When rehashing, does the hash remain the same or is a new one calculated for the key?

In rehashing in data structures such as hash tables, a new hash is computed for each key. This is because the size of the table changes, and to properly distribute the elements across the new buckets, the hash must be recalculated considering the new size.

That is, the old hash is not preserved; instead, the hash function is called again for each key, and the result is used to determine the new position in the expanded table.

Example in Go:

// When increasing the size of the table, rehash is called
func (ht *HashTable) rehash() {
    oldBuckets := ht.buckets
    ht.buckets = make([][]Entry, newSize)
    for _, bucket := range oldBuckets {
        for _, entry := range bucket {
            newIndex := hash(entry.key) % newSize
            ht.buckets[newIndex] = append(ht.buckets[newIndex], entry)
        }
    }
}

Thus, the hash for the key is recalculated during rehashing.