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Thread: Converting a piece of C# code to Java (unsigned longs, etc)

  1. #1
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    Default Converting a piece of C# code to Java (unsigned longs, etc)

    Hello!

    I'm trying to convert this piece of C# code to java, but I'm stumbling to problems because java doesn't seem to have unsigned long.

    public static string keyHexToKey(byte[] keyHex)
    {
            string arg = "";
            string text = "0123456789ABCDEFGHJKLMNPRSTVWXYZ";
            for (int i = 0; i < 3; i++)
            {
                    ulong num = 0uL;
                    for (int j = 0; j < 5; j++)
                    {
                            num <<= 8;
                            num |= (ulong)keyHex[i * 5 + j];
                    }
                    for (int j = 0; j < 8; j++)
                    {
                            ulong num2 = num >> j * 5 & 31uL;
                            char c = text[(int)num2];
                            arg += c;
                    }
            }
            return arg;
    }

    What I came up by myself, but doesn't seem to work (return wrong values).
    private static String keyHexToKey(byte[] keyHex)
    	{
    	        String arg = "";
    	        String text = "0123456789ABCDEFGHJKLMNPRSTVWXYZ";
    	        char[] text2 = text.toCharArray();
     
    	        for (int i = 0; i < 3; i++)
    	        {
    	                //long num = 0uL;
    	        		long num = (long)0xff;
    	                for (int j = 0; j < 5; j++)
    	                {
    	                        num <<= 8;
    	                        num |= (long)keyHex[i * 5 + j];
    	                }
    	                for (int j = 0; j < 8; j++)
    	                {
    	                        long num2 = num >> j * 5 & (long)31;
    	                        //char c = text[(int)num2];
    	                        char c = text2[(int)num2];
    	                        arg += c;
    	                }
    	        }
    	        return arg;
    	}


  2. #2
    Senile Half-Wit Freaky Chris's Avatar
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    Default Re: Converting a piece of C# code to Java (unsigned longs, etc)

    In Java a Long is a signed 64 bit value, and Java doesn't offer an unsigned version of this as they believe unsigned integers to be wrong, the only unsigned data type is a char.

    Also you do not need to cast literals to long, simple append "L" such as 32L would be the long value for 32.

    Your options are either, manually handle the sign, or use the BigInteger class

    BigInteger (Java Platform SE 7 )

    Chris
    chris[at]javaprogrammingforums[dot]com

    Prifysgol Bangor University, North Wales

  3. #3
    Super Moderator helloworld922's Avatar
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    Default Re: Converting a piece of C# code to Java (unsigned longs, etc)

    The best way to translate the code is to figure out fundamentally what the intent of the code is.

    For example:

    ulong num = 0uL;
    for (int j = 0; j < 5; j++)
    {
        num <<= 8;
        num |= (ulong)keyHex[i * 5 + j];
    }

    This code takes each byte value from keyHex and shifts it into the LSB of num. There are 5 bytes shifted in.

    for (int j = 0; j < 8; j++)
    {
        ulong num2 = num >> j * 5 & 31uL;
        char c = text[(int)num2];
        arg += c;
    }

    This selects 5 bit chunks from num, working from the LSB to the MSB. There are 8 5-bit chunks. It then maps that value to the appropriate character and appends it to the output string.

    So the sole purpose of this code is to take 5-bit compressed values and expand them out, then map the results appropriately.

    The question then becomes how can you replicate this behavior in Java.

    As it turns out, pretty much the same exact code will work in Java (with a few minor differences to account for library and language semantics). Why? Because you never use the msb of the unsigned long, so it doesn't matter that it's unsigned.

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