Units surrounded by a Hazard refuse to move

[whakomatic]

AI units surrounded by hazardous world effects (fire, poison, acid) become unable to move and will skip their movement

[whakomatic]

AI Hazard Escape Fix

AI units surrounded by hazardous world effects (fire, poison, acid) become unable to move and will skip their movement

Root Cause

The native pathfinding system (X2ReachableTilesCache.UpdateTileCacheIfNeeded()) excludes tiles hazardous to the unit from the reachable tiles cache. AI shouldn't voluntarily walk into fire so this is correct behavior, however, when a unit is surrounded by the hazard the path to a destination will fail.

The reason for failure is that the pathfinding system integrates a Units immunities to hazards into the resulting destinations and unit pathing. An immune unit will path out just fine, but a non immune unit will not.

Code Flow

1. BT_StartGetDestinations
  └─> [NATIVE] m_kReachableTilesCache.UpdateTileCacheIfNeeded()
      └─> Excludes hazardous tiles from cache
      └─> Result: Empty cache when unit is hazards

2. MoveToPoint
  └─> foreach m_kReachableTilesCache.CoverDestinations(Position)
      └─> Zero tiles to process when cache is empty
      └─> AI has no destination options

Solution Overview

The fix detects when a unit is in a hazard with an empty movement cache and enables special "hazard escape" pathfinding that bypasses the native cache filtering.

Key Components

1. Detection Flag (function NeedsHazardEscape): Set when cache is empty AND unit is in hazardous terrain
2. Validation Bypass: Skip IsTileReachable checks (which rely on the empty cache)
3. Alternative Pathfinding: Use BuildNonUnitPath instead of cache-based pathfinding
4. Unit Capabilities: Build traversal list from unit's character template (respects grapple, climb, etc.)

Change 1: Detection Flag

Detect when hazard escape mode is needed when there are no cover destinations in movement range.
If there is no cover in movement range, then standing still is not really a disadvantage.

function NeedsHazardEscape return m_kUnit.m_kReachableTilesCache.CoverDestinations.Length == 0 &&
                      class'XComPath'.static.TileContainsHazard(UnitState, UnitState.TileLocation));

Note: Uses TileContainsHazard (terrain hazards only) not IsInDangerousArea (includes flanking).

---

Change 2: Alternative Pathfinding

Use BuildNonUnitPath with unit's actual traversal capabilities that does not depend on Hazard Immunities

if (NeedsHazardEscape())
{
    // Build allowed traversals from unit's character template
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_Normal) AllowedTraversals.AddItem(eTraversal_Normal);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_ClimbOver) AllowedTraversals.AddItem(eTraversal_ClimbOver);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_ClimbOnto) AllowedTraversals.AddItem(eTraversal_ClimbOnto);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_ClimbLadder) AllowedTraversals.AddItem(eTraversal_ClimbLadder);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_DropDown) AllowedTraversals.AddItem(eTraversal_DropDown);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_Grapple) AllowedTraversals.AddItem(eTraversal_Grapple);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_Landing) AllowedTraversals.AddItem(eTraversal_Landing);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_BreakWindow) AllowedTraversals.AddItem(eTraversal_BreakWindow);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_KickDoor) AllowedTraversals.AddItem(eTraversal_KickDoor);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_JumpUp) AllowedTraversals.AddItem(eTraversal_JumpUp);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_WallClimb) AllowedTraversals.AddItem(eTraversal_WallClimb);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_BreakWall) AllowedTraversals.AddItem(eTraversal_BreakWall);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_Launch) AllowedTraversals.AddItem(eTraversal_Launch);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_Flying) AllowedTraversals.AddItem(eTraversal_Flying);
    if (UnitState.GetMyTemplate().bCanUse_eTraversal_Land) AllowedTraversals.AddItem(eTraversal_Land);

    class'X2PathSolver'.static.BuildNonUnitPath(UnitState.TileLocation, kTileDest, AllowedTraversals, Path);
}

Solution Flow

1. BT_StartGetDestinations
   └─> Cache is empty + unit in hazard
   └─> Set bNeedsHazardEscape = true
   └─> Get list of potential destinations between 3-5 tiles away `GatherHazardEscapeTiles`
   └─> return with highest rated destination
   
2. MoveToPoint
   └─> Skip IsTileReachable check (cache is empty)
   └─> Build AllowedTraversals from unit template
   └─> Use BuildNonUnitPath (bypasses cache/hazard filtering)
   └─> Get path to escape destination
   └─> Unit moves out of hazard ✓

Potential Issues

• Will it slow down the game? Search should be fast because its not every reachable point, only 96 tiles.
• What if BuildNonUnitPath also fails?: Path.Length < 2, unit stands still (existing behavior)
• What if destination tile is also hazardous?: AI scoring already heavily penalizes hazardous tiles, so unit will pick the "least bad" option
• Can this cause AI to voluntarily enter hazards?: No - NeedsHazardEscape() only triggers when unit is already in a hazard with no paths