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
parent d4b3a63615
commit a5fbf45971
24 changed files with 670 additions and 1 deletions
@@ -8,7 +8,8 @@ One module per article. This repository currently holds:

 | Module | Article | Status |
 |---|---|---|
-| [`collections`](collections/) | Top 40 Java Collections Interview Questions (HashMap Internals to Fail-Fast Iterators) | this commit |
+| [`collections`](collections/) | Top 40 Java Collections Interview Questions (HashMap Internals to Fail-Fast Iterators) | published |
+| [`lld-parking-lot`](lld-parking-lot/) | Low-Level Design Interview in Java: Parking Lot System, Step by Step | this commit |

 ## `collections` module

@@ -0,0 +1,53 @@
+# 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.
@@ -0,0 +1,16 @@
+== 1. entering and leaving ==
+   board: {SMALL=1, MEDIUM=2, LARGE=1}
+   board: {SMALL=1, MEDIUM=1, LARGE=1}
+   board: {SMALL=1, MEDIUM=1, LARGE=0}
+bike  -> F0-1, car -> F0-2, truck -> F0-3
+   board: {SMALL=1, MEDIUM=2, LARGE=0}
+car stayed 130 min, fee = 60
+
+== 2. failures ==
+second MH03-TRK : MH03-TRK is already inside the lot
+another truck  : no free spot for TRUCK MH04-TRK
+reused ticket  : unknown or already used ticket T-2
+
+== 3. the strategy decides where the motorcycle goes ==
+NearestFirstStrategy puts a motorcycle in F0-1
+BestFitStrategy puts a motorcycle in F1-1
@@ -0,0 +1,11 @@
+PASS truckIsRejectedWhenOnlyMediumSpotsAreFree
+PASS listenersSeeEveryChange
+PASS samePlateCannotBeInsideTwice
+PASS bestFitKeepsLargeSpotsFreeForTrucks
+PASS concurrentDuplicatePlateAdmitsExactlyOne
+PASS failedParkDoesNotLeaveThePlateReserved
+PASS nearestFirstTakesTheFirstFreeSpotThatFits
+PASS feeRoundsPartialHoursUpAndHasAGracePeriod
+PASS concurrentParkingNeverDoubleBooksASpot
+PASS exitFreesTheSpotAndTheTicketIsSingleUse
+10 tests, 0 failures
@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>com.ankurm.interviewlab</groupId>
+  <artifactId>lld-parking-lot</artifactId>
+  <version>1.0.0</version>
+  <name>lld-parking-lot</name>
+  <description>Low-level design interview: a thread-safe parking lot in plain Java</description>
+
+  <properties>
+    <maven.compiler.release>21</maven.compiler.release>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <junit.version>6.1.3</junit.version>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter</artifactId>
+      <version>${junit.version}</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>
@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Regenerates every transcript in output/. Needs JDK 21+ and Maven 3.9+.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+mkdir -p output
+./scripts/run.sh > output/01-walkthrough.txt 2>&1
+mvn -B -q test > /dev/null
+python3 - <<'PY' > output/02-test-results.txt
+import glob, xml.etree.ElementTree as ET
+total = fail = 0
+for f in sorted(glob.glob("target/surefire-reports/TEST-*.xml")):
+    for tc in ET.parse(f).getroot().iter("testcase"):
+        bad = tc.find("failure") is not None or tc.find("error") is not None
+        total += 1; fail += bad
+        print(("FAIL " if bad else "PASS ") + tc.get("name"))
+print(f"{total} tests, {fail} failures")
+PY
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# Compiles with plain javac (no Maven needed) and runs the walkthrough. Needs JDK 21 or newer.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+rm -rf target/plain && mkdir -p target/plain
+javac -d target/plain $(find src/main/java -name '*.java')
+java -cp target/plain com.ankurm.interviewlab.parking.Demo
@@ -0,0 +1,12 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.List;
+
+/**
+ * Strategy pattern: decides the ORDER in which spots are offered to a vehicle.
+ * It never claims a spot itself, because claiming must be atomic and belongs to {@link ParkingSpot}.
+ * The lot walks the returned list and keeps the first spot that {@code tryOccupy} wins.
+ */
+public interface AllocationStrategy {
+    List<ParkingSpot> candidates(List<ParkingFloor> floors, Vehicle vehicle);
+}
@@ -0,0 +1,9 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.Map;
+
+/** Observer pattern: the display board at the entrance implements this. */
+@FunctionalInterface
+public interface AvailabilityListener {
+    void changed(Map<SpotSize, Long> freeBySize);
+}
@@ -0,0 +1,17 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.Comparator;
+import java.util.List;
+
+/** Smallest spot that fits, nearest first among equals. Keeps large spots free for trucks. */
+public final class BestFitStrategy implements AllocationStrategy {
+    @Override
+    public List<ParkingSpot> candidates(List<ParkingFloor> floors, Vehicle vehicle) {
+        return floors.stream()
+                .flatMap(f -> f.spots().stream())
+                .filter(ParkingSpot::isFree)
+                .filter(s -> s.size().canHold(vehicle.type()))
+                .sorted(Comparator.comparing(ParkingSpot::size).thenComparingInt(ParkingSpot::floor))
+                .toList();
+    }
+}
@@ -0,0 +1,58 @@
+package com.ankurm.interviewlab.parking;
+
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.util.List;
+
+/** Prints the whole story once; the transcript is committed as output/01-walkthrough.txt. */
+public final class Demo {
+
+    /** A clock we can move by hand, so "2 hours later" needs no sleeping. */
+    static final class ManualClock extends Clock {
+        private Instant now = Instant.parse("2026-10-22T09:00:00Z");
+        void advance(Duration d) { now = now.plus(d); }
+        @Override public Instant instant() { return now; }
+        @Override public java.time.ZoneId getZone() { return ZoneOffset.UTC; }
+        @Override public Clock withZone(java.time.ZoneId zone) { return this; }
+    }
+
+    public static void main(String[] args) {
+        System.out.println("== 1. entering and leaving ==");
+        var clock = new ManualClock();
+        var lot = new ParkingLot(
+                List.of(ParkingFloor.of(0, SpotSize.SMALL, SpotSize.MEDIUM, SpotSize.LARGE),
+                        ParkingFloor.of(1, SpotSize.SMALL, SpotSize.MEDIUM)),
+                new BestFitStrategy(), HourlyPricing.standard(), clock);
+        lot.addListener(free -> System.out.println("   board: " + free));
+
+        Ticket bike = lot.park(new Vehicle("MH01-BIKE", VehicleType.MOTORCYCLE));
+        Ticket car = lot.park(new Vehicle("MH02-CAR", VehicleType.CAR));
+        Ticket truck = lot.park(new Vehicle("MH03-TRK", VehicleType.TRUCK));
+        System.out.println("bike  -> " + bike.spotId() + ", car -> " + car.spotId() + ", truck -> " + truck.spotId());
+
+        clock.advance(Duration.ofHours(2).plusMinutes(10));
+        Receipt r = lot.exit(car.id());
+        System.out.println("car stayed " + r.duration().toMinutes() + " min, fee = " + r.fee());
+
+        System.out.println();
+        System.out.println("== 2. failures ==");
+        try { lot.park(new Vehicle("MH03-TRK", VehicleType.TRUCK)); }
+        catch (ParkingException e) { System.out.println("second MH03-TRK : " + e.getMessage()); }
+        try { lot.park(new Vehicle("MH04-TRK", VehicleType.TRUCK)); }
+        catch (ParkingException e) { System.out.println("another truck  : " + e.getMessage()); }
+        try { lot.exit(car.id()); }
+        catch (ParkingException e) { System.out.println("reused ticket  : " + e.getMessage()); }
+
+        System.out.println();
+        System.out.println("== 3. the strategy decides where the motorcycle goes ==");
+        for (AllocationStrategy s : List.of(new NearestFirstStrategy(), new BestFitStrategy())) {
+            var l = new ParkingLot(
+                    List.of(ParkingFloor.of(0, SpotSize.LARGE), ParkingFloor.of(1, SpotSize.SMALL)),
+                    s, HourlyPricing.standard(), clock);
+            System.out.println(s.getClass().getSimpleName() + " puts a motorcycle in "
+                    + l.park(new Vehicle("MH05-BIKE", VehicleType.MOTORCYCLE)).spotId());
+        }
+    }
+}
@@ -0,0 +1,34 @@
+package com.ankurm.interviewlab.parking;
+
+import java.math.BigDecimal;
+import java.time.Duration;
+import java.util.Map;
+
+/**
+ * Per-hour rate by vehicle type, partial hours rounded UP, with a free grace period.
+ * Rates are whole rupees to keep the example free of rounding noise; money would be BigDecimal in production anyway.
+ */
+public final class HourlyPricing implements PricingStrategy {
+    private final Map<VehicleType, BigDecimal> ratePerHour;
+    private final Duration grace;
+
+    public HourlyPricing(Map<VehicleType, BigDecimal> ratePerHour, Duration grace) {
+        this.ratePerHour = Map.copyOf(ratePerHour);
+        this.grace = grace;
+    }
+
+    public static HourlyPricing standard() {
+        return new HourlyPricing(Map.of(
+                VehicleType.MOTORCYCLE, BigDecimal.valueOf(10),
+                VehicleType.CAR, BigDecimal.valueOf(20),
+                VehicleType.TRUCK, BigDecimal.valueOf(40)), Duration.ofMinutes(15));
+    }
+
+    @Override
+    public BigDecimal fee(Vehicle vehicle, Duration stayed) {
+        if (stayed.compareTo(grace) <= 0) return BigDecimal.ZERO;
+        long minutes = stayed.toMinutes() + (stayed.toSecondsPart() > 0 ? 1 : 0);
+        long hours = Math.max(1, (minutes + 59) / 60);
+        return ratePerHour.get(vehicle.type()).multiply(BigDecimal.valueOf(hours));
+    }
+}
@@ -0,0 +1,15 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.List;
+
+/** Lowest floor, then lowest spot number. Fast for the driver, wasteful for big spots. */
+public final class NearestFirstStrategy implements AllocationStrategy {
+    @Override
+    public List<ParkingSpot> candidates(List<ParkingFloor> floors, Vehicle vehicle) {
+        return floors.stream()
+                .flatMap(f -> f.spots().stream())
+                .filter(ParkingSpot::isFree)
+                .filter(s -> s.size().canHold(vehicle.type()))
+                .toList();
+    }
+}
@@ -0,0 +1,18 @@
+package com.ankurm.interviewlab.parking;
+
+/** Base type so callers can catch one thing; the nested types say what actually went wrong. */
+public abstract class ParkingException extends RuntimeException {
+    protected ParkingException(String message) { super(message); }
+
+    public static final class LotFull extends ParkingException {
+        public LotFull(Vehicle v) { super("no free spot for " + v.type() + " " + v.plate()); }
+    }
+
+    public static final class AlreadyParked extends ParkingException {
+        public AlreadyParked(String plate) { super(plate + " is already inside the lot"); }
+    }
+
+    public static final class UnknownTicket extends ParkingException {
+        public UnknownTicket(String id) { super("unknown or already used ticket " + id); }
+    }
+}
@@ -0,0 +1,19 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.List;
+
+/** A floor is just an ordered group of spots; the order is the "nearest to the entrance" order. */
+public record ParkingFloor(int number, List<ParkingSpot> spots) {
+    public ParkingFloor {
+        spots = List.copyOf(spots);
+    }
+
+    /** Convenience factory: {@code ParkingFloor.of(0, SMALL, SMALL, MEDIUM, LARGE)} creates spots F0-1..F0-4. */
+    public static ParkingFloor of(int number, SpotSize... sizes) {
+        var spots = new java.util.ArrayList<ParkingSpot>();
+        for (int i = 0; i < sizes.length; i++) {
+            spots.add(new ParkingSpot("F" + number + "-" + (i + 1), number, sizes[i]));
+        }
+        return new ParkingFloor(number, spots);
+    }
+}
@@ -0,0 +1,90 @@
+package com.ankurm.interviewlab.parking;
+
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.EnumMap;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.atomic.AtomicLong;
+
+/**
+ * The aggregate the interviewer is really asking about.
+ *
+ * Thread-safety without a big lock:
+ *  - a spot is claimed with one compare-and-set ({@link ParkingSpot#tryOccupy});
+ *  - "one plate inside at most once" is a concurrent set add;
+ *  - tickets live in a ConcurrentHashMap, and removing one is what makes a ticket single-use.
+ */
+public final class ParkingLot {
+    private final List<ParkingFloor> floors;
+    private final Map<String, ParkingSpot> spotsById = new HashMap<>();
+    private final AllocationStrategy allocation;
+    private final PricingStrategy pricing;
+    private final Clock clock;
+
+    private final Set<String> platesInside = ConcurrentHashMap.newKeySet();
+    private final Map<String, Ticket> activeTickets = new ConcurrentHashMap<>();
+    private final AtomicLong ticketSequence = new AtomicLong();
+    private final List<AvailabilityListener> listeners = new CopyOnWriteArrayList<>();
+
+    public ParkingLot(List<ParkingFloor> floors, AllocationStrategy allocation, PricingStrategy pricing, Clock clock) {
+        this.floors = List.copyOf(floors);
+        this.allocation = allocation;
+        this.pricing = pricing;
+        this.clock = clock;
+        for (ParkingFloor f : floors) for (ParkingSpot s : f.spots()) spotsById.put(s.id(), s);
+    }
+
+    public void addListener(AvailabilityListener l) { listeners.add(l); }
+
+    public Ticket park(Vehicle vehicle) {
+        if (!platesInside.add(vehicle.plate())) throw new ParkingException.AlreadyParked(vehicle.plate());
+        for (ParkingSpot spot : allocation.candidates(floors, vehicle)) {
+            if (spot.tryOccupy(vehicle)) {                       // lost a race? try the next candidate
+                Ticket t = new Ticket("T-" + ticketSequence.incrementAndGet(), vehicle, spot.id(), clock.instant());
+                activeTickets.put(t.id(), t);
+                publish();
+                return t;
+            }
+        }
+        platesInside.remove(vehicle.plate());                      // undo the plate reservation
+        throw new ParkingException.LotFull(vehicle);
+    }
+
+    public Receipt exit(String ticketId) {
+        Ticket t = activeTickets.remove(ticketId);                  // atomic: only one caller can get the ticket
+        if (t == null) throw new ParkingException.UnknownTicket(ticketId);
+        Instant now = clock.instant();
+        Duration stayed = Duration.between(t.entry(), now);
+        var receipt = new Receipt(t, now, stayed, pricing.fee(t.vehicle(), stayed));
+        spotsById.get(t.spotId()).release();
+        platesInside.remove(t.vehicle().plate());
+        publish();
+        return receipt;
+    }
+
+    public long freeSpots(SpotSize size) {
+        return floors.stream().flatMap(f -> f.spots().stream())
+                .filter(s -> s.size() == size && s.isFree()).count();
+    }
+
+    public Map<SpotSize, Long> freeBySize() {
+        Map<SpotSize, Long> m = new EnumMap<>(SpotSize.class);
+        for (SpotSize s : SpotSize.values()) m.put(s, freeSpots(s));
+        return m;
+    }
+
+    private void publish() {
+        if (listeners.isEmpty()) return;
+        Map<SpotSize, Long> snapshot = freeBySize();
+        for (AvailabilityListener l : listeners) l.changed(snapshot);
+    }
+
+    public List<ParkingFloor> floors() { return new ArrayList<>(floors); }
+}
@@ -0,0 +1,36 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.Optional;
+import java.util.concurrent.atomic.AtomicReference;
+
+/**
+ * One physical spot. Occupancy is a single compare-and-set, which is the whole concurrency story:
+ * two cars racing for the same spot cannot both win, and no lock is needed.
+ */
+public final class ParkingSpot {
+    private final String id;
+    private final int floor;
+    private final SpotSize size;
+    private final AtomicReference<Vehicle> occupant = new AtomicReference<>();
+
+    public ParkingSpot(String id, int floor, SpotSize size) {
+        this.id = id;
+        this.floor = floor;
+        this.size = size;
+    }
+
+    public String id() { return id; }
+    public int floor() { return floor; }
+    public SpotSize size() { return size; }
+    public boolean isFree() { return occupant.get() == null; }
+    public Optional<Vehicle> occupant() { return Optional.ofNullable(occupant.get()); }
+
+    /** Atomically claims the spot. Returns false if another thread got there first or the vehicle does not fit. */
+    public boolean tryOccupy(Vehicle v) {
+        return size.canHold(v.type()) && occupant.compareAndSet(null, v);
+    }
+
+    public void release() { occupant.set(null); }
+
+    @Override public String toString() { return id + "(" + size + ")"; }
+}
@@ -0,0 +1,9 @@
+package com.ankurm.interviewlab.parking;
+
+import java.math.BigDecimal;
+import java.time.Duration;
+
+/** Strategy pattern again: pricing rules change far more often than parking rules. */
+public interface PricingStrategy {
+    BigDecimal fee(Vehicle vehicle, Duration stayed);
+}
@@ -0,0 +1,8 @@
+package com.ankurm.interviewlab.parking;
+
+import java.math.BigDecimal;
+import java.time.Duration;
+import java.time.Instant;
+
+/** What the driver gets at the gate: how long they stayed and what they owe. */
+public record Receipt(Ticket ticket, Instant exit, Duration duration, BigDecimal fee) {}
@@ -0,0 +1,11 @@
+package com.ankurm.interviewlab.parking;
+
+/** Physical size of a spot. Ordered small to large so "fits" is a single comparison. */
+public enum SpotSize {
+    SMALL, MEDIUM, LARGE;
+
+    /** A spot can hold a vehicle when it is at least as big as the vehicle's minimum spot. */
+    public boolean canHold(VehicleType type) {
+        return this.ordinal() >= type.minimumSpot().ordinal();
+    }
+}
@@ -0,0 +1,6 @@
+package com.ankurm.interviewlab.parking;
+
+import java.time.Instant;
+
+/** Issued at entry, presented at exit. Immutable; the lot never mutates a ticket. */
+public record Ticket(String id, Vehicle vehicle, String spotId, Instant entry) {}
@@ -0,0 +1,12 @@
+package com.ankurm.interviewlab.parking;
+
+import java.util.Objects;
+
+/** Immutable. The plate is the identity: one plate can be inside the lot at most once. */
+public record Vehicle(String plate, VehicleType type) {
+    public Vehicle {
+        Objects.requireNonNull(plate, "plate");
+        Objects.requireNonNull(type, "type");
+        if (plate.isBlank()) throw new IllegalArgumentException("plate must not be blank");
+    }
+}
@@ -0,0 +1,14 @@
+package com.ankurm.interviewlab.parking;
+
+/** Vehicle categories and the smallest spot each one fits in. */
+public enum VehicleType {
+    MOTORCYCLE(SpotSize.SMALL),
+    CAR(SpotSize.MEDIUM),
+    TRUCK(SpotSize.LARGE);
+
+    private final SpotSize minimumSpot;
+
+    VehicleType(SpotSize minimumSpot) { this.minimumSpot = minimumSpot; }
+
+    public SpotSize minimumSpot() { return minimumSpot; }
+}
@@ -0,0 +1,169 @@
+package com.ankurm.interviewlab.parking;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.math.BigDecimal;
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.junit.jupiter.api.Test;
+
+class ParkingLotTest {
+
+    /** Hand-moved clock: tests never sleep. */
+    static final class ManualClock extends Clock {
+        private volatile Instant now = Instant.parse("2026-10-22T09:00:00Z");
+        void advance(Duration d) { now = now.plus(d); }
+        @Override public Instant instant() { return now; }
+        @Override public ZoneId getZone() { return ZoneOffset.UTC; }
+        @Override public Clock withZone(ZoneId zone) { return this; }
+    }
+
+    private static Vehicle car(String plate) { return new Vehicle(plate, VehicleType.CAR); }
+
+    private ParkingLot lot(AllocationStrategy s, ManualClock c, ParkingFloor... floors) {
+        return new ParkingLot(List.of(floors), s, HourlyPricing.standard(), c);
+    }
+
+    @Test
+    void bestFitKeepsLargeSpotsFreeForTrucks() {
+        var lot = lot(new BestFitStrategy(), new ManualClock(),
+                ParkingFloor.of(0, SpotSize.LARGE, SpotSize.MEDIUM));
+        assertEquals("F0-2", lot.park(car("A")).spotId());           // medium, not the nearer large
+        assertEquals(1, lot.freeSpots(SpotSize.LARGE));
+    }
+
+    @Test
+    void nearestFirstTakesTheFirstFreeSpotThatFits() {
+        var lot = lot(new NearestFirstStrategy(), new ManualClock(),
+                ParkingFloor.of(0, SpotSize.LARGE, SpotSize.MEDIUM));
+        assertEquals("F0-1", lot.park(car("A")).spotId());           // the large spot is nearer
+    }
+
+    @Test
+    void truckIsRejectedWhenOnlyMediumSpotsAreFree() {
+        var lot = lot(new BestFitStrategy(), new ManualClock(), ParkingFloor.of(0, SpotSize.MEDIUM));
+        assertThrows(ParkingException.LotFull.class, () -> lot.park(new Vehicle("T", VehicleType.TRUCK)));
+    }
+
+    @Test
+    void failedParkDoesNotLeaveThePlateReserved() {
+        var lot = lot(new BestFitStrategy(), new ManualClock(), ParkingFloor.of(0, SpotSize.MEDIUM));
+        var truck = new Vehicle("T", VehicleType.TRUCK);
+        assertThrows(ParkingException.LotFull.class, () -> lot.park(truck));
+        assertThrows(ParkingException.LotFull.class, () -> lot.park(truck));   // not AlreadyParked
+    }
+
+    @Test
+    void samePlateCannotBeInsideTwice() {
+        var lot = lot(new BestFitStrategy(), new ManualClock(), ParkingFloor.of(0, SpotSize.MEDIUM, SpotSize.MEDIUM));
+        lot.park(car("A"));
+        assertThrows(ParkingException.AlreadyParked.class, () -> lot.park(car("A")));
+    }
+
+    @Test
+    void feeRoundsPartialHoursUpAndHasAGracePeriod() {
+        var clock = new ManualClock();
+        var lot = lot(new BestFitStrategy(), clock, ParkingFloor.of(0, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM));
+        record Case(Duration stay, int expected) {}
+        for (var c : List.of(
+                new Case(Duration.ofMinutes(10), 0),             // inside grace
+                new Case(Duration.ofMinutes(15), 0),             // grace boundary is free
+                new Case(Duration.ofMinutes(16), 20),            // 1 started hour
+                new Case(Duration.ofMinutes(60), 20),            // exactly 1 hour
+                new Case(Duration.ofMinutes(61), 40))) {         // 2 started hours
+            Ticket t = lot.park(car("P" + c.stay().toMinutes()));
+            clock.advance(c.stay());
+            assertEquals(0, BigDecimal.valueOf(c.expected()).compareTo(lot.exit(t.id()).fee()), "stay " + c.stay());
+        }
+    }
+
+    @Test
+    void exitFreesTheSpotAndTheTicketIsSingleUse() {
+        var lot = lot(new BestFitStrategy(), new ManualClock(), ParkingFloor.of(0, SpotSize.MEDIUM));
+        Ticket t = lot.park(car("A"));
+        assertEquals(0, lot.freeSpots(SpotSize.MEDIUM));
+        lot.exit(t.id());
+        assertEquals(1, lot.freeSpots(SpotSize.MEDIUM));
+        assertThrows(ParkingException.UnknownTicket.class, () -> lot.exit(t.id()));
+        assertDoesNotThrow(() -> lot.park(car("B")));                     // spot is reusable
+    }
+
+    @Test
+    void listenersSeeEveryChange() {
+        var lot = lot(new BestFitStrategy(), new ManualClock(), ParkingFloor.of(0, SpotSize.MEDIUM, SpotSize.MEDIUM));
+        List<Long> seen = new ArrayList<>();
+        lot.addListener(free -> seen.add(free.get(SpotSize.MEDIUM)));
+        Ticket t = lot.park(car("A"));
+        lot.park(car("B"));
+        lot.exit(t.id());
+        assertEquals(List.of(1L, 0L, 1L), seen);
+    }
+
+    /** 64 threads race for 10 spots: exactly 10 win, and no two winners share a spot. */
+    @Test
+    void concurrentParkingNeverDoubleBooksASpot() throws Exception {
+        var lot = lot(new BestFitStrategy(), new ManualClock(),
+                ParkingFloor.of(0, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM),
+                ParkingFloor.of(1, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM));
+        int threads = 64;
+        var start = new CountDownLatch(1);
+        var parked = new AtomicInteger();
+        var full = new AtomicInteger();
+        Set<String> spots = ConcurrentHashMap.newKeySet();
+        try (var pool = Executors.newFixedThreadPool(threads)) {
+            List<Future<?>> fs = new ArrayList<>();
+            for (int i = 0; i < threads; i++) {
+                final int n = i;
+                fs.add(pool.submit(() -> {
+                    start.await();
+                    try {
+                        spots.add(lot.park(car("CAR-" + n)).spotId());
+                        parked.incrementAndGet();
+                    } catch (ParkingException.LotFull e) {
+                        full.incrementAndGet();
+                    }
+                    return null;
+                }));
+            }
+            start.countDown();
+            for (Future<?> f : fs) f.get();
+        }
+        assertEquals(10, parked.get());
+        assertEquals(54, full.get());
+        assertEquals(10, spots.size(), "every winner got a different spot");
+        assertEquals(0, lot.freeSpots(SpotSize.MEDIUM));
+    }
+
+    /** The same plate from many threads: exactly one gets in. */
+    @Test
+    void concurrentDuplicatePlateAdmitsExactlyOne() throws Exception {
+        var lot = lot(new BestFitStrategy(), new ManualClock(),
+                ParkingFloor.of(0, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM, SpotSize.MEDIUM));
+        var start = new CountDownLatch(1);
+        var ok = new AtomicInteger();
+        try (var pool = Executors.newFixedThreadPool(16)) {
+            List<Future<?>> fs = new ArrayList<>();
+            for (int i = 0; i < 16; i++) {
+                fs.add(pool.submit(() -> {
+                    start.await();
+                    try { lot.park(car("SAME")); ok.incrementAndGet(); }
+                    catch (ParkingException.AlreadyParked ignored) { }
+                    return null;
+                }));
+            }
+            start.countDown();
+            for (Future<?> f : fs) f.get();
+        }
+        assertEquals(1, ok.get());
+        assertEquals(3, lot.freeSpots(SpotSize.MEDIUM));
+    }
+}