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    GNU Libidn is a fully documented implementation of the Stringprep, Punycode and IDNA specifications. Libidn's purpose is to encode and decode internationalized domain names.

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    /**
 * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009  Free Software Foundation, Inc.
 *
 * Author: Oliver Hitz
 *
 * This file is part of GNU Libidn.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301
 * USA
 */

package gnu.inet.encoding;

/**
 * This class offers static methods for preparing internationalized
 * strings. It supports the following stringprep profiles:
 * 
      
 * 
    • RFC3491 nameprep
 * 
    • RFC3920 XMPP nodeprep and resourceprep
 * 
    
 * Note that this implementation only supports 16-bit Unicode code
 * points.
 */
public class Stringprep
{
  /**
   * Preps a name according to the Stringprep profile defined in
   * RFC3491. Unassigned code points are not allowed.
   *
   * @param input the name to prep.
   * @return the prepped name.
   * @throws StringprepException If the name cannot be prepped with
   * this profile.
   * @throws NullPointerException If the name is null.
   */
  public static String nameprep(String input)
    throws StringprepException,
	   NullPointerException
  {
    return nameprep(input, false);
  }

  /**
   * Preps a name according to the Stringprep profile defined in
   * RFC3491.
   *
   * @param input the name to prep.
   * @param allowUnassigned true if the name may contain unassigned
   * code points.
   * @return the prepped name.
   * @throws StringprepException If the name cannot be prepped with
   * this profile.
   * @throws NullPointerException If the name is null.
   */
  public static String nameprep(String input, boolean allowUnassigned)
    throws StringprepException,
	   NullPointerException
  {
    if (input == null) {
      throw new NullPointerException();
    }

    StringBuffer s = new StringBuffer(input);

    if (!allowUnassigned && contains(s, RFC3454.A1)) {
      throw new StringprepException(StringprepException.CONTAINS_UNASSIGNED);
    }

    filter(s, RFC3454.B1);
    map(s, RFC3454.B2search, RFC3454.B2replace);

    s = new StringBuffer(NFKC.normalizeNFKC(s.toString()));
    // B.3 is only needed if NFKC is not used, right?
    // map(s, RFC3454.B3search, RFC3454.B3replace);

    if (contains(s, RFC3454.C12) ||
	contains(s, RFC3454.C22) ||
	contains(s, RFC3454.C3) ||
	contains(s, RFC3454.C4) ||
	contains(s, RFC3454.C5) ||
	contains(s, RFC3454.C6) ||
	contains(s, RFC3454.C7) ||
	contains(s, RFC3454.C8)) {
      // Table C.9 only contains code points > 0xFFFF which Java
      // doesn't handle
      throw new StringprepException(StringprepException.CONTAINS_PROHIBITED);
    }

    // Bidi handling
    boolean r = contains(s, RFC3454.D1);
    boolean l = contains(s, RFC3454.D2);

    // RFC 3454, section 6, requirement 1: already handled above (table C.8)

    // RFC 3454, section 6, requirement 2
    if (r && l) {
      throw new StringprepException(StringprepException.BIDI_BOTHRAL);
    }

    // RFC 3454, section 6, requirement 3
    if (r) {
      if (!contains(s.charAt(0), RFC3454.D1) ||
	  !contains(s.charAt(s.length()-1), RFC3454.D1)) {
	throw new StringprepException(StringprepException.BIDI_LTRAL);
      }
    }

    return s.toString();
  }

  /**
   * Characters prohibited by RFC3920 nodeprep that aren't defined as
   * part of the RFC3454 tables.
   */
  private static final char [] RFC3920_NODEPREP_PROHIBIT = new char [] {
    '\u0022', '\u0026', '\'',     '\u002F',
    '\u003A', '\u003C', '\u003E', '\u0040'
  };

  /**
   * Preps a node name according to the Stringprep profile defined in
   * RFC3920. Unassigned code points are not allowed.
   *
   * @param input the node name to prep.
   * @return the prepped node name.
   * @throws StringprepException If the node name cannot be prepped
   * with this profile.
   * @throws NullPointerException If the node name is null.
   */
  public static String nodeprep(String input)
    throws StringprepException,
	   NullPointerException
  {
    return nodeprep(input, false);
  }

  /**
   * Preps a node name according to the Stringprep profile defined in
   * RFC3920.
   *
   * @param input the node name to prep.
   * @param allowUnassigned true if the node name may contain
   * unassigned code points.
   * @return the prepped node name.
   * @throws StringprepException If the node name cannot be prepped
   * with this profile.
   * @throws NullPointerException If the node name is null.
   */
  public static String nodeprep(String input, boolean allowUnassigned)
    throws StringprepException,
	   NullPointerException
  {
    if (input == null) {
      throw new NullPointerException();
    }

    StringBuffer s = new StringBuffer(input);

    if (!allowUnassigned && contains(s, RFC3454.A1)) {
      throw new StringprepException(StringprepException.CONTAINS_UNASSIGNED);
    }

    filter(s, RFC3454.B1);
    map(s, RFC3454.B2search, RFC3454.B2replace);

    s = new StringBuffer(NFKC.normalizeNFKC(s.toString()));
    
    if (contains(s, RFC3454.C11) ||
	contains(s, RFC3454.C12) ||
	contains(s, RFC3454.C21) ||
	contains(s, RFC3454.C22) ||
	contains(s, RFC3454.C3) ||
	contains(s, RFC3454.C4) ||
	contains(s, RFC3454.C5) ||
	contains(s, RFC3454.C6) ||
	contains(s, RFC3454.C7) ||
	contains(s, RFC3454.C8) ||
	contains(s, RFC3920_NODEPREP_PROHIBIT)) {                           
      // Table C.9 only contains code points > 0xFFFF which Java
      // doesn't handle
      throw new StringprepException(StringprepException.CONTAINS_PROHIBITED);
    }

    // Bidi handling
    boolean r = contains(s, RFC3454.D1);
    boolean l = contains(s, RFC3454.D2);

    // RFC 3454, section 6, requirement 1: already handled above (table C.8)

    // RFC 3454, section 6, requirement 2
    if (r && l) {
      throw new	StringprepException(StringprepException.BIDI_BOTHRAL);
    }
    
    // RFC 3454, section 6, requirement 3
    if (r) {
      if (!contains(s.charAt(0), RFC3454.D1) ||
	  !contains(s.charAt(s.length() - 1), RFC3454.D1)) {
	throw new StringprepException(StringprepException.BIDI_LTRAL);
      }
    }
    
    return s.toString();
  }

  /**
   * Preps a resource name according to the Stringprep profile defined
   * in RFC3920. Unassigned code points are not allowed.
   *
   * @param input the resource name to prep.
   * @return the prepped node name.
   * @throws StringprepException If the resource name cannot be prepped
   * with this profile.
   * @throws NullPointerException If the resource name is null.
   */
  public static String resourceprep(String input)
    throws StringprepException,
	   NullPointerException
  {
    return resourceprep(input, false);
  }

  /**
   * Preps a resource name according to the Stringprep profile defined
   * in RFC3920.
   *
   * @param input the resource name to prep.
   * @param allowUnassigned true if the resource name may contain
   * unassigned code points.
   * @return the prepped node name.
   * @throws StringprepException If the resource name cannot be prepped
   * with this profile.
   * @throws NullPointerException If the resource name is null.
   */
  public static String resourceprep(String input, boolean allowUnassigned)
    throws StringprepException,
	   NullPointerException
  {
    if (input == null) {
      throw new NullPointerException();
    }

    StringBuffer s = new StringBuffer(input);
    
    if (!allowUnassigned && contains(s, RFC3454.A1)) {
      throw new StringprepException(StringprepException.CONTAINS_UNASSIGNED);
    }

    filter(s, RFC3454.B1);
    
    s = new StringBuffer(NFKC.normalizeNFKC(s.toString()));
    
    if (contains(s, RFC3454.C12) ||
	contains(s, RFC3454.C21) ||
	contains(s, RFC3454.C22) ||
	contains(s, RFC3454.C3) ||
	contains(s, RFC3454.C4) ||
	contains(s, RFC3454.C5) ||
	contains(s, RFC3454.C6) ||
	contains(s, RFC3454.C7) ||
	contains(s, RFC3454.C8)) {
      // Table C.9 only contains code points > 0xFFFF which Java
      // doesn't handle
      throw new StringprepException(StringprepException.CONTAINS_PROHIBITED);
    }
    
    // Bidi handling
    boolean r = contains(s, RFC3454.D1);
    boolean l = contains(s, RFC3454.D2);
    
    // RFC 3454, section 6, requirement 1: already handled above (table C.8)
    
    // RFC 3454, section 6, requirement 2
    if (r && l) {
      throw new	StringprepException(StringprepException.BIDI_BOTHRAL);
    }

    // RFC 3454, section 6, requirement 3
    if (r) {
      if (!contains(s.charAt(0), RFC3454.D1) ||
	  !contains(s.charAt(s.length() - 1), RFC3454.D1)) {
	throw new StringprepException(StringprepException.BIDI_LTRAL);
      }
    }
    
    return s.toString();
  }

  static boolean contains(StringBuffer s, char[] p)
  {
    for (int i = 0; i < p.length; i++) {
      char c = p[i];
      for (int j = 0; j < s.length(); j++) {
	if (c == s.charAt(j)) {
	  return true;
	}
      }
    }
    return false;
  }

  static boolean contains(StringBuffer s, char[][] p)
  {
    for (int i = 0; i < p.length; i++) {
      char[] r = p[i];
      if (1 == r.length) {
	char c = r[0];
	for (int j = 0; j < s.length(); j++) {
	  if (c == s.charAt(j)) {
	    return true;
	  }
	}
      } else if (2 == r.length) {
	char f = r[0];
	char t = r[1];
	for (int j = 0; j < s.length(); j++) {
	  if (f <= s.charAt(j) && t >= s.charAt(j)) {
	    return true;
	  }
	}
      }
    }
    return false;
  }

  static boolean contains(char c, char[][] p)
  {
    for (int i = 0; i < p.length; i++) {
      char[] r = p[i];
      if (1 == r.length) {
	if (c == r[0]) {
	  return true;
	}
      } else if (2 == r.length) {
	char f = r[0];
	char t = r[1];
	if (f <= c && t >= c) {
	  return true;
	}
      }
    }
    return false;
  }

  static void filter(StringBuffer s, char[] f)
  {
    for (int i = 0; i < f.length; i++) {
      char c = f[i];

      int j = 0;
      while (j < s.length()) {
	if (c == s.charAt(j)) {
	  s.deleteCharAt(j);
	} else {
	  j++;
	}
      }
    }
  }

  static void filter(StringBuffer s, char[][] f)
  {
    for (int i = 0; i < f.length; i++) {
      char[] r = f[i];

      if (1 == r.length) {
	char c = r[0];

	int j = 0;
	while (j < s.length()) {
	  if (c == s.charAt(j)) {
	    s.deleteCharAt(j);
	  } else {
	    j++;
	  }
	}
      } else if (2 == r.length) {
	char from = r[0];
	char to = r[1];

	int j = 0;
	while (j < s.length()) {
	  if (from <= s.charAt(j) && to >= s.charAt(j)) {
	    s.deleteCharAt(j);
	  } else {
	    j++;
	  }
	}
      }
    }
  }

  static void map(StringBuffer s, char[] search, String[] replace)
  {
    for (int i = 0; i < search.length; i++) {
      char c = search[i];

      int j = 0;
      while (j < s.length()) {
	if (c == s.charAt(j)) {
	  s.deleteCharAt(j);
	  if (null != replace[i]) {
	    s.insert(j, replace[i]);
	    j += replace[i].length()-1;
	  }
	} else {
	  j++;
	}
      }
    }
  }
}




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