Which hash algorithm is best for speed and identical hashes. (a lot of collisions)

Question

I'm searching an existing hash function or trying to make a hash function that has a lot of collisions.

Regularly a hash is used for it's ability to create unique hashes for hash tables or security purposes, but I desire the opposite. I desire a hash function that has allot of collisions. Preferably a simplistic hash function so finding a collision is easier/faster.

By the definition of a hash function. The hashes also have a predetermined length. I'd like a hash function that has this as a variable.

I'm new to this subject so I'm searching sources to create a hash function myself or candidate's that might fulfill the requirements.

If I have definition or terms wrong, please correct me.

Does this answer your question? Which hashing algorithm is best for uniqueness and speed? — gnat, Jul 12 '20 at 12:53
No, that question asks for a hash function that produces a lot of unique hashes, I desire the opposite as my question says. A hash function that contains a lot of collisions... — NS - Not the train, Jul 12 '20 at 12:55
What kind of data are you hashing? If it is strings you could take the first character as your hash code. — Martin Maat, Jul 12 '20 at 13:14
Im hashing a string, but it's necessary to hash the whole string not just the first character — NS - Not the train, Jul 12 '20 at 13:51
XOR every byte together into a single byte result and you're done. Make it one bit instead of a byte for extra collisions! — whatsisname, Jul 12 '20 at 14:05
Why do you want collisions? The trivial hash function h(x) = 0 would have a colliding hash for everything. More reasonably, you can take a good hash function and chop off unneeded bits until you get the desired collision rate. — amon, Jul 12 '20 at 14:27

score 1 · Answer 1 · answered Jul 12 '20 at 13:03

1

The probability of collisions is ratio between how many items you have and what size the output hash is.

If you have 1024 items (10 bits) and hash outputs 8 bits, you would expect 4 items (2^(10-8)) to have same hash and thus 4 collisions.

And there are plenty of hash functions with low amount of output bits. Like a trivial Pearson Hash. And it being trivial and easy to understand, it should be easy to modify it to have variable bit output size.

answered Jul 12 '20 at 13:03

Euphoric

37,384

To clarify, it will be (on average) 4 items for each of the 256 possible hash values, not just 4 collisions total. – 8bittree Jul 13 '20 at 16:33

score 1 · Answer 2 · answered Jul 12 '20 at 14:27

Here is an example of a hash function that has the properties you desire:

H(message, outputLength) = 1 << outputLength

It is clearly a hash function, since it maps a larger input space to a smaller output space.
It has the maximum possible performance for every possible sequence of inputs.
It has the maximum possible amount of collisions for every possible sequence of inputs.

Which hash algorithm is best for speed and identical hashes. (a lot of collisions)

2 Answers2