Add lld-parking-lot module: thread-safe parking lot with Strategy, Observer and CAS-based spot claiming, 10 tests incl. 64-thread race

2026-10-22 08:01:55 +00:00
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+# lld-parking-lot
+
+A thread-safe parking lot in plain Java, built the way a low-level-design interview expects:
+requirements, entities, patterns, concurrency, extensions. Companion to
+[Low-Level Design Interview in Java: Parking Lot System, Step by Step](https://ankurm.com/low-level-design-interview-parking-lot-system-java/).
+
+No framework, no runtime dependencies. JUnit 6 is used for tests only.
+
+## Versions
+
+| Component | Version | Verified against |
+|---|---|---|
+| JDK | 25.0.4.1+1 (Temurin LTS); also compiled and run with `--release 21` on OpenJDK 21 | `java -version`; the walkthrough output is byte-identical on both |
+| JUnit Jupiter | 6.1.3 | `org.junit.jupiter:junit-jupiter` metadata on Maven Central |
+| Maven | 3.9.11 | `mvn -version` |
+
+## Run it
+
+```bash
+cd lld-parking-lot
+./scripts/run.sh          # compiles with javac, prints the walkthrough (no Maven needed)
+mvn test                  # 10 tests, including two concurrency tests
+./scripts/run-all.sh      # regenerates everything in output/
+```
+
+## Layout
+
+```
+src/main/java/com/ankurm/interviewlab/parking/
+  VehicleType, SpotSize           enums; SpotSize.canHold(VehicleType) is the only "does it fit" rule
+  Vehicle, Ticket, Receipt        immutable records
+  ParkingSpot                     occupancy is one AtomicReference compare-and-set
+  ParkingFloor                    ordered spots
+  AllocationStrategy              Strategy: NearestFirstStrategy, BestFitStrategy
+  PricingStrategy                 Strategy: HourlyPricing (grace period, partial hours round up)
+  AvailabilityListener            Observer: the entrance display board
+  ParkingException                LotFull, AlreadyParked, UnknownTicket
+  ParkingLot                      the aggregate: park(), exit(), freeSpots()
+  Demo                            prints output/01-walkthrough.txt
+src/test/java/.../ParkingLotTest  10 tests; Clock is injected so nothing sleeps
+output/                           captured real output
+```
+
+## Captured output
+
+| File | What it shows |
+|---|---|
+| `output/01-walkthrough.txt` | Enter, leave, fail three ways, and the two allocation strategies placing the same motorcycle differently |
+| `output/02-test-results.txt` | One line per test, generated from the Surefire XML report |
+
+## License
+
+MIT, see the repository root.