I'm trying to create a unique unsigned "long" number ($64$ bits) from a list of $4$ other numerical values. Some function $f(n_1, n_2, n_3, n_4) = x$. The order of the values is important so unfortunately I can't use this technique (calculating unique value from given numbers) which seems to give the same unique value from a set of numbers where order doesn't matter.

For example $n_1 = 10, n_2 = 14, n_3 = 18, n_4 = 21$ should be different than $m_1 = 10, m_2 = 1, m_3 = 418, m_4 = 19$. In addition, $o_1 = 10, o_2 = 14, o_3 = 21, o_4 = 18$ should also be different.

I realize it might not be possible to get a truly unique number due to the 64-bit limitation but if you can help me to find a number that's very unlikely to be unique, that would be very very nice:)

Other interesting suggestions that I've looked at:

Function for unique hash code

Deduce a unique number from number

Calculate unique Integer representing a pair of integers

Perhaps that last one could be applied three times? Since $4$ values are two pairs, each pair could generate a unique value and then the two unique values could be used in the function again?


The first rule of hash functions is that you probably shouldn't be designing hash functions on your own (most people design pretty bad ones).

A simple way is to take a string hash function, put the numbers in with separators, and then hash the string, like "n1,n2,n3,n4". The programming language you're using may have a built in hash function for this purpose (for example, Java has a hash function for strings built in, which you can use [or reimplement - I don't think Java requires the hash functions to be consistent on different runs of the program] but it is 32 bits). Obviously a 32 bit hash function can be mapped to 64 bits by ignoring 32 bits. Or, you can concatenate 32-bit hashes for "n1,n2" and "n3,n4" into a 64 bit long.

A list of hash functions on wikipedia may be useful.

Do you have any restrictions on what values the numbers can take?

  • $\begingroup$ Thanks for your input and I realise designing hash functions for security reasons is a bad idea. I didn't realise Java's hashCode() actually returns an int. Thanks. hashCode() -> s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]. Since I'm in c++ I guess I'll build a string and use this: std::hash<std::string>()("foo");. Funny how googling can be this random after searching for 1 hour without results :P $\endgroup$ – span Feb 11 '14 at 15:49
  • $\begingroup$ Well, even ignoring security, people don't necessarily get good collision resistance. There is a Boost::hash function/std::hash function in <functional> in C++ which you may want to read up on (important to note that passing it a char * will hash the pointer, not the thing it points to, so pass it a std::string (you will obviously need <string>). $\endgroup$ – Batman Feb 11 '14 at 15:55
  • $\begingroup$ Again, you need to check the consistency issue for different runs of the program and across different platforms, provided you are storing these hashes. So, finding a hash function online and doing it may be the best thing to do. $\endgroup$ – Batman Feb 11 '14 at 16:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.