OpenTTD

I found a strange bug during testing the physics patch. As the fault appears to be in trunk code, I don't suspect the patch causes this.

I can't confirm it, but this bugs seems to happen only with trains (electric or not ) on elrail. No occurance noticed on normal rail.

The symptoms are that trains suddenly have 0 hp, which sometimes fixes itself magically. As the train power is cached and all, it normal not to encounter this symptom. The physics patch reuses a part of the power calculations for tractive effort (TE), which goes to 0 sometimes, too; only it's much more visible then as trains will suddenly move very slowly for no apparent reason.

Calculation and caching of these values happens in TrainCargoChanged(). To debug it, I added a line to GetTrainAcceleration() that calls TrainCargoChanged() every time power or TE are 0. It's very likely that bridges (and maybe tunnels) have something to with it, as I observed a train that passed below a bridge and fixed itself, while it wasn't able to do so on the about 15 tiles of elrail before.

I was able to pin it down to the following code part from train_cmd.c line 92 inside TrainPowerChanged(). In most cases, all test should be negative an no variable setto false. The bug occurs when one of these test produces a false positive.

if (IsBridgeTile(u->tile) && IsBridgeMiddle(u->tile) && DiagDirToAxis(DirToDiagDir(u->direction)) == GetBridgeAxis(u->tile)) {
if (!HasPowerOnRail(u->u.rail.railtype, GetRailTypeOnBridge(u->tile))) engine_has_power = false;
if (!HasPowerOnRail(v->u.rail.railtype, GetRailTypeOnBridge(u->tile))) wagon_has_power = false;
} else if (IsLevelCrossing(u->tile)) {
if (!HasPowerOnRail(u->u.rail.railtype, GetRailTypeCrossing(u->tile))) engine_has_power = false;
if (!HasPowerOnRail(v->u.rail.railtype, GetRailTypeCrossing(u->tile))) wagon_has_power = false;
} else {
if (!HasPowerOnRail(u->u.rail.railtype, GetRailType(u->tile))) engine_has_power = false;
if (!HasPowerOnRail(v->u.rail.railtype, GetRailType(u->tile))) wagon_has_power = false;
}