodeTax = doc.getElementsByTagName("tax").item(0);
			runningTotal += doubleValue(nodeTax);
			
			// Add shipping and handling
			Node nodeShippingAndHandling =
			    doc.getElementsByTagName("shippingAndHandling").item(0);
				
			runningTotal += doubleValue(nodeShippingAndHandling);
			
			// Get invoice total
			Node nodeTotalCost = 
			    doc.getElementsByTagName("totalCost").item(0);
			double total = doubleValue(nodeTotalCost);
			
			// Use delta equality to prevent cumulative binary arithmetic errors. 
			// Choose delta = 1/2 of one cent
			if (Math.abs(runningTotal - total) >= .005) {
				throw new Exception(
				  "Invoice error: total is " + Double.toString(total) + 
				  " while our calculation shows a total of " + 
				  Double.toString(Math.round(runningTotal * 100) / 100.0));
				  
			}		
			
	}
	
	/** 
	 * Extract a double value from the text of a DOM node
	 * @param node A DOM node with character content
	 * @return the double representation of the node's content
	 * @exception Exception could be the result of either a node
	 * that doesn't have text content or a node whose text content
	 * is not a number. 
	 */ 
	 
	 private double doubleValue(Node node) throws Exception {
		 // Get the character data from the node and parse it
		 String value = ((CharacterData) node.getFirstChild()).getData();
		 return Double.valueOf(value).doubleValue();
	 }
}