Sina Weibo mid encryption/decryption.

gistfile1.java

    /**
     * A helper class to do the convert between id and mid in Sina Weibo.
     * Based on BASE62 encryption.
     */
    public static class Base62Util {  
        static String[] str62key = {"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" };  
	   
        /** 
         * Convert mid to id.
         * @param mid 
         * @return 
         */  
        	public static String mid2id(String mid) {
		String id = "";
		String k = mid.toString().substring(3, 4);	//will be used when the 4th char is 0
		if (!k.equals("0")) {
			for (int i = mid.length() - 4; i > -4; i = i - 4) {	//4 chars as a group
				int offset1 = i < 0 ? 0 : i;
				int offset2 = i + 4;
				String str = mid.toString().substring(offset1, offset2);
				str = str62to10(str);	//convert string to decimal base
				// if not the 1st group, add '0's if length < 7
				if (offset1 > 0) {
					while (str.length() < 7) {
						str = '0' + str;
					}
				}
				id = str + id;
			}
		} else {
			for (int i = mid.length() - 4; i > -4; i = i - 4) {
				int offset1 = i < 0 ? 0 : i;
				int offset2 = i + 4;
				if (offset1 > -1 && offset1 < 1 || offset1 > 4) {
					String str = mid.toString().substring(offset1, offset2);
					str = str62to10(str);
					// if not the 1st group, add '0's if length < 7
					if (offset1 > 0) {
						while (str.length() < 7) {
							str = '0' + str;
						}
					}
					id = str + id;
				} else {
					String str = mid.toString().substring(offset1 + 1, offset2);
					str = str62to10(str);
					// if not the 1st group, add '0's if length < 7
					if (offset1 > 0) {
						while (str.length() < 7) {
							str = '0' + str;
						}
					}
					id = str + id;
				}
			}
		}
		return id;
	}


	/**
	 * Convert id to mid.
	 * 
	 * @param id
	 * @return
	 */
	public static String id2mid(String id) {
		String mid = "";
		for (int j = id.length() - 7; j > -7; j = j - 7) {	// 7 chars as a group
			int offset3 = j < 0 ? 0 : j;
			int offset4 = j + 7;
			if ((j > 0 && j < 6) && (id.substring(id.length() - 14, id.length() - 13).equals("0") && id.length() == 19)) {
				String num = id.toString().substring(offset3 + 1, offset4);
				num = int10to62(Integer.valueOf(num));	// convert 10 base to 62 base
				mid = 0 + num + mid;
				if (mid.length() == 9) {
					mid = mid.substring(1, mid.length());
				}
			} else {
				String num = id.toString().substring(offset3, offset4);
				num = int10to62(Integer.valueOf(num));
				mid = num + mid;
			}
		}
    return mid;
	}

	/**
	 * Convert 62 base to 10 base.
	* 
	 * @param str
	* @return
	*/
	public static String str62to10(String str) {
		String i10 = "0";
		int c = 0;
		for (int i = 0; i < str.length(); i++) {
			int n = str.length() - i - 1;
			String s = str.substring(i, i + 1);
			for (int k = 0; k < str62key.length; k++) {
				if (s.equals(str62key[k])) {
					int h = k;
					c += (int) (h * Math.pow(62, n));
					break;
				}
			}
			i10 = String.valueOf(c);
		}
		return i10;
	}

	/**
	 * Convert 10 base to 62 base.
	 * 
	 * @param int10
	 * @return
	 */
	public static String int10to62(double int10) {
		String s62 = "";
		int w = (int) int10;
		int r = 0;
		int a = 0;
		while (w != 0) {
			r = (int) (w % 62);
			s62 = str62key[r] + s62;
			a = (int) (w / 62);
			w = (int) Math.floor(a);
		}
		return s62;
	}
}