Implemented SequentialFillingPicker

app/src/main/java/ru/dragonestia/picker/model/type/SlotLimit.java

@@ -7,7 +7,7 @@ import java.beans.Transient;
 @Getter
 public class SlotLimit {
 
-    private final static int UNLIMITED_VALUE = -1;
+    public final static int UNLIMITED_VALUE = -1;
 
     private final int slots;
 

app/src/main/java/ru/dragonestia/picker/service/impl/picker/ItemWrapper.java

@@ -0,0 +1,26 @@
+package ru.dragonestia.picker.service.impl.picker;
+
+import ru.dragonestia.picker.model.type.SlotLimit;
+
+public interface ItemWrapper<ITEM> {
+
+    String getIdentifier();
+
+    int countUnits();
+
+    int maxUnits();
+
+    default boolean isFull() {
+        return maxUnits() != SlotLimit.UNLIMITED_VALUE && countUnits() >= maxUnits();
+    }
+
+    default boolean isEmpty() {
+        return countUnits() == 0;
+    }
+
+    default boolean canAddUnits(int amount) {
+        return maxUnits() == SlotLimit.UNLIMITED_VALUE || countUnits() + amount < maxUnits();
+    }
+
+    ITEM getItem();
+}

app/src/main/java/ru/dragonestia/picker/service/impl/picker/Picker.java

@@ -0,0 +1,12 @@
+package ru.dragonestia.picker.service.impl.picker;
+
+import java.util.Collection;
+
+public interface Picker<ITEM, UNIT> {
+
+    void add(ITEM item);
+
+    void remove(ITEM item);
+
+    ITEM pick(Collection<UNIT> units);
+}

app/src/main/java/ru/dragonestia/picker/service/impl/picker/RoomWrapper.java

@@ -0,0 +1,41 @@
+package ru.dragonestia.picker.service.impl.picker;
+
+import ru.dragonestia.picker.model.Room;
+
+import java.util.function.Supplier;
+
+public class RoomWrapper implements ItemWrapper<Room> {
+
+    private final Room room;
+    private final Supplier<Integer> userCountSupplier;
+
+    public RoomWrapper(Room room, Supplier<Integer> userCountSupplier) {
+        this.room = room;
+        this.userCountSupplier = userCountSupplier;
+    }
+
+    @Override
+    public String getIdentifier() {
+        return room.getId();
+    }
+
+    @Override
+    public int countUnits() {
+        return userCountSupplier.get();
+    }
+
+    @Override
+    public int maxUnits() {
+        return room.getSlots().getSlots();
+    }
+
+    @Override
+    public Room getItem() {
+        return room;
+    }
+
+    @Override
+    public boolean canAddUnits(int amount) {
+        return ItemWrapper.super.canAddUnits(amount) && !room.isLocked();
+    }
+}

app/src/main/java/ru/dragonestia/picker/service/impl/picker/SequentialFillingPicker.java

@@ -0,0 +1,48 @@
+package ru.dragonestia.picker.service.impl.picker;
+
+import ru.dragonestia.picker.model.Room;
+import ru.dragonestia.picker.model.User;
+import ru.dragonestia.picker.repository.UserRepository;
+
+import java.util.Collection;
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+public class SequentialFillingPicker implements Picker<Room, User> {
+
+    private final UserRepository userRepository;
+    private final Map<String, RoomWrapper> wrappers = new LinkedHashMap<>();
+
+    public SequentialFillingPicker(UserRepository userRepository) {
+        this.userRepository = userRepository;
+    }
+
+    @Override
+    public void add(Room room) {
+        synchronized (wrappers) {
+            wrappers.put(room.getId(), new RoomWrapper(room, () -> userRepository.usersOf(room).size()));
+        }
+    }
+
+    @Override
+    public void remove(Room room) {
+        synchronized (wrappers) {
+            wrappers.remove(room.getId());
+        }
+    }
+
+    @Override
+    public Room pick(Collection<User> users) {
+        int amount = users.size();
+
+        synchronized (wrappers) {
+            for (var wrapper: wrappers.values()) {
+                if (wrapper.canAddUnits(amount)) continue;
+
+                return wrapper.getItem();
+            }
+        }
+
+        throw new RuntimeException("There are no rooms available");
+    }
+}