What is the concept of rehashing in hashing?

Rehashing is a concept in hashing that involves the process of creating a new hash table and transferring the elements from the old hash table to the new one. It is typically performed when the load factor of the hash table exceeds a certain threshold, causing the hash table to become inefficient in terms of time complexity.

The main purpose of rehashing is to maintain a balanced and efficient hash table by redistributing the elements. When the load factor exceeds the threshold, it means that the number of elements stored in the hash table is approaching or exceeding the number of available slots. This can lead to an increase in collisions, which in turn affects the performance of hash table operations such as insertion, deletion, and retrieval.

During rehashing, a new hash table with a larger size is created. The size of the new hash table is typically chosen to be a prime number to minimize collisions. Then, each element from the old hash table is rehashed and inserted into the new hash table based on the new hash function and the new size.

The process of rehashing involves the following steps:
1. Create a new hash table with a larger size.
2. Initialize the new hash table with empty slots.
3. Iterate through each element in the old hash table.
4. Rehash each element using the new hash function and calculate its new position in the new hash table.
5. Insert the element into the new hash table at its new position.
6. Repeat steps 3-5 for all elements in the old hash table.
7. Once all elements have been rehashed and inserted into the new hash table, the old hash table is discarded and the new hash table becomes the active hash table.

Rehashing helps in maintaining a low load factor, which ensures that the hash table remains efficient in terms of time complexity. By increasing the size of the hash table, rehashing reduces the chances of collisions and improves the overall performance of hash table operations. However, rehashing can be an expensive operation in terms of time and memory, especially when dealing with a large number of elements. Therefore, it is important to choose an appropriate threshold and size for the hash table to minimize the frequency of rehashing.