Problem 4.11 — Ride Booking (LLD)

Nearest-first scan plus the atomic 'claim this driver' step, guarded exactly like a parking spot.

public final class Driver {
  private final String id;
  private volatile Location location;
  private final AtomicBoolean available = new AtomicBoolean(true);

  public Driver(String id, Location loc) { this.id = id; this.location = loc; }

  public boolean tryClaim() { return available.compareAndSet(true, false); }  // atomic: only one rider wins
  public void release()     { available.set(true); }
  public Location location() { return location; }
  public String id() { return id; }
}

public final class DriverMatcher {
  private final Map<String, Driver> byId = new ConcurrentHashMap<>();
  private final GeoIndex geoIndex;                 // grid or geohash bucket lookup, O(nearby) not O(all)

  public DriverMatcher(GeoIndex geoIndex) { this.geoIndex = geoIndex; }

  public Optional<Driver> match(Location pickup, double radiusKm) {
    return geoIndex.nearby(pickup, radiusKm).stream()
        .map(byId::get)
        .filter(Objects::nonNull)
        .sorted(Comparator.comparingDouble(d -> pickup.distanceKm(d.location())))
        .filter(Driver::tryClaim)                  // first driver whose CAS succeeds wins the race
        .findFirst();
    // Two riders calling match() at the same instant will iterate the same sorted list, but
    // compareAndSet ensures only one of them gets true from tryClaim() on any given driver.
  }
}