On Fri, Sep 01, 2023 at 01:56:18PM +0000, Andrew Poelstra via bitcoin-dev wrote:
> We can swag what the space savings would be: there are 122MM utxos right
> now, which is a bit under 2^27. So assuming a uniform distribution of
> prefixes we'd need to specify 28 bits to identify a UTXO. To contrast,
> to identify a blockheight we need 20 bits and then maybe 12 more bits to
> specify a TXO within a block. Plus whatever varint overhead we have.
> (I've been working on this project but busy with family stuff and don't
> remember exactly where we landed on the varints for this. I think we
> agreed that there was room for improvement but didn't want to hold up
> posting the rest of the concept because of it.)

Since most transactions spend txouts that are similar in height to each other,
you could save further bits by specifying a reference height and then encoding
the exact txout with a delta.

If you're sending multiple txins or multiple transactions in a single packet,
you could achieve this by starting the packet with the reference block height.

If your application tends to send just a single transaction, you could use a
reference height that is a function of the current time. Since sender and
receiver might not agree on the exact time, you could try slightly difference
reference heights via bruteforcing until the transaction signatures validate.

-- 
https://petertodd.org 'peter'[:-1]@petertodd.org