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Fix reversed path searches from inaccessible locations.

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The Harvester trait and MoveAdjacentTo activity called the pathfinder but had a single source and multiple targets. The pathfinder interface only allows for the opposite: multiple sources and a single target. To work around this they would swap the inputs. This works in most cases but not all cases. One aspect of asymmetry is that an actor may move out of an inaccessible source cell, but not onto an inaccessible target cell.

Searches that involved an inaccessible source cell and that applied this swapping method would therefore fail to return a path, when a valid path was possible. Although a rare case, once good way to reproduce is to use a production building that spawns actors on inaccessible cells around it, such as the RA naval yard. A move order uses the pathfinder correctly and the unit will move out. Using a force attack causes the unit to use the broken "swapped" mechanism in MoveAdjacentTo and it will be stuck.

This asymmetry has been longstanding but the pathfinding infrastructure only sporadically accounted for it. It is now documented and applied consistently. Create a new overload on the pathfinder trait that allows a single source and multiple targets, so callers have an overload that does what they need and won't be tempted to swap the positions and run into this issue.

Internally, this requires us to teach Locomotor to ignore the self actor when performing movement cost checks for these "in reverse" searches so the unit doesn't consider the cell blocked by itself.
This commit is contained in:
RoosterDragon
2023-03-27 18:32:51 +01:00
committed by Paul Chote
parent e4ba9733fe
commit 4ec5a4b34a
11 changed files with 171 additions and 38 deletions
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12
OpenRA.Mods.Common/Pathfinder/HierarchicalPathFinder.cs
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@@ -73,13 +73,13 @@ namespace OpenRA.Mods.Common.Pathfinder
/// nodes, but uses a heuristic informed from the previous level to guide the search in the right direction.</para>
///
/// <para>This implementation is aware of movement costs over terrain given by
/// <see cref="Locomotor.MovementCostToEnterCell(Actor, CPos, CPos, BlockedByActor, Actor)"/>. It is aware of
/// <see cref="Locomotor.MovementCostToEnterCell(Actor, CPos, CPos, BlockedByActor, Actor, bool)"/>. It is aware of
/// changes to the costs in terrain and able to update the abstract graph when this occurs. It is able to search
/// the abstract graph as if <see cref="BlockedByActor.None"/> had been specified. If
/// <see cref="BlockedByActor.Immovable"/> is given in the constructor, the abstract graph will additionally
/// account for a subset of immovable actors using the same rules as
/// <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor)"/>. It will be aware of
/// changes to actors on the map and update the abstract graph when this occurs. Other types of blocking actors
/// <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor, bool)"/>. It will be aware
/// of changes to actors on the map and update the abstract graph when this occurs. Other types of blocking actors
/// will not be accounted for in the heuristic.</para>
///
/// <para>If the obstacle on the map is from terrain (e.g. a cliff or lake) the heuristic will work well. If the
@@ -633,14 +633,14 @@ namespace OpenRA.Mods.Common.Pathfinder
/// <summary>
/// <see cref="BlockedByActor.Immovable"/> defines immovability based on the mobile trait. The blocking rules
/// in <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor)"/> allow units to
/// pass these immovable actors if they are temporary blockers (e.g. gates) or crushable by the locomotor.
/// in <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor, bool)"/> allow units
/// to pass these immovable actors if they are temporary blockers (e.g. gates) or crushable by the locomotor.
/// Since our abstract graph must work for any actor, we have to be conservative and can only consider a subset
/// of the immovable actors in the graph - ones we know cannot be passed by some actors due to these rules.
/// Both this and <see cref="ActorCellIsBlocking"/> must be true for a cell to be blocked.
///
/// This method is dependant on the logic in
/// <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor)"/> and
/// <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor, bool)"/> and
/// <see cref="Locomotor.UpdateCellBlocking"/>. This method must be kept in sync with changes in the locomotor
/// rules.
/// </summary>