C#

static int decodeSerial(String serial, byte[] result, int ResLen)
        {

            int m;
            int digit;


            Array.Clear(result, 0, ResLen);

            for (int i = 0; i < serial.Length; i++)
            {

                digit = serial[i] - '0';

                
                m = 0;

                do
                {
                    digit += result[m] * 10;
                    result[m++] = (byte)(digit % 255);
                    digit >>= 8;
                }

                while (m < ResLen);

               
                
            }

            return 0;
        }

private static void runAllTests() {
	test("1");
	test("1000");
	test("1000000");
	test("1000000000");
	long j=1;
	for(long i=0; i<63; i++) {
		test(j.ToString());
		j+=j;
	}
	test("-1");
	test("12345678901234567890123456789012345678901234567890123456789012345678901234567890");
}

private static void test(string decimalDigits) {
	try {
		byte[] bytes=decodeSerial(decimalDigits, 10);
		string s=decimalDigits+" = ";
		foreach (byte b in bytes) s+=" "+((int)b).ToString("X2");
		log(s);
	} catch (Exception exc) {
		log(exc.ToString());
	}
}

// returns a little-endian byte array of given size, and representing the multi-precision
// integer value of a positive integer (entered as decimal string) assuming the size suffices.
// example: decodeSerial("1000", 4) returns E8 03 00 00 (4 bytes shown in hex).
private static byte[] decodeSerial(string decimalDigits, int numberOfBytes) {
	byte[] bytes=new byte[numberOfBytes];
	foreach (char c in decimalDigits) {
		int digit="0123456789".IndexOf(c);
		if (digit<0) throw new ArgumentException("bad char in decodeSerial("+decimalDigits+")");
		for (int m=0; m< numberOfBytes; m++) {
			digit+=bytes[m]*10;
			bytes[m]=(byte)(digit&0xFF);
			digit>>=8;
		}
		if (digit!=0) throw new OverflowException("decodeSerial("+decimalDigits+","+numberOfBytes+")");
	}
	return bytes;
}

stephen.darling wrote:

It was hard to explain

stephen.darling wrote:

I have been on with this problem for over 2 months before asking for help

Convert.ToString(n, 16)

stephen.darling wrote:

while (m < ResLen);

stephen.darling wrote:

If I enter 123456789 on my calculator and press the "hex" button, it gives:

459A717

T M Gray wrote:

Why are you subtracting a string?

MessageBox.Show(("5"[0] - '0' + 1).ToString()); // Displays: 6.

Create a multiplier array. Set it to a value of 1 (i.e., a single element with the value set to 1).
Create a result array. Set it to a value of 0 (i.e., a single element with the value set to 0).
Create a secondary array. It's value will be set later.
Loop from the last char to the first char.
	Create a product array by multiplying the char value by the multiplier array (note: 5x = x + x + x + x + x).
		Initialize product array to the size of the multiplier array plus 1 (set each element to a value of 0).
		Loop from 1 to char value.
			Assign the product array the result of binary adding the product array and the multiplier array.
	Binary add the product array to the value array and assign it to the value array, adding a head byte if necessary.
	Multiply the multiplier array by 10 (note: 10x = x+x+x+x+x+x+x+x+x+x) (optimization: 10x = 8x + 2x).
		Copy the multiplier array to the secondary array.
		Multiply secondary array by 2.
			Prefix a head byte to the secondary array.
			Shift all the bits in the secondary array to the left by 1 bit (discard the first bit).
		Multiply multiplier array by 8.
			Prefix a head byte to the multiplier array.
			Shift all the bits in the multiplier array to the left by 3 bits (discard the first 3 bits).
		Binary add the secondary array and the multiplier array and assign the result to the multiplier array.
		If the first byte in the multiplier is 0, remove it (this isn't necessary, but will reduce memory used).
Repeat loop until first char is reached and processed.
Remove any bytes from the head of the value array that are 0 (they are meaningless, just as the zeroes on 00000123 are meaningless).