fix: keep passenger car couplers connected through bends

[ryanbarlow97]

Passenger cars currently separate from the locomotive through bends because each body follows its own track tangent and spacing includes coupler height. Place each trailing car against the preceding car’s planned rear anchor, using the model pivot, scale and base rotation; keep route coordinates for track traversal and use the corrected pose for collision checks. Also give the passenger car a nonzero forward direction using its connector bones.

Validation: all 496 Maven tests pass, including new bend/stop/reverse coupling coverage and real model pivot/scale transforms. On an isolated CachyOS Paper 1.21.10 + ModelEngine server with a vanilla RTX 3070 client, the same radius-12 bend measured a maximum connector gap of 1.076495 blocks before and 0.000004 after (135 moving samples per build); before/after video was reviewed. No model edits are needed. Matching installed config is tracked in https://github.com/TF-Minecraft/ServerAssets/pull/17.

Review follow-up: retain the sampled pose if connector blueprint/model data is unavailable. Three regression cases reproduce missing parent, child, or both connector blueprints and verify placement recovers once they load.

Summary by CodeRabbit

• Improvements
  • Train cars now align their couplers more accurately on curved tracks, including when trains stop or reverse.
  • Coupling accounts for vehicle scale and connector position, helping keep adjacent cars connected.
  • Car placement handles missing connector points and avoids moving the train tail through nearby obstacles.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 7d13e0d7-636d-4bfe-88cd-658eb79b110f

📥 Commits

Reviewing files that changed from the base of the PR and between 7910896 and 3619167.

📒 Files selected for processing (2)

• src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java
• src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainReversePlacementTest.java

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📝 Walkthrough

Walkthrough

The change adds connector reach and pose calculations. Train placement uses these calculations to align car couplers when planning a train. Tests cover connector alignment and train movement on curved tracks, including stops, reversals, and collisions.

Changes

Train Coupler Alignment

Layer / File(s)	Summary
Connector geometry and reach src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/train/Connector.java, src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/train/ConnectorTest.java	Connector calculates track reach, connector position at a TrackPose, and a pose that aligns the connector with a target. Tests check reach across model scales and alignment across pose and rotation values.
Coupler-based train placement src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java, src/main/resources/vehicles/passenger_car.yml, src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainReversePlacementTest.java	Train placements store sampled poses. When adjacent cars have couplers and no spacing is missing, placement aligns the car’s front coupler with the preceding car’s back-coupler position. The passenger car behavior vector uses connector bones. Tests cover curved-track alignment, missing connector bones, train movement, and collision checks.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant TrainHandler
  participant BackConnector
  participant FrontConnector
  TrainHandler->>BackConnector: positionAt(parent pose)
  BackConnector-->>TrainHandler: parent back-coupler position
  TrainHandler->>FrontConnector: coupledPose(nominal pose, target position)
  FrontConnector-->>TrainHandler: aligned car pose
  TrainHandler->>TrainHandler: store placement with aligned pose

Loading

Merge Risk: ⚪ Minimal · up to 36191

No actionable merge-blocking issue remains in the supplied evidence. The train-placement fallback is retained when connector blueprints are unavailable.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 36191

The coupling change affects train placement and collision behavior, but the reviewed path does not show a new way to control trains or a verified security issue. Some runtime and change-scope evidence remains incomplete.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The identified effect is confined to the existing train-movement and in-world collision path; the reviewed changes do not establish a new external service or privileged sink.

Trust Boundaries and Controls

• observed — Connector alignment runs on the existing parent-child consist and requires attachment eligibility and complete spacing; the reviewed code does not relink cars or change those eligibility checks.

Resilience and Maintainability Implications

• observed — Missing connector data falls back to a sampled pose, while rejected movement steps do not commit their proposed placements. These controls limit the observed failure path, without proving every entity-lifecycle state safe.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 9.38% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 32 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: maintaining passenger car coupler connections through track bends.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches 💡 1

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_{A rabbit checks the couplers meet,
Along the bends, through stops, retreat.
The front finds where the back has been,
And scale-aware anchors line up clean.
The train rolls on; the tracks curve by,
While carrots wait beneath the sky.}

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
In
@src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java:
- Around line 809-814: Preserve the sampled carPose when connector blueprints
are unavailable: guard the coupled-pose adjustment in the planCars flow around
parentTrain.getBack().positionAt and carTrain.getFront().coupledPose, and retain
carPose if those calls fail while the model is unloaded.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: b23676f7-1283-4eac-b5db-a30789dbc714

📥 Commits

Reviewing files that changed from the base of the PR and between 931ea8c and 7910896.

📒 Files selected for processing (5)

• src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java
• src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/train/Connector.java
• src/main/resources/vehicles/passenger_car.yml
• src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainReversePlacementTest.java
• src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/train/ConnectorTest.java

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 9 remain after this review.

[ryanbarlow97]

Reviewed the final placement/collision diff and fixed the one actionable finding about unavailable connector blueprints, with a failing-before/passing-after regression. All 496 tests pass. The CachyOS comparison exercised the normal loaded-model path; the follow-up only restores the unloaded-model fallback.

The advisory docstring coverage warning includes existing methods and test helpers; broad documentation churn is outside this bug fix. New coordinate-transform intent is documented inline. The architecture suggestion about unusually large pivots is a future hardening case: the supplied locomotive/passenger dimensions and multiple scales are covered, and no bypass was established by review. No remaining blocking findings in the reviewed diff.