code: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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.apache.commons.lang;
  
  import java.util.Random;
  /**
   * <p>Operations for random <code>String</code>s.</p>
   * <p>Currently <em>private high surrogate</em> characters are ignored. 
   * These are unicode characters that fall between the values 56192 (db80)
   * and 56319 (dbff) as we don't know how to handle them. 
   * High and low surrogates are correctly dealt with - that is if a 
   * high surrogate is randomly chosen, 55296 (d800) to 56191 (db7f) 
   * then it is followed by a low surrogate. If a low surrogate is chosen, 
   * 56320 (dc00) to 57343 (dfff) then it is placed after a randomly 
   * chosen high surrogate. </p>
   *
   * @author <a href="mailto:steven@caswell.name">Steven Caswell</a>
   * @author Stephen Colebourne
   * @author Gary Gregory
   * @author Phil Steitz
   * @since 1.0
   * @version $Id: RandomStringUtils.java 471626 2006-11-06 04:02:09Z bayard $
   */
  public class RandomStringUtils {
  
      /**
       * <p>Random object used by random method. This has to be not local
       * to the random method so as to not return the same value in the 
       * same millisecond.</p>
       */
      private static final Random RANDOM = new Random();
  
      /**
       * <p><code>RandomStringUtils</code> instances should NOT be constructed in
       * standard programming. Instead, the class should be used as
       * <code>RandomStringUtils.random(5);</code>.</p>
       *
       * <p>This constructor is public to permit tools that require a JavaBean instance
       * to operate.</p>
       */
      public RandomStringUtils() {
        super();
      }
  
      // Random
      //-----------------------------------------------------------------------
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of all characters.</p>
       *
       * @param count  the length of random string to create
       * @return the random string
       */
      public static String random(int count) {
          return random(count, false, false);
      }
  
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of characters whose
       * ASCII value is between <code>32</code> and <code>126</code> (inclusive).</p>
       *
       * @param count  the length of random string to create
       * @return the random string
       */
      public static String randomAscii(int count) {
          return random(count, 32, 127, false, false);
      }
      
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of alphabetic
       * characters.</p>
       *
       * @param count  the length of random string to create
       * @return the random string
       */
      public static String randomAlphabetic(int count) {
          return random(count, true, false);
      }
      
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of alpha-numeric
       * characters.</p>
       *
       * @param count  the length of random string to create
       * @return the random string
       */
      public static String randomAlphanumeric(int count) {
          return random(count, true, true);
      }
      
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of numeric
       * characters.</p>
       *
       * @param count  the length of random string to create
       * @return the random string
       */
      public static String randomNumeric(int count) {
          return random(count, false, true);
      }
  
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of alpha-numeric
       * characters as indicated by the arguments.</p>
       *
       * @param count  the length of random string to create
       * @param letters  if <code>true</code>, generated string will include
       *  alphabetic characters
       * @param numbers  if <code>true</code>, generated string will include
       *  numeric characters
       * @return the random string
       */
      public static String random(int count, boolean letters, boolean numbers) {
          return random(count, 0, 0, letters, numbers);
      }
      
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of alpha-numeric
       * characters as indicated by the arguments.</p>
       *
       * @param count  the length of random string to create
       * @param start  the position in set of chars to start at
       * @param end  the position in set of chars to end before
       * @param letters  if <code>true</code>, generated string will include
       *  alphabetic characters
       * @param numbers  if <code>true</code>, generated string will include
       *  numeric characters
       * @return the random string
       */
      public static String random(int count, int start, int end, boolean letters, boolean numbers) {
          return random(count, start, end, letters, numbers, null, RANDOM);
      }
  
      /**
       * <p>Creates a random string based on a variety of options, using
       * default source of randomness.</p>
       *
       * <p>This method has exactly the same semantics as
       * {@link #random(int,int,int,boolean,boolean,char[],Random)}, but
       * instead of using an externally supplied source of randomness, it uses
       * the internal static {@link Random} instance.</p>
       *
       * @param count  the length of random string to create
       * @param start  the position in set of chars to start at
       * @param end  the position in set of chars to end before
       * @param letters  only allow letters?
       * @param numbers  only allow numbers?
       * @param chars  the set of chars to choose randoms from.
       *  If <code>null</code>, then it will use the set of all chars.
       * @return the random string
       * @throws ArrayIndexOutOfBoundsException if there are not
       *  <code>(end - start) + 1</code> characters in the set array.
       */
      public static String random(int count, int start, int end, boolean letters, boolean numbers, char[] chars) {
          return random(count, start, end, letters, numbers, chars, RANDOM);
      }
  
      /**
       * <p>Creates a random string based on a variety of options, using
       * supplied source of randomness.</p>
       *
       * <p>If start and end are both <code>0</code>, start and end are set
       * to <code>' '</code> and <code>'z'</code>, the ASCII printable
       * characters, will be used, unless letters and numbers are both
       * <code>false</code>, in which case, start and end are set to
       * <code>0</code> and <code>Integer.MAX_VALUE</code>.
       *
       * <p>If set is not <code>null</code>, characters between start and
       * end are chosen.</p>
       *
       * <p>This method accepts a user-supplied {@link Random}
       * instance to use as a source of randomness. By seeding a single 
       * {@link Random} instance with a fixed seed and using it for each call,
       * the same random sequence of strings can be generated repeatedly
       * and predictably.</p>
       *
       * @param count  the length of random string to create
       * @param start  the position in set of chars to start at
       * @param end  the position in set of chars to end before
       * @param letters  only allow letters?
       * @param numbers  only allow numbers?
       * @param chars  the set of chars to choose randoms from.
       *  If <code>null</code>, then it will use the set of all chars.
       * @param random  a source of randomness.
       * @return the random string
       * @throws ArrayIndexOutOfBoundsException if there are not
       *  <code>(end - start) + 1</code> characters in the set array.
       * @throws IllegalArgumentException if <code>count</code> &lt; 0.
       * @since 2.0
       */
      public static String random(int count, int start, int end, boolean letters, boolean numbers,
                                  char[] chars, Random random) {
          if (count == 0) {
              return "";
          } else if (count < 0) {
              throw new IllegalArgumentException("Requested random string length " + count + " is less than 0.");
          }
          if ((start == 0) && (end == 0)) {
              end = 'z' + 1;
              start = ' ';
              if (!letters && !numbers) {
                  start = 0;
                  end = Integer.MAX_VALUE;
              }
          }
  
          char[] buffer = new char[count];
          int gap = end - start;
  
          while (count-- != 0) {
              char ch;
              if (chars == null) {
                  ch = (char) (random.nextInt(gap) + start);
              } else {
                  ch = chars[random.nextInt(gap) + start];
              }
              if ((letters && Character.isLetter(ch))
                  || (numbers && Character.isDigit(ch))
                  || (!letters && !numbers)) 
              {
                  if(ch >= 56320 && ch <= 57343) {
                      if(count == 0) {
                          count++;
                      } else {
                          // low surrogate, insert high surrogate after putting it in
                          buffer[count] = ch;
                          count--;
                          buffer[count] = (char) (55296 + random.nextInt(128));
                      }
                  } else if(ch >= 55296 && ch <= 56191) {
                      if(count == 0) {
                          count++;
                      } else {
                          // high surrogate, insert low surrogate before putting it in
                          buffer[count] = (char) (56320 + random.nextInt(128));
                          count--;
                          buffer[count] = ch;
                      }
                  } else if(ch >= 56192 && ch <= 56319) {
                      // private high surrogate, no effing clue, so skip it
                      count++;
                  } else {
                      buffer[count] = ch;
                  }
              } else {
                  count++;
              }
          }
          return new String(buffer);
      }
  
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of characters
       * specified.</p>
       *
       * @param count  the length of random string to create
       * @param chars  the String containing the set of characters to use,
       *  may be null
       * @return the random string
       * @throws IllegalArgumentException if <code>count</code> &lt; 0.
       */
      public static String random(int count, String chars) {
          if (chars == null) {
              return random(count, 0, 0, false, false, null, RANDOM);
          }
          return random(count, chars.toCharArray());
      }
  
      /**
       * <p>Creates a random string whose length is the number of characters
       * specified.</p>
       *
       * <p>Characters will be chosen from the set of characters specified.</p>
       *
       * @param count  the length of random string to create
       * @param chars  the character array containing the set of characters to use,
       *  may be null
       * @return the random string
       * @throws IllegalArgumentException if <code>count</code> &lt; 0.
       */
      public static String random(int count, char[] chars) {
          if (chars == null) {
              return random(count, 0, 0, false, false, null, RANDOM);
          }
          return random(count, 0, chars.length, false, false, chars, RANDOM);
      }
      
  }
  