Base63.java

import java.math.BigInteger;

final public class Base63 {

    static public final char[] CHARSET = 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','_'};
    static private final BigInteger BASE = BigInteger.valueOf(CHARSET.length);

    public static String encode(BigInteger integer) {
        final StringBuilder sb = new StringBuilder();
        BigInteger[] r = new BigInteger[]{integer, null};

        do {
            r = r[0].divideAndRemainder(BASE);
            sb.insert(0, CHARSET[r[1].intValue()]);
        }while(r[0].compareTo(BigInteger.ZERO) != 0);

        return sb.toString();
    }

    static public BigInteger decode(String value) {
        final int size = value.length();
        BigInteger result = BigInteger.ZERO;
        BigInteger m = BigInteger.ONE;
        for(int i = 0; i < size; i++) {
            char c = value.charAt(size - 1 - i);
            if(c == '_')
                result = result.add(BigInteger.valueOf(62).multiply(m));
            else if(c <= '9')
                result = result.add(BigInteger.valueOf(c - '0').multiply(m));
            else if(c <= 'Z')
                result = result.add(BigInteger.valueOf(c - 'A' + 10).multiply(m));
            else if(c <= 'z')
                result = result.add(BigInteger.valueOf(c - 'a' + 36).multiply(m));
            else
                throw new IllegalArgumentException("illegal character: "+c);
            m = m.multiply(BASE);
        }
        return result;
    }
}