String to Integer Hashing Function with Precision

Question

I want to hash a char array in to an int or a long. The resulting value has to adhere to a given precision value. The function I\'ve been using is given below:

Answer 1

MD5 or SHA. There are many open implementations, and the outcome is very unlikely to produce a duplicate result.

Answer 2

The very definition of a hash is that it produces duplicate values for some values, due to hash value range being smaller than the space of the hashed data.

In theory, a 32-bit hash has enough range to hash all ~6 character strings (A-Z,a-z,0-9 only), without causing a collision. In practice, hashes are not a perfect permutation of the input. Given a 32-bit hash, you can expect to get hash collisions after hashing ~16 bit of random inputs, due to the birthday paradox.

Given a static set of data values, it's always possible to construct a hash function designed specifically for them, which will never collide with itself (of course, size of its output will be at least log(|data set|). However, it requires you to know all the possible data values ahead of time. This is called perfect hashing.

That being said, here are a few alternatives which should get you started (they are designed to minimize collisions)

Answer 3

Hashes generate the same value for different inputs -- that's what they do. All you can do is create a hash function with sufficient distribution or bit depth (or both) to minimize those collisions. Since you have this additional constraint of precision (0-5 ?) then you are going to hit collisions far more often.

Answer 4

Every hash will have collisions. Period. That's called a Birthday Problem.

You may want to check cryptographic has functions like MD5 (relatively fast and you don't care that it's insecure) but it also will have collisions.