src/main/java/com/magamochi/mangamochi/model/dto/MangaChapterImagesDTO.java

@@ -10,11 +10,15 @@ import java.util.List;
 public record MangaChapterImagesDTO(
     @NotNull Long id,
     @NotBlank String mangaTitle,
+    Long previousChapterId,
+    Long nextChapterId,
     @NotNull List<@NotBlank String> chapterImageKeys) {
-  public static MangaChapterImagesDTO from(MangaChapter mangaChapter) {
+  public static MangaChapterImagesDTO from(MangaChapter mangaChapter, Long prevId, Long nextId) {
     return new MangaChapterImagesDTO(
         mangaChapter.getId(),
         mangaChapter.getTitle(),
+        prevId,
+        nextId,
         mangaChapter.getMangaChapterImages().stream()
             .sorted(Comparator.comparing(MangaChapterImage::getPosition))
             .map(mangaChapterImage -> mangaChapterImage.getImage().getFileKey())

src/main/java/com/magamochi/mangamochi/service/MangaChapterService.java

@@ -99,7 +99,26 @@ public class MangaChapterService {
   public MangaChapterImagesDTO getMangaChapterImages(Long chapterId) {
     var chapter = getMangaChapterThrowIfNotFound(chapterId);
 
-    return MangaChapterImagesDTO.from(chapter);
+    var chapters = chapter.getMangaProvider().getMangaChapters();
+    Long prevId = null;
+    Long nextId = null;
+
+    // TODO: this doesn't perform well for large datasets
+    for (var i = 0; i < chapters.size(); i++) {
+      if (chapters.get(i).getId().equals(chapterId)) {
+        if (i > 0) {
+          prevId = chapters.get(i - 1).getId();
+        }
+
+        if (i < chapters.size() - 1) {
+          nextId = chapters.get(i + 1).getId();
+        }
+
+        break;
+      }
+    }
+
+    return MangaChapterImagesDTO.from(chapter, prevId, nextId);
   }
 
   public void markAsRead(Long chapterId) {

src/main/java/com/magamochi/mangamochi/util/JwtUtil.java

@@ -31,9 +31,9 @@ public class JwtUtil {
     return extractClaim(token, Claims::getSubject, getAccessSigningKey());
   }
 
-    public String extractUsernameFromRefreshToken(String token) {
-        return extractClaim(token, Claims::getSubject, getRefreshSigningKey());
-    }
+  public String extractUsernameFromRefreshToken(String token) {
+    return extractClaim(token, Claims::getSubject, getRefreshSigningKey());
+  }
 
   public String generateAccessToken(UserDetails userDetails) {
     Map<String, Object> claims = new HashMap<>();