BIP8: directly support special cases

[ajtowns]

Rather than always having the genesis block be DEFINED, allow {0,0,true} to act as ALWAYS_ACTIVE and {0,0,false} to act as NEVER_ACTIVE, and {n,n,true} to act as a buried deployment.

This cleans up the implementation a bit I think, removing the need for special case handling. (Note that in the {n,n,true} case n will still be rounded up to the next retarget period however)

[ajtowns]

Could also make it:

    } else if (block.height >= startheight) {
        if (lockinontimeout && block.height + 2016 >= timeoutheight) {
           return MUST_SIGNAL;
        } else {
           return STARTED;
        }
    }

which would also remove the edge case from #1019.

[luke-jr]

I wonder if this is needed in the BIP, or just implementation details that complicate the BIP.

[ajtowns]

Fair question. I think it makes the special cases ("what if timeout is less than start? what if they're not equal but in the same retarget phase?") a bit clearer, without complicating the BIP very much. It also might let the implementation follow the BIP pseudocode more exactly which might make it easier to see both are correct.

[jonasnick]

The genesis block handling is slightly better than before, but imo this is more difficult to understand than the current "timoutheight must be 2 retarget periods after startheight".

[ajtowns]

Closing this -- it only covers cases that can't reasonably arise in practice on either mainnet or testnet, and can be treated as an implementation detail elsewhere.