Algorithm to convert a String of decimal digits to BCD

I am looking a way to convert a string to BCD equivalent. I use Java, but it is not a question of the language indeed. I am trying to understand step by step how to convert a string to BCD.

For example, suppose I have the following string;

"0200" (This string has four ASCII characters, if we were in java this string had been contained in a byte[4] where byte[0] = 48, byte[1] = 50, byte[2] = 48 and byte[3] = 48)

In BCD (according this page: http://es.wikipedia.org/wiki/Decimal_codificado_en_binario):

Ok, I think the conversion is correct but I have to save this in a byte[2]. What Should I have to do? After, I have to read the BCD and convert it to the original string "0200" but first I have to resolve String to BCD.

Solution

Find a utility class to do this for you. Surely someone out there has written a BCD conversion utility for Java.

Here you go. I Googled "BCD Java" and got this as the first result. Copying code here for future reference.

public class BCD {

    /*
     * long number to bcd byte array e.g. 123 --> (0000) 0001 0010 0011
     * e.g. 12 ---> 0001 0010
     */
    public static byte[] DecToBCDArray(long num) {
        int digits = 0;

        long temp = num;
        while (temp != 0) {
            digits++;
            temp /= 10;
        }

        int byteLen = digits % 2 == 0 ? digits / 2 : (digits + 1) / 2;
        boolean isOdd = digits % 2 != 0;

        byte bcd[] = new byte[byteLen];

        for (int i = 0; i < digits; i++) {
            byte tmp = (byte) (num % 10);

            if (i == digits - 1 && isOdd)
                bcd[i / 2] = tmp;
            else if (i % 2 == 0)
                bcd[i / 2] = tmp;
            else {
                byte foo = (byte) (tmp << 4);
                bcd[i / 2] |= foo;
            }

            num /= 10;
        }

        for (int i = 0; i < byteLen / 2; i++) {
            byte tmp = bcd[i];
            bcd[i] = bcd[byteLen - i - 1];
            bcd[byteLen - i - 1] = tmp;
        }

        return bcd;
    }

    public static String BCDtoString(byte bcd) {
        StringBuffer sb = new StringBuffer();

        byte high = (byte) (bcd & 0xf0);
        high >>>= (byte) 4; 
        high = (byte) (high & 0x0f);
        byte low = (byte) (bcd & 0x0f);

        sb.append(high);
        sb.append(low);

        return sb.toString();
    }

    public static String BCDtoString(byte[] bcd) {

        StringBuffer sb = new StringBuffer();

        for (int i = 0; i < bcd.length; i++) {
            sb.append(BCDtoString(bcd[i]));
        }

        return sb.toString();
    }
}

There's also this question: Java code or lib to decode a binary-coded decimal (BCD) from a String.