package com.atlassian.plugin.servlet.util;
   
   import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;
   import java.util.Date;
   
   /**
    * This class manages the last modified date of a single HTTP resource.
    */
  public class LastModifiedHandler {
      private long lastModified;
      private String etag;
      private static final int ONE_SECOND_MILLIS = 1000;
  
      public LastModifiedHandler() {
          modified(new Date());
      }
  
      public LastModifiedHandler(Date lastModifiedDate) {
          modified(lastModifiedDate);
      }
  
      /**
       * Check whether we need to generate a response for this request. Set the necessary headers on the response, and if
       * we don't need to provide content, set the response status to 304.
       *
       * If this method returns true, the caller should not perform any more processing on the request.
       *
       * @return true if we don't need to provide any data to satisfy this request
       */
      public boolean checkRequest(HttpServletRequest request, HttpServletResponse response) {
          return checkRequest(request, response, lastModified, etag);
      }
  
      /**
       * The content has changed, reset the modified date and the etag
       */
      public void modified() {
          modified(new Date());
      }
  
      private void modified(Date date) {
          lastModified = calculateLastModifiedDate(date);
          etag = calculateEtag(lastModified);
      }
  
      private static long calculateLastModifiedDate(Date lastModifiedDate) {
          long lastModified = lastModifiedDate.getTime();
          // resolution of 1 second
          lastModified -= lastModified % ONE_SECOND_MILLIS;
          return lastModified;
      }
  
      private static String calculateEtag(long lastModified) {
          return "\"" + lastModified + "\"";
      }
  
      /**
       * This static method is used when the resource being served by the servlet keeps track of the last modified date,
       * and so no state needs to be maintained by this handler.
       */
      public static boolean checkRequest(HttpServletRequest request, HttpServletResponse response, Date lastModifiedDate) {
          long lastModified = calculateLastModifiedDate(lastModifiedDate);
          return checkRequest(request, response, lastModified, calculateEtag(lastModified));
      }
  
      private static boolean checkRequest(HttpServletRequest request, HttpServletResponse response, long lastModified, String etagString) {
          if ("true".equals(System.getProperty("atlassian.disable.caches", "false")))
              return false;
  
          response.setDateHeader("Last-Modified", lastModified);
          response.setHeader("ETag", etagString);
  
          long ifModifiedSince = request.getDateHeader("If-Modified-Since");
          String ifNoneMatch = request.getHeader("If-None-Match");
          if (noConditionalGetHeadersFound(ifModifiedSince, ifNoneMatch)
                  || isContentModifiedSince(ifModifiedSince, lastModified)
                  || !etagMatches(ifNoneMatch, etagString)) {
              return false;
          }
          response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
          return true;
      }
  
      private static boolean etagMatches(String ifNoneMatch, String etagString) {
          return ifNoneMatch != null && ifNoneMatch.equals(etagString);
      }
  
      private static boolean isContentModifiedSince(long ifModifiedSince, long lastModified) {
          return ifModifiedSince != -1 && ifModifiedSince < lastModified;
      }
  
      private static boolean noConditionalGetHeadersFound(long ifModifiedSince, String ifNoneMatch) {
          return ifModifiedSince == -1 && ifNoneMatch == null;
      }
  }