This hash table uses less space than the items it stores

Isn’t this just a trie with only two levels? It seems to me that this technique isn’t particularly unknown, as it’s taught in most elementary CS courses.

Maybe I'm missing something obvious, but if I insert all 32 bit keys, does that not take 32*2^32 space?

Kind of reminds me of a patricia trie, which is used to store routing tables. Each prefix exists as a path you take from one node to the other.

Neat. I have an order-of-gigabytes hash table I'd love to shrink. Will experiment with this technique.

This is just a classic combination of trie with hashmap.