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Packs and migration

Loose zombies clump into packs, packs walk the world on their own errand, and a pack nobody is looking at is dematerialised until someone comes near. None of it is aimed at a player: a pack picks where to go from its own heading and a seeded random walk, never from where anyone is standing. The rule is enforced by a test, PackNoPlayerAccessTest, which fails the build if anything under pack/ so much as reaches for a player position.

The consequence is the one worth knowing before reading the numbers: a pack that arrives at your base did not come for you. It was passing.

Forming, holding, breaking

A lone zombie only considers a pack when it has at least packMinNeighbours neighbour, and it takes that decision one activation out of packDecisionDivisor, inspecting at most packScanCap neighbours. Those three numbers are the whole cost of the system in a dense horde.

A brand-new pack needs 3 zombies in the neighbourhood to appear out of nothing. Once it exists a zombie joins it whenever the pack centroid is within 24 blocks, up to a hard 24 members.

Leaving is deliberately harder than joining. A member has to drift past 40 blocks from the centroid, well beyond the 24 it joined at, and stay out there for packStrayActivations activations in a row before it actually leaves. Without that hysteresis a zombie walking around a wall would lose its pack. Below 2 members a pack dissolves, but only after 200 ticks of grace.

Merging

Two centroids closer than 32 blocks are merge candidates, and they only fuse if their headings agree: the dot product of the two directions has to reach packMergeHeadingDot. Without that second test, packs crossing in opposite directions would still fuse, and the whole population of the world collapses into one mass.

Migration

A pack picks a destination between 96 and 384 blocks away, turning at most 45 degrees from its current heading. Bounding the turn is what makes the path read as a migration; unbounded it reads as brownian motion. It counts as arrived within 8 blocks, then pauses 200 ticks plus a jitter of up to 100 so packs do not all move in lockstep.

Off view the pack is simulated coarsely at packVirtualSpeed, about 40 percent of a zombie's walking speed, which pays back the terrain detours a straight line never takes. Members are aimed at a waypoint 24 blocks ahead of the centroid rather than at the destination itself: the level-of-detail system classifies on the distance to the point being walked to, so a destination hundreds of blocks away would freeze the very zombies it was meant to move.

When there is simply no path, packStuckActivations activations without headway make the pack give up and pick somewhere else. And by default packs travel by night: packMigrateAtDay is off, so the mod's own day-sleep wins and a pack does not burn under the sun below the immunity phase.

Out of sight, dematerialised

A pack nobody can see stops existing as entities and keeps existing as a pack. Every 20 ticks a sweep looks at each pack, not at each zombie, and a pack found not entity-ticking for 40 consecutive sweeps is dematerialised. That is a grace period rather than a timer on purpose: chunk unloading has been measured in this project at 2, 35, 272 and once over 1200 ticks, so a single observation proves nothing.

On approach the members come back scattered over 6 blocks so the pack does not reappear in one column, and a returning member re-joins its pack from up to 64 blocks away. A ghost that fails to restore 5 times is given up on.

What survives a restart

Packs are written to the world save, both when it closes and at every autosave, so a crash no longer loses them. Membership, centroid, heading and destination come back as they were.

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