10进制与16，32，62进制相互转换

/*

• Copyright (c) 2010 Ant Kutschera, maxant
• The code below is free software: you can redistribute it and/or modify
• it under the terms of the Lesser GNU General Public License as published by
• the Free Software Foundation, either version 3 of the License, or
• (at your option) any later version.
• The code in this file 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
• Lesser GNU General Public License for more details.
• You should have received a copy of the Lesser GNU General Public License
• along with Foobar. If not, see http://www.gnu.org/licenses/.
  */

package uk.co.maxant.util;

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**

• allows you to convert a whole number into a compacted representation of that number,

• based upon the dictionary you provide. very similar to base64 encoding, or indeed hex

• encoding.
  */
  public class BaseX {

  /**

  • contains hexadecimals 0-F only.
    */
    public static final char[] DICTIONARY_16 =
    new char[]{'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};

  /**

  • contains only alphanumerics, in capitals and excludes letters/numbers which can be confused,
  • eg. 0 and O or L and I and 1.
    */
    public static final char[] DICTIONARY_32 =
    new char[]{'1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','J','K','M','N','P','Q','R','S','T','U','V','W','X','Y','Z'};

  /**

  • contains only alphanumerics, including both capitals and smalls.
    */
    public static final char[] DICTIONARY_62 =
    new char[]{'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};

  /**

  • contains alphanumerics, including both capitals and smalls, and the following special chars:
  • +"@*#%&/|()=?'~[!]{}-:.,; (you might not be able to read all those using a browser!
    /
    public static final char[] DICTIONARY_89 =
    new char[]{'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','+','"','@','','#','%','&','/','|','(',')','=','?','~','[',']','{','}','$','-','','.',':',',',';','<','>'};

  protected char[] dictionary;

  /**

  • create an encoder with the given dictionary.
  • @param dictionary the dictionary to use when encoding and decoding.
    */
    public BaseX(char[] dictionary){
    this.dictionary = dictionary;
    }

  /**

  • creates an encoder with the {@link #DICTIONARY_62} dictionary.
  • @param dictionary the dictionary to use when encoding and decoding.
    */
    public BaseX(){
    this.dictionary = DICTIONARY_62;
    }

  /**

  • tester method.
    */
    public static void main(String[] args) {
    String original = "123456789012345678901234567890";
    System.out.println("Original: " + original);
    BaseX bx = new BaseX(DICTIONARY_62);
    String encoded = bx.encode(new BigInteger(original));
    System.out.println("encoded: " + encoded);
    BigInteger decoded = bx.decode(encoded);
    System.out.println("decoded: " + decoded);
    if(original.equals(decoded.toString())){
    System.out.println("Passed! decoded value is the same as the original.");
    }else{
    System.err.println("FAILED! decoded value is NOT the same as the original!!");
    }
    }

  /**

  • encodes the given string into the base of the dictionary provided in the constructor.

  • @param value the number to encode.

  • @return the encoded string.
    */
    public String encode(BigInteger value) {

    List<Character> result = new ArrayList<Character>();
    BigInteger base = new BigInteger("" + dictionary.length);
    int exponent = 1;
    BigInteger remaining = value;
    while(true){
    BigInteger a = base.pow(exponent); //16^1 = 16
    BigInteger b = remaining.mod(a); //119 % 16 = 7 | 112 % 256 = 112
    BigInteger c = base.pow(exponent - 1);
    BigInteger d = b.divide(c);

     //if d > dictionary.length, we have a problem. but BigInteger doesnt have
     //a greater than method :-(  hope for the best. theoretically, d is always 
     //an index of the dictionary!
     result.add(dictionary[d.intValue()]);
     remaining = remaining.subtract(b); //119 - 7 = 112 | 112 - 112 = 0
    
     //finished?
     if(remaining.equals(BigInteger.ZERO)){
         break;
     }
    
     exponent++;
    

    }

    //need to reverse it, since the start of the list contains the least significant values
    StringBuffer sb = new StringBuffer();
    for(int i = result.size()-1; i >= 0; i--){
    sb.append(result.get(i));
    }
    return sb.toString();
    }

  /**

  • decodes the given string from the base of the dictionary provided in the constructor.

  • @param str the string to decode.

  • @return the decoded number.
    */
    public BigInteger decode(String str) {

    //reverse it, coz its already reversed!
    char[] chars = new char[str.length()];
    str.getChars(0, str.length(), chars, 0);

    char[] chars2 = new char[str.length()];
    int i = chars2.length -1;
    for(char c : chars){
    chars2[i--] = c;
    }

    //for efficiency, make a map
    Map<Character, BigInteger> dictMap = new HashMap<Character, BigInteger>();
    int j = 0;
    for(char c : dictionary){
    dictMap.put(c, new BigInteger("" + j++));
    }

    BigInteger bi = BigInteger.ZERO;
    BigInteger base = new BigInteger("" + dictionary.length);
    int exponent = 0;
    for(char c : chars2){
    BigInteger a = dictMap.get(c);
    BigInteger b = base.pow(exponent).multiply(a);
    bi = bi.add(new BigInteger("" + b));
    exponent++;
    }

    return bi;

  }

}