Using a reversible hash function to store encoded keys is certainly a domain-specific optimization, but integer keys are so common that I don't understand why I

Using a reversible hash function to store encoded keys is certainly a domain-specific optimization, but integer keys are so common that I don't understand why I don't see this optimization more often. For non-integer keys, you can just store truncated hash codes to eliminate hash function calls in most cases (note that the hash value is needed both to calculate the preferred bucket and to determine the order of entries in that bucket). The smaller the table, the smaller the truncated hash codes can be. When you resize the table, you can increase the truncated hash code size if necessary (one bit for each doubling, which you can amortize by adding a byte every 8 doublings). This of course requires rehashing all keys, but conventional hash tables already rehash all keys on resizing.