Question

I have implemented my own hash table functions in C, but currently it doesn’t support resizing. I was wondering what algorithms do exist apart from the brute-force way of creating a new empty hash table and moving everything there?

Answer 1

There is incremental resizing.

From Wikipedia:

Incremental resizing

Some hash table implementations,
notably in real-time systems, cannot
pay the price of enlarging the hash
table all at once, because it may
interrupt time-critical operations. If
one cannot avoid dynamic resizing, a
solution is to perform the resizing
gradually:

During the resize, allocate the new
hash table, but keep the old table
unchanged.
In each lookup or delete operation, check both tables.
Perform insertion operations only in the new table.
At each insertion also move r elements from the old table to the new
table.
When all elements are removed from the old table, deallocate it.

To ensure that the old table will be
completely copied over before the new
table itself needs to be enlarged, it
is necessary to increase the size of
the table by a factor of at least (r +
1)/r during the resizing.

So this is not some clever way of moving all of the elements from the old table into the new table (and if there is one, I haven’t seen it); rather, it eases the burden of resizing by allowing the migration to happen gradually.