LNPBP-5: Unset value of tx position and input/output index fields

[inaltoasinistra]

hello! the two fields 32-47 and 49-63 use 0 to encode no value. This has a couple of drawbacks:

• the coinbase transaction cannot be represented (as @dr-orlovsky specified);
• the input/output index values need an aritmetic operation (-1) before be used as index.

I think that the encoding would be simpler if N is encoded as N, and INT_MAX(bits) is used to encode UNSET.

I would define the constants

• UNSET_TX_INDEX = INT_MAX(16) = 0xffff
• UNSET_IO_INDEX = INT_MAX(15) = 0x7fff

I would redefine the entity Block; On-chain

First bit set to 0, bits 32-47 set to UNSET_TX_INDEX, bit 48 set to 0, bits 49-63 set to UNSET_IO_INDEX

and the entity Transaction; On-chain to

First bit set to 0, bit 48 set to 0, bits 49-63 set to UNSET_IO_INDEX

[dr-orlovsky]

@cdecker what do you think on this enhancement to universal bitcoin ids?