The AI uses HarvesterBotModule to check for idle harvesters, and give them harvest orders. By default it scans every 50 ticks (2 seconds at normal speed), and for any idle harvesters locates an ore patch and issues a harvest order. This FindNextResource to scan for a suitable ore path is quite expensive. If the AI has to scan for ore patches for several harvesters, then this can produce a noticeable lag spike. Additionally, when there are no available ore patches, the scan will just keep repeating since the harvesters will always be idle - thus the lag spikes repeat every 50 ticks.
To reduce the impact, there already exists a randomization on the first scan interval so that multiple different AIs scan on different ticks. By ensuring the AI players scan at different times, we avoid a huge lag spike where they all operate on the same tick.
To reduce the impact even more, we make four additional changes:
- Scans continue to be done every 50 ticks to detect harvesters. But we spread out the searches for ore patches over multiple later ticks. We'll only perform one ore patch search per tick. This means instead of ordering e.g. 30 harvesters on a single tick and creating a spike, we order one on each tick over the next 30 ticks instead. This spreads out the performance impact.
- When a harvester fails to locate any suitable ore patch, we put it on a longer cooldown, by default 5x the regular cooldown. We don't need to scan as often for these harvesters, since it'll take time for new resources to appear.
- We change the path search in FindNextResource from FindPathToTargetCellByPredicate to FindPathToTargetCells. The format in an undirected path search that must flood fill from the start location. If ore is on the other side of the map, this entails searching the whole map which is very expensive. By maintaining a lookup of resource types per cell, we can instead give the target locations directly to the path search. This lookup requires a small overhead to maintain, but allows for a far more efficient path search to be carried out. The search can be directed towards the target locations, and the hierarchical path finder can be employed resulting in a path search that explores far fewer cells. A few tweaks are made to ResourceClaimLayer to avoid it creating empty list entries when this can be avoided.
- We adjust how the enemy avoidance cost is done. Previously, this search used world.FindActorsInCircle to check for nearby enemies, but this check was done for every cell that was searched, and is itself quite expensive. Now, we create a series of "bins" and cache the additional cost for that bin. This is a less fine grained approach but is sufficient for our intended goal of "avoid resource patches with too many enemies nearby". The customCost function is now less expensive so we can reuse the avoidance cost stored for each bin, rather than calculating fresh for every cell.
* Maintains lists of claims, and only restricts
reservations for friendly units.
* Removes OnNotifyResourceClaimLost; it's not
clear whether that is still useful, and it
prevents future necessary cleanups.
* Moves other code without changing behaviour.
This fixed stale claims from dead units and enemy
claims from preventing otherwise valid harvest
activities.
