Shift a 128-bit signed BigInteger to always be positive

Question

I'm converting a Guid to a BigInteger so I can base62 encode it. This works well, however, I can get negative numbers in BigInterger. How do I shift the BigInteger so the number is positive. I'll also need to be able to shift it back so I can convert back to a Guid.

// GUID is a 128-bit signed integer 
Guid original = new Guid("{35db5c21-2d98-4456-88a0-af263ed87bc2}");
BigInteger b = new BigInteger(original.ToByteArray());

// shift so its a postive number?

Just curious: What's wrong with using the GUID string itself? — Pat, May 26 '11 at 16:45

score 2 · Accepted Answer · answered May 26 '11 at 16:39

Note: For url-safe version of Base64 consider using modifyed set of characters for Base64 ( http://en.wikipedia.org/wiki/Base64#URL_applications) instead of custom Base62.

I believe you can append 0 to the array first (will make higest byte always not to contain 1 in the highest bit) and then convert to BigInteger if you really need positive BigInteger.

FRL · Answer 2 · 2019-08-04T23:19:07.957

This article I think it can give you the solution: In summary it is to add one more byte, to 0, if the most significant bit of the last byte is a 1

Guid original = Guid.NewGuid();
byte[] bytes = original.ToByteArray();
if ((bytes[bytes.Length - 1] & 0x80) > 0) 
{
   byte[] temp = new byte[bytes.Length];
   Array.Copy(bytes, temp, bytes.Length);
   bytes = new byte[temp.Length + 1];
   Array.Copy(temp, bytes, temp.Length);
}

BigInteger guidPositive = new BigInteger(bytes);

score 0 · Answer 3 · answered May 26 '11 at 16:22

0

do you mean base64 encode?

Convert.ToBase64String(Guid.NewGuid().ToByteArray());

answered May 26 '11 at 16:22

Jay

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No, I have a custom base62 encoding. Simply need a way to reliably have the BigInteger always be positive. – Paul Welter May 26 '11 at 16:29

score 0 · Answer 4 · edited May 23 '17 at 10:34

If you sometimes get negative numbers, it means that your GUID value is large enough to fill all 128 bits of the BigInteger or else the BigInteger byte[] ctor is interpreting the data as such. To make sure your bytes are actually positive, check that you are getting <= 16 bytes (128 bits) and that the most-significant bit of the last byte (because it's little endian) is zero. If you have <16 bytes, you can simply append a zero byte to your array (again, append because it is little endian) to make sure the BigInteger ctor treats it as a positive number.

Shift a 128-bit signed BigInteger to always be positive

4 Answers4