Define Hash Function. List out properties of Hash function.

A hash function is a function that is used to convert a large amount of data into a small fixed size, which is typically a hash value or key. The main purpose of a hash function is to ensure data retrieval in an efficient manner. It is used in various fields like databases, password security, data retrieval, and more.

Properties of Hash Function:

1. Deterministic: For a given input, the output (hash value) will always be the same. This means if you hash a particular key today, you should get the same hash value when you hash it after a year.

2. Fixed Size: No matter how big or small your input is, the output hash value will always be of a fixed size.

3. Uniform Distribution: A good hash function should distribute data uniformly across the array. This means every hash value in the output range should be equally likely, which helps in avoiding clustering.

4. Collision-Resistant: Although it's impossible to avoid collisions completely, a good hash function should minimize them. A collision occurs when two different keys hash to the same output.

5. Fast Computation: The hash function should be capable of returning the hash value quickly for a given input. If it's slow, then it defeats the purpose of hashing, which is to speed up data retrieval times.

6. Preimage Resistance: It should be computationally hard to retrieve the original input value given only the hash output. This property is especially important in cryptographic hashing.