Initial commitHEADmain

9 files changed, 715 insertions, 0 deletions

diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionController.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionController.java
new file mode 100644
index 0000000..1ffe137
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionController.java
@@ -0,0 +1,155 @@
+package org.psesquared.server.subscriptions.api.controller;
+
+import java.util.List;
+import lombok.RequiredArgsConstructor;
+import org.psesquared.server.subscriptions.api.service.SubscriptionService;
+import org.psesquared.server.util.UpdateUrlsWrapper;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.PutMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+
+/**
+ * This is a controller class for the Subscription API that handles the requests
+ * from the client concerning adding subscriptions, removing subscriptions and
+ * getting all current subscriptions.
+ * In the end an appropriate response is sent back to the user.
+ */
+@RestController
+@RequiredArgsConstructor
+public class SubscriptionController {
+
+  /**
+   * The response for uploading subscriptions successfully.
+   */
+  private static final String UPLOAD_SUCCESS = "";
+
+  /**
+   * The service class that this controller calls to further process requests.
+   */
+  private final SubscriptionService subscriptionService;
+
+  /**
+   * It takes a list of strings containing the URLs of all subscribed podcasts,
+   * and saves them to the database.
+   *
+   * @param username      The username of the user
+   * @param deviceId      The device ID of the device that is uploading the
+   *                      subscriptions (will be ignored in this implementation)
+   * @param subscriptions A list of strings, each string is the URL to a podcast
+   *                      RSS-Feed that was subscribed
+   * @return The response containing an empty String with
+   *        <br>
+   *        {@link HttpStatus#OK} on success,
+   *        <br>
+   *        {@link HttpStatus#NOT_FOUND} user not found
+   */
+  @PutMapping(path = "/subscriptions/{username}/{deviceId}.json")
+  public ResponseEntity<String> uploadSubscriptions(
+      @PathVariable final String username,
+      @PathVariable final String deviceId,
+      @RequestBody final List<String> subscriptions) {
+    HttpStatus status
+        = subscriptionService.uploadSubscriptions(username, subscriptions);
+    return new ResponseEntity<>(UPLOAD_SUCCESS, status);
+  }
+
+  /**
+   * This function returns a list of subscriptions for a given user.
+   *
+   * @param username      The username of the user whose subscriptions you want
+   *                      to retrieve
+   * @param deviceId      This is the unique identifier for the device of the
+   *                      user whose subscriptions are asked for
+   *                      (will be ignored in this implementation)
+   * @param functionJsonp This parameter is not supported in this implementation
+   *                      and is thus ignored
+   * @return A list of strings containing the RSS-Feed URLs of all subscribed
+   *        podcasts
+   */
+  @GetMapping(path = {"/subscriptions/{username}.json",
+      "/subscriptions/{username}/{deviceId}.json"})
+  public ResponseEntity<List<String>> getSubscriptions(
+      @PathVariable final String username,
+      @PathVariable(required = false) final String deviceId,
+      @RequestParam(value = "jsonp",
+          required = false) final String functionJsonp) {
+    List<String> subscriptions = subscriptionService.getSubscriptions(username);
+    return ResponseEntity.ok(subscriptions);
+  }
+
+  /**
+   * This function takes the information of added and removed podcasts in the
+   * form of a SubcriptionDelta as a JSON object.
+   * <br>
+   * After that, it applies the changes to the given user in the database.
+   *
+   * @param username The username of the user who is making changes to their
+   *                 subscriptions
+   * @param deviceId The device ID of the device that is requesting the update
+   *                 (will be ignored in this implementation)
+   * @param delta    Contains all the changes that were made to the
+   *                 subscriptions of the user
+   * @return The response containing a placeholder for not
+   *        supported function with
+   *        <br>
+   *        {@link HttpStatus#OK} on success,
+   *        <br>
+   *        {@link HttpStatus#NOT_FOUND} user or subscription not found
+   */
+  @PostMapping(path = "/api/2/subscriptions/{username}/{deviceId}.json")
+  public ResponseEntity<UpdateUrlsWrapper> applySubscriptionDelta(
+      @PathVariable final String username,
+      @PathVariable final String deviceId,
+      @RequestBody final SubscriptionDelta delta) {
+    subscriptionService.applySubscriptionDelta(username, delta);
+    return ResponseEntity.ok(new UpdateUrlsWrapper());
+  }
+
+  /**
+   * It returns a list of all the changes to the subscriptions of a user since a
+   * given time.
+   *
+   * @param username The username of the user whose SubscriptionDeltas are being
+   *                 requested
+   * @param deviceId The device ID of the device that is requesting the delta
+   *                 (will be ignored in this implementation)
+   * @param since    The timestamp of the last time the client checked for
+   *                 updates
+   * @return A response containing the SubscriptionDelta of all changes that
+   *        were made in a JSON format
+   */
+  @GetMapping(path = "/api/2/subscriptions/{username}/{deviceId}.json")
+  public ResponseEntity<SubscriptionDelta> getSubscriptionDelta(
+      @PathVariable final String username,
+      @PathVariable final String deviceId,
+      @RequestParam("since") final long since) {
+    SubscriptionDelta delta
+        = subscriptionService.getSubscriptionDelta(username, since);
+    return ResponseEntity.ok(delta);
+  }
+
+  /**
+   * This function returns a list of podcasts a user is subscribed to.
+   * <br>
+   * This includes not only the podcast itself, but also the latest 20 Episodes
+   * of the podcast.
+   *
+   * @param username The username of the user whose podcasts are being requested
+   * @return A response containing a List of podcasts and their episodes the
+   *        user is subscribed to
+   */
+  @GetMapping(path = "/subscriptions/titles/{username}.json")
+  public ResponseEntity<List<SubscriptionTitles>> getTitles(
+      @PathVariable final String username) {
+    List<SubscriptionTitles> responseBody
+        = subscriptionService.getTitles(username);
+    return ResponseEntity.ok(responseBody);
+  }
+
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionDelta.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionDelta.java
new file mode 100644
index 0000000..7611b05
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionDelta.java
@@ -0,0 +1,80 @@
+package org.psesquared.server.subscriptions.api.controller;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.util.List;
+import lombok.NonNull;
+
+/**
+ * SubscriptionDeltas contain all changes that were made to the subscriptions
+ * of a user (added / removed podcasts) at a certain time.
+ */
+public class SubscriptionDelta {
+
+  /**
+   * The list of recently subscribed podcasts.
+   */
+  @JsonProperty(required = true)
+  @NonNull
+  private final List<String> add;
+
+  /**
+   * The list of recently unsubscribed podcasts.
+   */
+  @JsonProperty(required = true)
+  @NonNull
+  private final List<String> remove;
+
+  /**
+   * The timestamp of the delta.
+   */
+  @JsonProperty(access = JsonProperty.Access.READ_ONLY)
+  private final long timestamp;
+
+  /**
+   * Instantiates a new SubscriptionDelta with a current timestamp.
+   *
+   * @param addedPodcastUrls    List of Strings containing the RSS-Feed URLs of
+   *                            all added podcasts
+   * @param removedPodcastUrls List of Strings containing the RSS-Feed URLs of
+   *                           all removed podcasts
+   */
+  public SubscriptionDelta(
+      @org.springframework.lang.NonNull final List<String> addedPodcastUrls,
+      @org.springframework.lang.NonNull final List<String> removedPodcastUrls) {
+    this.add = addedPodcastUrls;
+    this.remove = removedPodcastUrls;
+    this.timestamp = LocalDateTime.now().toEpochSecond(ZoneOffset.UTC);
+  }
+
+  /**
+   * Returns the list of RSS-Feed URLs of all added podcasts.
+   *
+   * @return RSS-Feed URLs of all added podcasts
+   */
+  @org.springframework.lang.NonNull
+  public List<String> getAdd() {
+    return add;
+  }
+
+  /**
+   * Returns the list of RSS-Feed URLs of all removed podcasts.
+   *
+   * @return RSS-Feed URLs of all removed podcasts
+   */
+  @org.springframework.lang.NonNull
+  public List<String> getRemove() {
+    return remove;
+  }
+
+  /**
+   * Returns the timestamp of when this Subscription Delta was uploaded.
+   *
+   * @return The timestamp of when this Subscription Delta was uploaded
+   */
+  public long getTimestamp() {
+    return timestamp;
+  }
+
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionTitles.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionTitles.java
new file mode 100644
index 0000000..682497b
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/SubscriptionTitles.java
@@ -0,0 +1,17 @@
+package org.psesquared.server.subscriptions.api.controller;
+
+import com.fasterxml.jackson.annotation.JsonUnwrapped;
+import java.util.List;
+import org.psesquared.server.episode.actions.api.controller.EpisodeActionPost;
+import org.psesquared.server.model.Subscription;
+
+/**
+ * Contains a podcast and its latest 20 Episodes.
+ *
+ * @param subscription The podcast
+ * @param episodes     The episodes of the podcast
+ */
+public record SubscriptionTitles(
+    @JsonUnwrapped Subscription subscription,
+    List<EpisodeActionPost> episodes) {
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/package-info.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/package-info.java
new file mode 100644
index 0000000..0238039
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/controller/package-info.java
@@ -0,0 +1,13 @@
+/**
+ * This package represents the highest logical layer of the subscription API
+ * ({@link org.psesquared.server.subscriptions.api}) - the controller layer.
+ * <br>
+ * It contains the
+ * {@link
+ * org.psesquared.server.subscriptions.api.controller.SubscriptionController}
+ * along with some wrapper classes for JSON request and response bodies.
+ *
+ * @author PSE-Squared Team
+ * @version 1.0
+ */
+package org.psesquared.server.subscriptions.api.controller;
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/SubscriptionActionDao.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/SubscriptionActionDao.java
new file mode 100644
index 0000000..5d91453
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/SubscriptionActionDao.java
@@ -0,0 +1,91 @@
+package org.psesquared.server.subscriptions.api.data.access;
+
+import java.util.List;
+import java.util.Optional;
+import org.psesquared.server.model.Subscription;
+import org.psesquared.server.model.SubscriptionAction;
+import org.psesquared.server.model.User;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+/**
+ * A DAO interface responsible for transactions involving SubscriptionActions.
+ */
+@Repository
+public interface SubscriptionActionDao
+    extends JpaRepository<SubscriptionAction, Long> {
+
+  /**
+   * True, if the given user is already subscribed to the given Subscription.
+   *
+   * @param user         The user that could be subscribed
+   * @param subscription The subscription the user could be subscribed to
+   * @return A boolean value signifying whether the user is subscribed to the
+   *        given subscription
+   */
+  boolean existsByUserAndSubscription(User user, Subscription subscription);
+
+  /**
+   * Find the SubscriptionAction signifying that the user is subscribed to the
+   * given Subscription.
+   *
+   * @param user         The user who is subscribed to the subscription
+   * @param subscription The subscription that the user is subscribed to
+   * @return Contains the relevant SubscriptionAction. Could also be NULL if
+   *        none was found.
+   */
+  Optional<SubscriptionAction> findByUserAndSubscription(
+      User user,
+      Subscription subscription);
+
+  /**
+   * Find the SubscriptionAction for a {@link User} with the given username
+   * and for a {@link Subscription} with the given URL.
+   *
+   * @param username         The username of the user who is subscribed to
+   *                         the subscription
+   * @param subscriptionUrl The URL of the subscription that the user is
+   *                        subscribed to
+   * @return Contains the relevant SubscriptionAction. Could also be NULL if
+   *        none was found.
+   */
+  Optional<SubscriptionAction> findByUserUsernameAndSubscriptionUrl(
+      String username, String subscriptionUrl);
+
+  /**
+   * All SubscriptionActions of a given user that were applied since a given
+   * timestamp are searched for and returned.
+   *
+   * @param username  The username of the user whose SubscriptionActions are
+   *                  requested
+   * @param timestamp The timestamp signifying how old the SubscriptionActions
+   *                  are allowed to be
+   * @return A list of SubscriptionActions that have since been applied
+   */
+  List<SubscriptionAction> findByUserUsernameAndTimestampGreaterThanEqual(
+      String username,
+      long timestamp);
+
+  /**
+   * Returns a List of all Subscriptions the user is subscribed to.
+   *
+   * @param username The username of the user whose subscriptions are requested
+   * @return A list of subscriptions the user is subscribed to
+   */
+  List<SubscriptionAction> findByUserUsernameAndAddedTrue(String username);
+
+  /**
+   * Returns a List of RSS-Feed URLs of all podcasts the given user is
+   * subscribed to since a given timestamp.
+   *
+   * @param username  The username of the user whose subscriptions are requested
+   * @param timestamp The timestamp signifying the time since when the user must
+   *                  have been subscribed
+   * @return List of RSS-Feed URLs
+   */
+  List<SubscriptionAction>
+      findByUserUsernameAndAddedTrueAndTimestampGreaterThanEqual(
+          String username,
+          long timestamp);
+
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/SubscriptionDao.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/SubscriptionDao.java
new file mode 100644
index 0000000..d3d1fbf
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/SubscriptionDao.java
@@ -0,0 +1,34 @@
+package org.psesquared.server.subscriptions.api.data.access;
+
+import java.util.Optional;
+import org.psesquared.server.model.Subscription;
+import org.springframework.data.jpa.repository.EntityGraph;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+/**
+ * A DAO interface responsible for transactions involving Subscriptions.
+ */
+@Repository
+public interface SubscriptionDao extends JpaRepository<Subscription, Long> {
+
+  /**
+   * Find a subscription by its URL.
+   *
+   * @param url The URL of the subscription
+   * @return The found subscription (could be NULL, if there was no match)
+   */
+  @EntityGraph(value = "graph.Subscription.episodes")
+  Optional<Subscription> findByUrl(String url);
+
+  /**
+   * Returns true if the database already has a Subscription that has the given
+   * URL.
+   *
+   * @param url The URL of the Subscription that could already exist in the
+   *            database
+   * @return A boolean value signifying the existence of the Subscription
+   */
+  boolean existsByUrl(String url);
+
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/package-info.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/package-info.java
new file mode 100644
index 0000000..db23d5f
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/data/access/package-info.java
@@ -0,0 +1,13 @@
+/**
+ * This package represents the lowest logical layer of the subscription API
+ * ({@link org.psesquared.server.subscriptions.api}) - the data-access layer.
+ * <br>
+ * It features the interfaces {@link
+ * org.psesquared.server.subscriptions.api.data.access.SubscriptionActionDao}
+ * and {@link
+ * org.psesquared.server.subscriptions.api.data.access.SubscriptionDao}.
+ *
+ * @author PSE-Squared Team
+ * @version 1.0
+ */
+package org.psesquared.server.subscriptions.api.data.access;
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/SubscriptionService.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/SubscriptionService.java
new file mode 100644
index 0000000..08dc1f9
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/SubscriptionService.java
@@ -0,0 +1,299 @@
+package org.psesquared.server.subscriptions.api.service;
+
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import lombok.RequiredArgsConstructor;
+import org.psesquared.server.authentication.api.data.access.AuthenticationDao;
+import org.psesquared.server.episode.actions.api.controller.EpisodeActionPost;
+import org.psesquared.server.episode.actions.api.data.access.EpisodeActionDao;
+import org.psesquared.server.episode.actions.api.service.EpisodeActionService;
+import org.psesquared.server.model.Subscription;
+import org.psesquared.server.model.SubscriptionAction;
+import org.psesquared.server.model.User;
+import org.psesquared.server.subscriptions.api.controller.SubscriptionDelta;
+import org.psesquared.server.subscriptions.api.controller.SubscriptionTitles;
+import org.psesquared.server.subscriptions.api.data.access.SubscriptionActionDao;
+import org.psesquared.server.subscriptions.api.data.access.SubscriptionDao;
+import org.psesquared.server.util.RssParser;
+import org.springframework.http.HttpStatus;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * This service class manages all business logic associated with the
+ * episode action API.
+ * <br>
+ * It is called from the
+ * {@link
+ * org.psesquared.server.subscriptions.api.controller.SubscriptionController}
+ * and passes on requests concerning data access mainly to the
+ * {@link SubscriptionDao} and {@link SubscriptionActionDao}.
+ */
+@Service
+@Transactional
+@RequiredArgsConstructor
+public class SubscriptionService {
+
+  /**
+   * The error message that is logged if no subscription exists
+   * for a remove action.
+   */
+  private static final String NO_SUB_WARNING
+      = "Subscription for remove action does not exist!";
+
+  /**
+   * The logger for logging some warnings.
+   */
+  private static final Logger LOGGER
+      = Logger.getLogger(SubscriptionService.class.getName());
+
+  /**
+   * The class for fetching data from RSS feeds.
+   */
+  private final RssParser rssParser;
+
+  /**
+   * The JPA repository that handles all user related database requests.
+   */
+  private final AuthenticationDao authenticationDao;
+
+  /**
+   * The JPA repository that handles all subscription related database requests.
+   */
+  private final SubscriptionDao subscriptionDao;
+
+  /**
+   * The JPA repository that handles all subscription action related database
+   * requests.
+   */
+  private final SubscriptionActionDao subscriptionActionDao;
+
+  /**
+   * The JPA repository that handles all episode action related database
+   * requests.
+   */
+  private final EpisodeActionDao episodeActionDao;
+
+  /**
+   * The service class of the episode action API.
+   */
+  private final EpisodeActionService episodeActionService;
+
+  /**
+   * It takes a list of podcast URLs in the form of strings and checks if they
+   * exist in the database.
+   * If they do not exist yet, it creates them.
+   * <br>
+   * Then it checks, if the user already has a subscription action for each
+   * subscription, separately.
+   * If not, it creates one.
+   * If yes, it updates the action.
+   *
+   * @param username            The username of the user
+   * @param subscriptionStrings List of Strings, each String is a URL of a
+   *                            podcast
+   * @return {@link HttpStatus#OK} on success,
+   *        <br>
+   *        {@link HttpStatus#NOT_FOUND} user not found
+   */
+  public HttpStatus uploadSubscriptions(
+      final String username,
+      final List<String> subscriptionStrings) {
+    User user;
+    try {
+      user = authenticationDao.findByUsername(username)
+          .orElseThrow();
+    } catch (NoSuchElementException e) {
+      return HttpStatus.NOT_FOUND;
+    }
+
+    Subscription subscription;
+    for (String subscriptionString : subscriptionStrings) {
+
+      try {
+        subscription
+            = subscriptionDao.findByUrl(subscriptionString).orElseThrow();
+      } catch (NoSuchElementException e) {
+        subscription = Subscription.builder()
+            .url(subscriptionString)
+            .timestamp(LocalDateTime.now().toEpochSecond(ZoneOffset.UTC))
+            .build();
+        subscriptionDao.save(subscription);
+        rssParser.validate(subscription);
+      }
+
+      try {
+        SubscriptionAction subscriptionAction
+            = subscriptionActionDao
+            .findByUserAndSubscription(user, subscription)
+            .orElseThrow();
+        subscriptionAction.setAdded(true);
+        subscriptionAction.setTimestamp(
+            LocalDateTime.now().toEpochSecond(ZoneOffset.UTC));
+        subscriptionActionDao.save(subscriptionAction);
+      } catch (NoSuchElementException e) {
+        SubscriptionAction subscriptionAction = SubscriptionAction.builder()
+            .user(user)
+            .added(true)
+            .subscription(subscription)
+            .timestamp(LocalDateTime.now().toEpochSecond(ZoneOffset.UTC))
+            .build();
+        subscriptionActionDao.save(subscriptionAction);
+      }
+    }
+
+    return HttpStatus.OK;
+  }
+
+  /**
+   * It returns a URL List of all podcasts the user is subscribed to in the form
+   * of a String list.
+   *
+   * @param username The username of the user whose subscriptions are being
+   *                 requested
+   * @return A list of RSS-Feed URLs of all subscribed podcasts
+   */
+  public List<String> getSubscriptions(final String username) {
+    List<SubscriptionAction> subscriptionActions
+        = subscriptionActionDao.findByUserUsernameAndAddedTrue(username);
+    List<String> subscriptionUrls = new ArrayList<>();
+    for (SubscriptionAction subscriptionAction : subscriptionActions) {
+      subscriptionUrls.add(subscriptionAction.getSubscription().getUrl());
+    }
+    return subscriptionUrls;
+  }
+
+  /**
+   * All subscription changes of the user are uploaded to the database.
+   *
+   * @param username The username of the user uploading their subscription
+   *                 changes
+   * @param delta    The subscription changes in the form of a SubscriptionDelta
+   *                 containing the added / removed subscriptions
+   */
+  public void applySubscriptionDelta(
+      final String username,
+      final SubscriptionDelta delta) {
+
+    SubscriptionDelta minimizedDelta = minimizeDelta(delta);
+
+    uploadSubscriptions(username, minimizedDelta.getAdd());
+    for (String removeSub : minimizedDelta.getRemove()) {
+      try {
+        SubscriptionAction subscriptionAction
+            = subscriptionActionDao
+            .findByUserUsernameAndSubscriptionUrl(username, removeSub)
+            .orElseThrow();
+        subscriptionAction.setAdded(false);
+        subscriptionAction.setTimestamp(
+            LocalDateTime.now().toEpochSecond(ZoneOffset.UTC));
+        subscriptionActionDao.save(subscriptionAction);
+        episodeActionDao
+            .deleteByUserUsernameAndEpisodeSubscriptionUrl(username, removeSub);
+      } catch (NoSuchElementException e) {
+        LOGGER.log(Level.WARNING, NO_SUB_WARNING);
+      }
+    }
+  }
+
+  private SubscriptionDelta minimizeDelta(SubscriptionDelta oldDelta){
+    SubscriptionDelta minimizedDelta = new SubscriptionDelta(new ArrayList<>(), new ArrayList<>());
+
+    Map<String, Integer> deltaMap = new HashMap<>();
+    for (String addString : oldDelta.getAdd()) {
+      if(deltaMap.containsKey(addString)) {
+        deltaMap.put(addString, deltaMap.get(addString) + 1);
+      }
+      else{
+        deltaMap.put(addString, 1);
+      }
+    }
+
+    for (String removeString : oldDelta.getRemove()) {
+      if(deltaMap.containsKey(removeString)) {
+        deltaMap.put(removeString, deltaMap.get(removeString) - 1);
+      }
+      else{
+        deltaMap.put(removeString, -1);
+      }
+    }
+
+    for(Map.Entry<String, Integer> deltaEntry : deltaMap.entrySet()) {
+      if(deltaEntry.getValue() > 0) {
+        minimizedDelta.getAdd().add(deltaEntry.getKey());
+      } else if(deltaEntry.getValue() < 0) {
+        minimizedDelta.getRemove().add(deltaEntry.getKey());
+      }
+    }
+
+    return minimizedDelta;
+  }
+
+  /**
+   * Returns a SubscriptionDelta of all changes made to a users subscriptions
+   * since a given point in time.
+   *
+   * @param username The username of the user whose subscription changes are
+   *                 being requested
+   * @param since    The timestamp signifying how old the changes are allowed to
+   *                 be
+   * @return The SubscriptionDelta of all changes made since the given timestamp
+   */
+  public SubscriptionDelta getSubscriptionDelta(
+      final String username,
+      final long since) {
+    List<String> added = new ArrayList<>();
+    List<String> removed = new ArrayList<>();
+
+    List<SubscriptionAction> subscriptionActions = subscriptionActionDao
+        .findByUserUsernameAndTimestampGreaterThanEqual(username, since);
+    for (SubscriptionAction subscriptionAction : subscriptionActions) {
+      if (subscriptionAction.isAdded()) {
+        added.add(subscriptionAction.getSubscription().getUrl());
+      } else {
+        removed.add(subscriptionAction.getSubscription().getUrl());
+      }
+    }
+
+    return new SubscriptionDelta(added, removed);
+  }
+
+  /**
+   * Returns all Subscriptions and their 20 latest episodes of a given user as a
+   * List of SubscriptionTitles.
+   *
+   * @param username The username of the user whose subscriptions are being
+   *                 requested
+   * @return A list of SubscriptionTitles containing each Subscription and their
+   *        20 latest Episodes
+   */
+  public List<SubscriptionTitles> getTitles(final String username) {
+    List<SubscriptionAction> subscriptionActions
+        = subscriptionActionDao.findByUserUsernameAndAddedTrue(username);
+    List<Subscription> subscriptions = new ArrayList<>();
+    List<SubscriptionTitles> subscriptionTitlesList = new ArrayList<>();
+
+    for (SubscriptionAction subscriptionAction : subscriptionActions) {
+      subscriptions.add(subscriptionAction.getSubscription());
+    }
+
+    for (Subscription subscription : subscriptions) {
+      List<EpisodeActionPost> episodeActionPosts
+          = episodeActionService
+          .getEpisodeActionsOfPodcast(username, subscription.getUrl());
+      SubscriptionTitles subscriptionTitles
+          = new SubscriptionTitles(subscription, episodeActionPosts);
+      subscriptionTitlesList.add(subscriptionTitles);
+    }
+
+    return subscriptionTitlesList;
+  }
+
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/package-info.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/package-info.java
new file mode 100644
index 0000000..9e2a598
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/package-info.java
@@ -0,0 +1,13 @@
+/**
+ * This package represents the logical middle layer of the subscription API
+ * ({@link org.psesquared.server.subscriptions.api}) - the service layer.
+ * <br>
+ * All business logic is handled here with the
+ * {@link
+ * org.psesquared.server.subscriptions.api.service.SubscriptionService}
+ * class.
+ *
+ * @author PSE-Squared Team
+ * @version 1.0
+ */
+package org.psesquared.server.subscriptions.api.service;