A robust transpiler to add extra pathing costs to FindPath in RimWorld

RimworldAddingExtraPathCostsSafeTranspiler.cs

[HarmonyPatch(typeof(PathFinder), "FindPath")]
[HarmonyPatch(new Type[] { typeof(IntVec3), typeof(LocalTargetInfo), typeof(TraverseParms), typeof(PathEndMode) })]
public static class PathFinder_FindPath_Patch
{
	static int GetExtraCosts(Pawn pawn, int gridIdx)
	{
		return 0; // extra costs to add for gridIdx
	}

	static readonly MethodInfo m_CellToIndex_int_int = AccessTools.Method(typeof(CellIndices), nameof(CellIndices.CellToIndex), new Type[] { typeof(int), typeof(int) });
	static readonly FieldInfo f_TraverseParms_pawn = AccessTools.Field(typeof(TraverseParms), nameof(TraverseParms.pawn));
	static readonly MethodInfo m_GetExtraCosts = SymbolExtensions.GetMethodInfo(() => GetZombieCosts(null, 0));
	static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions, MethodBase original)
	{
		var list = instructions.ToList();
		while (true)
		{
			var t_PathFinderNodeFast = AccessTools.Inner(typeof(PathFinder), "PathFinderNodeFast");
			var f_knownCost = AccessTools.Field(t_PathFinderNodeFast, "knownCost");
			if (f_knownCost == null)
			{
				Log.Error($"Cannot find field Verse.AI.PathFinder.PathFinderNodeFast.knownCost");
				break;
			}

			var idx = list.FirstIndexOf(ins => ins.Calls(m_CellToIndex_int_int));
			if (idx < 0 || idx >= list.Count() || list[idx + 1].opcode != OpCodes.Stloc_S)
			{
				Log.Error($"Cannot find CellToIndex(n,n)/Stloc_S in {original.FullDescription()}");
				break;
			}
			var gridIdx = list[idx + 1].operand;

			var insertLoc = list.FirstIndexOf(ins => ins.opcode == OpCodes.Ldfld && (FieldInfo)ins.operand == f_knownCost);
			while (insertLoc >= 0 && insertLoc < list.Count)
			{
				if (list[insertLoc].opcode == OpCodes.Add) break;
				insertLoc++;
			}
			if (insertLoc < 0 || insertLoc >= list.Count())
			{
				Log.Error($"Cannot find Ldfld knownCost ... Add in {original.FullDescription()}");
				break;
			}

			var traverseParmsIdx = original.GetParameters().FirstIndexOf(info => info.ParameterType == typeof(TraverseParms)) + 1;

			list.Insert(insertLoc++, new CodeInstruction(OpCodes.Add));
			list.Insert(insertLoc++, new CodeInstruction(OpCodes.Ldarga_S, traverseParmsIdx));
			list.Insert(insertLoc++, new CodeInstruction(OpCodes.Ldfld, f_TraverseParms_pawn));
			list.Insert(insertLoc++, new CodeInstruction(OpCodes.Ldloc_S, gridIdx));
			list.Insert(insertLoc++, new CodeInstruction(OpCodes.Call, m_GetExtraCosts));
			break;
		}

		foreach (var instr in list)
			yield return instr;
	}
}