The Nest that said 440/440 and wouldn’t die

The report came in from a phone, on a world that had been running since 1.1. A large Nest called Kestrelhurst sat on the board. Its chip at the top of the screen read:

Kestrelhurst (L) 440/440 cleared

And it was still pulsing.

What a Nest is

A Nest is a named knot of Creep. A mine hit can wake one, and so can old Creep festering on its own. It erupts over a square ring of ground around its core. Every few minutes it pulses Creep outward from the core. A large Nest’s ring is 440 cells.

You kill a Nest by clearing its ring: reveal every safe cell in it. The chip counts your progress. Each ring cell that is revealed, flagged or a crater counts toward the total, so you can watch the count climb as you work through the ring.

At 440/440 the count said the work was done. The Nest disagreed.

The two definitions of done

The first suspect was the settlers. They dig up flags, and a dig changes the board without the player revealing anything. If a settler’s dig had completed the ring, maybe the check that kills a Nest never ran.

It wasn’t that. The cause was a flag, and the flag was wrong.

The chip and the kill were counting different things. The chip counted every ring cell that was revealed, flagged or a crater. The kill waited until every safe cell in the ring was revealed. Those agree as long as your flags are right. When one flag in the ring sits on a safe cell, the chip counts it, because it’s flagged, and the kill doesn’t, because a safe cell under a flag isn’t revealed. The count reaches 440 and the Nest never dies.

From the player’s side, nothing could be done. You can’t find the wrong flag from the chip. The game never tells you outright that a flag is wrong, because that would turn every flag into a free question: plant it, look for a reaction, move it. The only things that expose a wrong flag are the Creep breaking it, a chord through it opening the mine underneath, and a settler digging it up and finding nothing. Inside a Nest’s ring, with everything else solved, none of those was going to happen. The world was stuck.

The fix

There’s an obvious fix that we couldn’t use: make the chip leave wrong flags out of its count. Then the count would read 439/440, and you’d know one of your flags in that ring was a lie. With a few flags in the ring, that’s most of the way to knowing which.

So it went the other way. There is now one definition of a cleared ring, used by both the chip and the kill: the count is full, or no safe cell in the ring is left covered. If a wrong flag is what completes the count, the Nest dies anyway, and that flag opens to show its true number. At that point the Nest is gone, so revealing the flag gives nothing away.

The check now runs every time a ring changes, after every step of the simulation, and for every Nest when a world loads. That last part mattered for the world in the report. A save stuck at a full count by an older build has its Nest killed the moment it loads in 1.2.1 or later. Nobody had to do anything to unstick it.

We also added a check to our long simulation runs that complains if it ever sees a live Nest with a complete ring. A 3-hour run of the fixed build with a lazy scripted player, played twice from the same seed to make sure it plays out identically, never tripped it. 247 tests passed.

The fix shipped as 1.2.1. The rule it settled is now part of the board’s rules: a Nest dies when its count is full, even if a flag in it is wrong, and the wrong flag then opens to its true number.