[linked_list]

Because a hash is by definition a one-way mapping, so then you'd have to keep a map of the reverse mapping hash -> state, which obviously gets impractical with state such as page index or search terms. Better just make two-way "compression" mapping

[yreg]

They probably have meant something like base64 encode

[linked_list]

If you base64 encode an ascii string it gets 33% longer