[50.02] Bridges should require connected roads in map validator (or rails, once rail terrain is implemented)

[gsparks3]

Environment

Standalone Megamek.
Windows 10
Java 11.0.10

Description

A Height 1 bridge across Depth 2 water, bordered by Level 1 hexes, does not seem to be passable by either a wheeled vehicle or a mech. Discovered during a campaign game. It is possible that the issue is due to the custom board file (included below).

Reproduction steps:

1. Load the provided savegame and join as either "Rahne Militia" for the wheeled vehicle or "Rahne Milita" [sic] (for the mech.
2. Use \skip to bypass all other player turns.
3. Attempt to move a unit across the bridge in hexes 0234/0335. Note that the hovercraft can succeed while the others fail.

Wheeled Vehicle (Mobile HQ):

Mech (Dervish-7D):

Files

Game save file:
bridgeBug.sav.gz
Custom board file:
Lyon_III.zip

Nothing in the logs.

[KyleM5]

The high cost of movement is because he is moving off of a rail tile onto a bridge tile (they can't connect). If the tile the Dervish or APC were standing on was connected road, it would work. If it is intended for rail tiles to connect to road bridges, than it is a bug. If not, this should be closed.

[gsparks3]

Rail hexes are not actually implemented in Megamek; the "rail hex" is a clear hex with a fluff image applied (which has no gameplay differences from a clear hex). This is still true as of version Megamek 50.02.

I managed to recreate the unexpectedly high MP cost in version 50.02 using the same custom board. Savegame attached:
bridgeBug50.02.sav.gz

I did notice that a very large number of bridges in other maps have roads running into them, while this one does not (since the "rails" are fluff-image-only). As a quick check, I set up another board to verify that a bridge between two elevated clear hexes (not over water) has no issues, so it's not simply that bridges are required to connect to roads:

Adjusting the map further, it seems that it is treating the unit as if it is moving through the water, despite being on a bridge above the water. Moving across a bridge over depth 1 water requires 2 MP:

Moving across a bridge over depth 2 water requires 4 MP:

This matches the expected MP cost for entering a depth 1 or depth 2 water hex without making a level change, except that the unit is on a bridge above the water and should not be affected by the water. This also explains why the units in the original example could not enter the bridge hex at all (due to the additional MP required for the Dervish to turn around on top of the 4 MP for the bridge hex over depth 2 water, it would have to Run to walk onto the bridge, and "entering a water hex" cannot be done while Running; similarly the vehicle is above its Cruise MP). The vehicle may also simply be excluded entirely, since water hexes are prohibited terrain for vehicles. Further testing confirms that this is the case:

Adjusting the map even further on a hunch, I can confirm that if there are roads leading into the bridge, this allows the units to "ignore the terrain in the hex" from following a road - masking the bug entirely. Mech:

Vehicle:

Save for testing purposes: bridgeBugTesting50.02.sav.gz

It is very likely that this is related to #6373 - both involve units on bridges being affected by terrain underneath the bridge.

[psikomonkie]

There’s a few different issues and non-issues at play here.

First, invalid maps:
“Every bridge starts and ends with a road hex that has the same elevation as the underlying terrain.” TO:AR 115

I think your test bridges are not valid per TO:AR rules. That’s why crossing the water works if there’s a road on both sides - it’s a valid bridge.

The map in the original report has rails on either side. Once Rails are implemented properly, and placed across the bridge if necessary, it should work.

Should be fixed by #6303 once rails are implemented.

Don't close this as a duplicate, this should be retested and verified once 6303's rails fully are implemented.

[gsparks3]

Interesting, apparently I missed that line about bridges needing to connect to roads in TO:AR. With that implemented in the map validator, the existing road code would be enough to ensure that units following the bridge are unaffected by terrain below them, even if it feels weird that the code doesn't actually check if the unit is touching the terrain or not.