xxHash convert resulting in hash too long

Question

I'm using xxHash for C# to hash a value for consistency. ComputeHash returns a byte[], but I need to store the results in a long.

I'm able to convert the results into an int32 using the BitConverter. Here is what I've tried:

var xxHash = new System.Data.HashFunction.xxHash();
byte[] hashedValue = xxHash.ComputeHash(Encoding.UTF8.GetBytes(valueItem));
long value = BitConverter.ToInt64(hashedValue, 0);

When I use int this works fine, but when I change to ToInt64 it fails.

Here's the exception I get:

Destination array is not long enough to copy all the items in the collection. Check array index and length.

Sorry I forgot to include the exception that I get: "Destination array is not long enough to copy all the items in the collection. Check array index and length." — Desmond Nzuza, Jun 18 '15 at 09:43
@DesmondNzuza hashedValue must be at least 8 bytes long to convert to `long` — xanatos, Jun 18 '15 at 09:50

score 8 · Answer 1 · answered Jun 18 '15 at 09:53

8

When you construct your xxHash object, you need to supply a hashsize:

var hasher = new xxHash(32);

valid hash sizes are 32 and 64.

See https://github.com/brandondahler/Data.HashFunction/blob/master/src/xxHash/xxHash.cs for the source.

answered Jun 18 '15 at 09:53

spender

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@leppie: Yep, it's almost worth writing `var hasher = new xxHash((int)32);` so a casual inspection of the call-site is unambiguous. – spender Jun 18 '15 at 09:57
Thanks for the answer spender. I specified the hash size and that sorted the issue out for me. I did not want to have to check the resulting byte[] this does it for me. Thanks. – Desmond Nzuza Jun 18 '15 at 11:37
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@DesmondNzuza Don't forget to check this answer then ;) – Binkan Salaryman Jun 18 '15 at 11:38

score 2 · Accepted Answer · answered Feb 16 '20 at 16:07

Adding a new answer because current implementation of xxHash from Brandon Dahler uses a hashing factory where you initialize the factory with a configuration containing hashsize and seed:

using System.Data.HashFunction.xxHash;

//can also set seed here, (ulong) Seed=234567
xxHashConfig config = new xxHashConfig() { HashSizeInBits = 64 };
var factory = xxHashFactory.Instance.Create(config);
byte[] hashedValue = factory.ComputeHash(Encoding.UTF8.GetBytes(valueItem)).Hash;

score 0 · Answer 3 · answered Jun 18 '15 at 09:51

0

BitConverter.ToInt64 expects hashedValue to have 8 bytes (= 64bits). You could manually extend, and then pass it.

answered Jun 18 '15 at 09:51

Binkan Salaryman

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I know the reason why it was failing was becuase the length is too short. I don't have the luxury to inspect or expand the resulting byte[] becuase I don't want to affect performance. Specifying a hash size did the trick for me. Thanks. – Desmond Nzuza Jun 18 '15 at 11:35

xxHash convert resulting in hash too long

3 Answers3