simple base64 decode snippet

base64.java

import java.util.Arrays;

public class main {
    private static byte base64_reverse_table[] = {
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -1, -1, -2, -2, -1, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -1, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, 62, -2, -2, -2, 63,
            52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -2, -2, -2, -2, -2, -2,
            -2,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
            15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -2, -2, -2, -2, -2,
            -2, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
            41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
            -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2
    };
    public static byte[] base64Decode(String data) throws Exception {
        byte[] bs = new byte[(data.length() + 2)/4*3];
        int k = 0;
        for (int i = 0, j = 0; i < data.length(); ++i) {
            char ch = data.charAt(i);
            if (ch == '=') {
                int tail = j & 3;
                if (tail == 0 || tail == 1||
                    (tail == 2 && (i != data.length() - 2 || data.charAt(i + 1) != '=')) ||
                    (tail == 3 && i != data.length() - 1)
                ) {
                    throw new Exception("malformed base64 encoding string");
                }
                break;
            }

            byte ck = base64_reverse_table[ch];
            if (ck == -1) {
                continue;
            } else if (ck == -2) {
                throw new Exception("malformed base64 encoding string");
            }

            switch(j & 3) {
                case 0:
                    bs[k] = (byte)(ck << 2);
                    break;
                case 1:
                    bs[k++] |= ck >> 4;
                    bs[k] = (byte)((ck & 0x0f) << 4);
                    break;
                case 2:
                    bs[k++] |= ck >>2;
                    bs[k] = (byte)((ck & 0x03) << 6);
                    break;
                case 3:
                    bs[k++] |= ck;
                    break;
            }
            j++;
        }
        return Arrays.copyOfRange(bs, 0, k);
    }

    public static void main(String[] args) {
        try {
            System.out.println(new String(base64Decode("MQ==")));
            System.out.println(new String(base64Decode("MTI=")));
            System.out.println(new String(base64Decode("MTIz")));
            System.out.println(new String(base64Decode("MTIzNA==")));
            System.out.println(new String(base64Decode("MTIzNDU=")));
            System.out.println(new String(base64Decode("5rWL6K+V5pWw5o2u")));
            System.out.println(new String(base64Decode("JXU2RDRCJXU4QkQ1JXU2NTcwJXU2MzZF")));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}