fix: keep trains in place when track under them is rebuilt

[ryanbarlow97]

Problem

Removing rail with the track remover could teleport a train on the same track, even one some distance away. A train stores its position as a spline id plus arc length s. Digging the middle of a spline keeps the id on the first piece and gives the far piece a new id, so a train on the far piece kept a stale s. That s was clamped to the end of the first piece, which is the cut. Trimming the first sample shifted every train on the spline forward by one block.

Punching and explosions only mark segments broken. They don't change geometry and weren't affected.

Change

• TrackRegistry.onRebuilt reports each rebuilt spline with the splines that now carry its track. This covers replace() (trim, extend, join, loop close, turnout removal, piece trimming after paid lays), mid-dig splits, and the dropped side of a join.
• TrainHandler.retrack moves each car on the old spline to the matching point on those replacements. Matching uses the TrackClearance overlap thresholds and ignores unrelated nearby track such as crossings or branches. Parked trains update straight away, so saved consist data stays correct.
• The remover refuses to dig track that a bound consist occupies, measured from each car out to its couplers. It tells the player: "A train is on this track. Move it before removing the rail."
• TrackRegistry.digTarget reports what a dig would remove before it happens. The unused dig(...) overloads are removed, and the turnout check is shared between digTarget and digAt.
• spacing and the new reach share one connector-offset helper.

Known limitations (not changed here)

• Joining track at a spline's first sample, or closing a loop, can reverse that spline. Trains on it keep their world position, but the model faces the new +s direction, so the loco turns round and its cars move to the other side. This bug predates this PR; the old code moved the train to a mirrored position instead.
• The occupancy check covers the dug spline. Track removed as a side effect, such as a branch turnout dropped because its stem piece became too short, isn't checked.

Testing

• mvn test: 445 tests pass.
• New tests in TrainReversePlacementTest cover: a split behind the train, driving on after a split, trimming the first sample, saving a parked train after a split, deleted track, a split at a crossing, deleting a track that has a crossing, breaking a segment, occupancy out to the couplers, and the dig span. On the old code, the split test shows the train jumping from z=60 to the cut at z=19.
• Not yet tested on a live server.

Docs: TF-Minecraft/Docs#61

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Trains remain aligned with track after nearby track sections are changed, split, or connected.
  • Trains retain their junction route when a track is split, and cars trailing on a branch keep their position.
  • Track sections occupied by a train cannot be dug up, and players receive a warning when removal is blocked.
  • Digging at a turnout removes the turnout section more consistently.
  • Train connector spacing is handled more reliably when connector offsets are unavailable.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 1adf7593-2616-40b2-94f5-b9999357f7c9

📥 Commits

Reviewing files that changed from the base of the PR and between b155175 and b6ae26f.

📒 Files selected for processing (2)

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

🚧 Files skipped from review as they are similar to previous changes (2)

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

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📝 Walkthrough

Walkthrough

Track rebuilds now trigger train retracking. Track digging resolves a target span and checks train occupancy before removing track. Tests cover retracking, occupancy, and dig-target selection.

Changes

Track Rebuild and Digging

Layer / File(s)	Summary
Track rebuild and train retracking src/main/java/net/tfminecraft/vehicleframework/VehicleFramework.java, src/main/java/net/tfminecraft/vehicleframework/tracks/TrackRegistry.java, src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java, src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainReversePlacementTest.java	TrackRegistry notifies listeners when it replaces, connects, or splits splines. VehicleFramework registers TrainHandler as a listener. Train handlers select nearby rebuilt splines within clearance limits and update bindings and junction-route state. Tests cover train positions and bindings after track changes.
Dig targets and train occupancy src/main/java/net/tfminecraft/vehicleframework/tracks/TrackRegistry.java, src/main/java/net/tfminecraft/vehicleframework/tracks/TrackCommands.java, src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java, src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainReversePlacementTest.java	TrackRegistry selects dig targets and reports their removal spans. TrackCommands blocks removal when a root train occupies the target span. TrainHandler checks consist positions using coupler reach and loop-track distance. Tests cover occupancy and dig-target values.

Priority: ➖ Normal

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

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant TrackRegistry
  participant TrainHandler
  participant VehicleManager
  TrackRegistry->>TrainHandler: Notify listener with old and rebuilt splines
  TrainHandler->>VehicleManager: Get registered train vehicles
  TrainHandler->>TrainHandler: Retrack each vehicle to a nearby rebuilt spline

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sequenceDiagram
  participant TrackCommands
  participant TrackRegistry
  participant VehicleManager
  participant TrainHandler
  TrackCommands->>TrackRegistry: Resolve dig target and removal span
  TrackCommands->>VehicleManager: Get root train vehicles
  TrackCommands->>TrainHandler: Check occupancy of target track and span
  alt Target is occupied
    TrackCommands->>TrackCommands: Warn player and leave rail in place
  else Target is not occupied
    TrackCommands->>TrackRegistry: Dig at target spline and index
  end

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Merge Risk: ⚪ Minimal · up to b6ae2

No actionable merge-blocking issue was established; the change is ready for normal merge checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to b1551

The change adds protection for occupied track and preserves the positions of active trains during rebuilds. No new security exploit was established, but the protection has not been shown to cover trains outside the active vehicle set.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The examined effect is confined to plugin-managed track and train state; no cross-service dependency or deployment change is established by the supplied architecture map.

Trust Boundaries and Controls

• observed — The new occupancy control is enforced by the player-facing remover path, not by every registry mutation. The separate administrative delete method calls registry deletion without that check; the evidence does not show that this existing path became more reachable in this PR.

Hardening Proposals

• proposed — Define whether occupied-track protection must include saved but inactive trains and administrative or direct registry deletions; if it must, enforce that policy at a mutation boundary with access to the required vehicle state.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 10.87% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 46 functions across 5 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: preserving train positions when track geometry is rebuilt.
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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---

_{I’m a rabbit by the rails,
I watch the train find new trails.
Tracks rebuild; the cars align,
Digging waits when trains are in line.
I thump my paws and hop away,
Safe rails carry trains today.}

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

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@src/main/java/net/tfminecraft/vehicleframework/tracks/TrackRegistry.java`:
- Line 264: In TrackRegistry, defer rebuild callbacks until junction rehoming is
complete: in the split path, call rehomeJunctions before rebuilt.accept; in the
connect path, rehome both saved junction sets before retracking either replaced
spline. Use a non-notifying replacement there so replace(next) does not fire
early, then invoke both rebuild callbacks after both rehomeJunctions calls.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 6b129569-3206-464f-9177-67b92c6dcc08

📥 Commits

Reviewing files that changed from the base of the PR and between 25c3b52 and b155175.

📒 Files selected for processing (5)

• src/main/java/net/tfminecraft/vehicleframework/VehicleFramework.java
• src/main/java/net/tfminecraft/vehicleframework/tracks/TrackCommands.java
• src/main/java/net/tfminecraft/vehicleframework/tracks/TrackRegistry.java
• src/main/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainHandler.java
• src/test/java/net/tfminecraft/vehicleframework/vehicles/handlers/TrainReversePlacementTest.java

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review.