Document & bolster calculate_fees

[DanGould]

calculate fees seems to work for a subset of possible transaction creations. What do the magic numbers mean? I'd like to add the ability to batch another arbitrary output. Nolooking also failed to meet min relayfee in an off by one error when sending from sparrow (payjoin/nolooking#34)

loptos/src/main.rs

Lines 187 to 195 in 4390bb0

	fn calculate_fees(channel_count: u64, fee_rate: u64, has_additional_output: bool) -> bitcoin::Amount {
	let additional_vsize = if has_additional_output {
	channel_count * (8 + 1 + 1 + 32)
	} else {
	(channel_count - 1) * (8 + 1 + 1 + 32) + 12
	};
	bitcoin::Amount::from_sat(fee_rate * additional_vsize)
	}

[DanGould]

channel_count * (8 + 1 + 1 + 32) // <8 invariant bytes = 4 version + 4 locktime> OP_0 OP_PUSHBYTES_32 <32 byte script>

[DanGould]

This check_fees fn in the bip78 crate is relevant. I'm still not sure what those +12 bytes are for when the output has no additional output 🤔

https://github.com/Kixunil/payjoin/blob/150dd33b9c60a4e1c219955b9af2e5aca0a43bec/bip78/src/sender/mod.rs#L196-L225

[DanGould]

The public key hash in P2WPKH is 20 bytes; The script hash in P2WSH is 32 bytes. 32-20 is 12 when the static reserve output is substituted with a channel funding tx

[DanGould]

I think this fee calculation assumes only + 1 additional input and + 1 additional output while the args support more than one channel to be queued.

[DanGould]

Could 8 + 1 + 1 + 32 be wrong? I think OP_0 OP_PUSHBYTES_32 <32 byte script> is actually 34 bytes

[Kixunil]

Oh, yeah, it seems to be wrong. 8 is for amount, 1 should be for the length of the script (compact size), 1 for OP_0, 1 for OP_PUSHBYTES32 and 32 for the hash. So off-by one.

But this still assumes there are less than 253 channels which is fine for now but this needs to be implemented correctly long term. It should go to payjoin crate anyway.