हैश (en. Hash)

Ideally, the hash function will assign each key to a unique bucket, but most hash table designs employ an imperfect hash function, which might cause hash collisions where the hash function generates the same index for more than one key.

You obviously cannot re-create the original data from the hash, but you can hash the data again to see if the same hash value is generated.

Hash tables are commonly used to implement many types of in-memory tables.

A hash table is an unordered data structure and there are many types of queries which hash indexes cannot even help with.

In many situations, hash tables turn out to be on average more efficient than search trees or any other table lookup structure.

Changing every value in a hash in Ruby.

How to replace a hash key with another key.