feat: implement waitlist promotion service

src/main/java/com/slotline/reservation/WaitlistService.java

@@ -0,0 +1,80 @@
+package com.slotline.reservation;
+
+import com.slotline.common.InvalidReservationException;
+import com.slotline.notification.NotificationMessage;
+import com.slotline.notification.NotificationService;
+import com.slotline.notification.NotificationType;
+import com.slotline.reservation.dto.WaitlistRequest;
+import com.slotline.reservation.dto.WaitlistResponse;
+import com.slotline.resource.Resource;
+import com.slotline.resource.ResourceService;
+import com.slotline.security.AuthenticatedUser;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.time.Instant;
+import java.util.List;
+
+@Service
+public class WaitlistService {
+
+    private final WaitlistRepository waitlist;
+    private final ReservationRepository reservations;
+    private final ResourceService resources;
+    private final NotificationService notifications;
+
+    public WaitlistService(WaitlistRepository waitlist, ReservationRepository reservations, ResourceService resources,
+                           NotificationService notifications) {
+        this.waitlist = waitlist;
+        this.reservations = reservations;
+        this.resources = resources;
+        this.notifications = notifications;
+    }
+
+    @Transactional
+    public WaitlistResponse join(AuthenticatedUser actor, WaitlistRequest request) {
+        if (!request.startTime().isBefore(request.endTime())) {
+            throw new InvalidReservationException("startTime must be strictly before endTime");
+        }
+        Resource resource = resources.require(actor.organizationId(), request.resourceId());
+        WaitlistEntry entry = new WaitlistEntry(resource.getId(), resource.getOrganizationId(), actor.id(),
+                request.startTime(), request.endTime(), Instant.now());
+        return WaitlistResponse.from(waitlist.save(entry));
+    }
+
+    @Transactional(readOnly = true)
+    public List<WaitlistResponse> listForUser(Long userId) {
+        return waitlist.findByUserIdOrderByCreatedAtDesc(userId).stream()
+                .map(WaitlistResponse::from)
+                .toList();
+    }
+
+    @Transactional
+    public void promoteAfterCancellation(Reservation cancelled) {
+        Resource resource = cancelled.getResource();
+        List<WaitlistEntry> candidates =
+                waitlist.findWaitingOverlaps(resource.getId(), cancelled.getStartTime(), cancelled.getEndTime());
+        for (WaitlistEntry candidate : candidates) {
+            boolean stillFree = reservations
+                    .findActiveOverlaps(resource.getId(), candidate.getStartTime(), candidate.getEndTime())
+                    .isEmpty();
+            if (!stillFree) {
+                continue;
+            }
+            Reservation promoted = new Reservation(resource, candidate.getUserId(), candidate.getStartTime(),
+                    candidate.getEndTime(), Instant.now());
+            reservations.save(promoted);
+            candidate.promote(promoted.getId());
+            notifications.dispatch(new NotificationMessage(
+                    NotificationType.WAITLIST_PROMOTED,
+                    resource.getOrganizationId(),
+                    candidate.getUserId(),
+                    resource.getId(),
+                    resource.getName(),
+                    promoted.getStartTime(),
+                    promoted.getEndTime(),
+                    "A waitlisted slot became available and was booked for you"));
+            return;
+        }
+    }
+}

src/main/java/com/slotline/reservation/dto/WaitlistRequest.java

@@ -0,0 +1,11 @@
+package com.slotline.reservation.dto;
+
+import jakarta.validation.constraints.NotNull;
+
+import java.time.Instant;
+
+public record WaitlistRequest(
+        @NotNull Long resourceId,
+        @NotNull Instant startTime,
+        @NotNull Instant endTime) {
+}

src/main/java/com/slotline/reservation/dto/WaitlistResponse.java

@@ -0,0 +1,29 @@
+package com.slotline.reservation.dto;
+
+import com.slotline.reservation.WaitlistEntry;
+import com.slotline.reservation.WaitlistStatus;
+
+import java.time.Instant;
+
+public record WaitlistResponse(
+        Long id,
+        Long resourceId,
+        Long userId,
+        Instant startTime,
+        Instant endTime,
+        WaitlistStatus status,
+        Long promotedReservationId,
+        Instant createdAt) {
+
+    public static WaitlistResponse from(WaitlistEntry entry) {
+        return new WaitlistResponse(
+                entry.getId(),
+                entry.getResourceId(),
+                entry.getUserId(),
+                entry.getStartTime(),
+                entry.getEndTime(),
+                entry.getStatus(),
+                entry.getPromotedReservationId(),
+                entry.getCreatedAt());
+    }
+}