content: /*
   * $Header$
   * $Revision$
   * $Date$
   *
   * ====================================================================
   *
   * Copyright 2000-2002 bob mcwhirter & James Strachan.
   * All rights reserved.
   *
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are
   * met:
   * 
   *   * Redistributions of source code must retain the above copyright
   *     notice, this list of conditions and the following disclaimer.
   * 
   *   * Redistributions in binary form must reproduce the above copyright
   *     notice, this list of conditions and the following disclaimer in the
   *     documentation and/or other materials provided with the distribution.
   * 
   *   * Neither the name of the Jaxen Project nor the names of its
   *     contributors may be used to endorse or promote products derived 
   *     from this software without specific prior written permission.
   * 
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
   * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
   * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
   * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
   * OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
   * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   *
   * ====================================================================
   * This software consists of voluntary contributions made by many
   * individuals on behalf of the Jaxen Project and was originally
   * created by bob mcwhirter <bob@werken.com> and
   * James Strachan <jstrachan@apache.org>.  For more information on the
   * Jaxen Project, please see <http://www.jaxen.org/>.
   *
   */
  package org.jaxen.function;
  
  import java.util.List;
  
  import org.jaxen.Context;
  import org.jaxen.Function;
  import org.jaxen.FunctionCallException;
  import org.jaxen.Navigator;
  /**
   * <p>
   * <b>4.2</b>
   * <code><i>string</i> substring(<i>string</i>,<i>number</i>,<i>number?</i>)</code>
   * </p>
   * 
   * <blockquote src="http://www.w3.org/TR/xpath"> 
   * <p>The <b>substring</b> function returns the
   * substring of the first argument starting at the position specified in
   * the second argument with length specified in the third argument. For
   * example,
   * 
   * <code>substring("12345",2,3)</code> returns <code>"234"</code>.
   * If the third argument is not specified, it returns the substring
   * starting at the position specified in the second argument and
   * continuing to the end of the string. For example,
   * <code>substring("12345",2)</code> returns <code>"2345"</code>.
   * </p>
   * 
   * <p>
   * More precisely, each character in the string (see <a
   * href="http://www.w3.org/TR/xpath#strings">[<b>3.6 Strings</b>]</a>) is considered to have a
   * numeric position: the position of the first character is 1, the
   * position of the second character is 2 and so on.
   * </p>
   * 
   * <blockquote> <b>NOTE: </b>This differs from Java and ECMAScript, in
   * which the <code>String.substring</code> method treats the position
   * of the first character as 0.</blockquote>
   * 
   * <p>
   * The returned substring contains those characters for which the
   * position of the character is greater than or equal to the rounded
   * value of the second argument and, if the third argument is specified,
   * less than the sum of the rounded value of the second argument and the
   * rounded value of the third argument; the comparisons and addition
   * used for the above follow the standard IEEE 754 rules; rounding is
   * done as if by a call to the <b><a href="#function-round">round</a></b>
   * function. The following examples illustrate various unusual cases:
   * </p>
   * 
   * <ul>
   * 
   * <li>
   * <p>
   * <code>substring("12345", 1.5, 2.6)</code> returns
   * <code>"234"</code>
   * </p>
   * </li>
   * 
   * <li>
   * <p>
   * <code>substring("12345", 0, 3)</code> returns <code>"12"</code>
   * 
   * </p>
   * </li>
   * 
   * <li>
   * <p>
   * <code>substring("12345", 0 div 0, 3)</code> returns <code>""</code>
   * </p>
   * </li>
   * 
   * <li>
   * <p>.
   * <code>substring("12345", 1, 0 div 0)</code> returns
   * 
   * <code>""</code>
   * </p>
   * </li>
   * 
   * <li>
   * <p>
   * <code>substring("12345", -42, 1 div 0)</code> returns
   * <code>"12345"</code>
   * </p>
   * </li>
   * 
   * <li>
   * <p>
   * 
   * <code>substring("12345", -1 div 0, 1 div 0)</code> returns
   * <code>""</code> </blockquote>
   * 
   * @author bob mcwhirter (bob @ werken.com)
   * 
   * @see <a href="http://www.w3.org/TR/xpath#function-substring"
   *      target="_top">Section 4.2 of the XPath Specification</a>
   */
  public class SubstringFunction implements Function
  {
  
      /**
       * Create a new <code>SubstringFunction</code> object.
       */
      public SubstringFunction() {}
  
      
      /** Returns a substring of an XPath string-value by character index.
       *
       * @param context the context at the point in the
       *         expression when the function is called
       * @param args a list that contains two or three items
       * 
       * @return a <code>String</code> containing the specifed character subsequence of 
       *     the original string or the string-value of the context node
       * 
       * @throws FunctionCallException if <code>args</code> has more than three
       *     or less than two items
       */
      public Object call(Context context,
                         List args) throws FunctionCallException
      {
          final int argc = args.size();
          if (argc < 2 || argc > 3){
              throw new FunctionCallException( "substring() requires two or three arguments." );
          }
  
          final Navigator nav = context.getNavigator();
  
          final String str = StringFunction.evaluate(args.get(0), nav );
          // The spec doesn't really address this case
          if (str == null) {
              return "";
          }
  
          final int stringLength = (StringLengthFunction.evaluate(args.get(0), nav )).intValue();
  
          if (stringLength == 0) {
              return "";
          }
  
          Double d1 = NumberFunction.evaluate(args.get(1), nav);
  
          if (d1.isNaN()){
              return "";
          }
          // Round the value and subtract 1 as Java strings are zero based
          int start = RoundFunction.evaluate(d1, nav).intValue() - 1;
  
          int substringLength = stringLength;
          if (argc == 3){
              Double d2 = NumberFunction.evaluate(args.get(2), nav);
  
              if (!d2.isNaN()){
                  substringLength = RoundFunction.evaluate(d2, nav ).intValue();
              }
              else {
                  substringLength = 0;
              }
          }
          
          if (substringLength < 0) return "";
  
          int end = start + substringLength;
          if (argc == 2) end = stringLength;
              
          // negative start is treated as 0
          if ( start < 0){
              start = 0;
          }
          else if (start > stringLength){
              return "";
          }
  
          if (end > stringLength){
              end = stringLength;
          }
          else if (end < start) return "";
          
          if (stringLength == str.length()) {
              // easy case; no surrogate pairs
              return str.substring(start, end);
          }
          else {
              return unicodeSubstring(str, start, end);
          }
          
      }
  
      private static String unicodeSubstring(String s, int start, int end) {
  
          StringBuffer result = new StringBuffer(s.length());
          for (int jChar = 0, uChar=0; uChar < end; jChar++, uChar++) {
              char c = s.charAt(jChar);
              if (uChar >= start) result.append(c);
              if (c >= 0xD800) { // get the low surrogate
                  // ???? we could check here that this is indeed a low surroagte
                  // we could also catch StringIndexOutOfBoundsException
                  jChar++;
                  if (uChar >= start) result.append(s.charAt(jChar));
              }
          }
          return result.toString();
      }
  }
  