fix: refactor the pagination prefix and token processing (#1476)

Encapsulate pageTokenPrefix and paginationToken within a unified
PageToken class or utility functions to simplify and generalize
pagination logic.
Issue : The logic to validate and extract the pageToken from the request
is not standardized and has repetition for every case which makes it
error prone.

The change was already done for exporting release. 

This change adds generic methods to the StringPaginationToken class and
refactors the export playlist method.

Author: Shashank1116

extensions/data-transfer/portability-data-transfer-google/src/main/java/org/datatransferproject/datatransfer/google/music/GoogleMusicExporter.java

@@ -132,42 +132,25 @@ public ExportResult<MusicContainerResource> export(
     }
   }
 
-  // TODO(@jzacsh): Replace pageTokenPrefix and paginationToken with simplified and general class or
-  // functions.
   @VisibleForTesting
   ExportResult<MusicContainerResource> exportPlaylists(
       TokensAndUrlAuthData authData, Optional<PaginationData> paginationData, UUID jobId)
       throws IOException, InvalidTokenException, PermissionDeniedException {
-    Optional<String> paginationToken = Optional.empty();
-    String pageTokenPrefix = "";
-    if (paginationData.isPresent()) {
-      String token = ((StringPaginationToken) paginationData.get()).getToken();
-      Preconditions.checkArgument(
-          token.startsWith(PLAYLIST_TOKEN_PREFIX), "Invalid pagination token %s", token);
-      pageTokenPrefix = token.substring(0, getTokenPrefixLength(token));
-      if (getTokenPrefixLength(token) < token.length()) {
-        paginationToken = Optional.of(token.substring(getTokenPrefixLength(token)));
-      }
-    }
-
+    Optional<String> paginationToken = getToken(PLAYLIST_TOKEN_PREFIX, paginationData);
     PlaylistExportResponse playlistExportResponse =
         getOrCreateMusicHttpApi(authData).exportPlaylists(paginationToken);
 
-    PaginationData nextPageData;
-    String token = playlistExportResponse.getNextPageToken();
+    PaginationData nextPageData = new StringPaginationToken("");
+    String nextPageToken = playlistExportResponse.getNextPageToken();
     List<MusicPlaylist> playlists = new ArrayList<>();
     GooglePlaylist[] googlePlaylists = playlistExportResponse.getPlaylists();
     ResultType resultType = ResultType.END;
 
-    if (Strings.isNullOrEmpty(token)) {
-      nextPageData =
-          new StringPaginationToken(pageTokenPrefix.substring(PLAYLIST_TOKEN_PREFIX.length()));
-    } else {
-      nextPageData = new StringPaginationToken(pageTokenPrefix + token);
+    if (!Strings.isNullOrEmpty(nextPageToken)) {
+      nextPageData = new StringPaginationToken(PLAYLIST_TOKEN_PREFIX + nextPageToken);
       resultType = ResultType.CONTINUE;
     }
     ContinuationData continuationData = new ContinuationData(nextPageData);
-
     if (googlePlaylists != null && googlePlaylists.length > 0) {
       for (GooglePlaylist googlePlaylist : googlePlaylists) {
         Instant createTime = googlePlaylist.getCreateTime() == null ? null
@@ -275,37 +258,13 @@ ExportResult<MusicContainerResource> exportReleases(TokensAndUrlAuthData authDat
   private Optional<String> getToken(String prefix, Optional<PaginationData> paginationData){
     Optional<String> paginationToken = Optional.empty();
     if (paginationData.isPresent()) {
-      String token = ((StringPaginationToken) paginationData.get()).getToken();
-      Preconditions.checkArgument(
-          token.startsWith(prefix), "Invalid pagination token %s",
-          token);
-      if (prefix.length() < token.length()) {
-        paginationToken = Optional.of(token.substring(prefix.length()));
-      }
+      StringPaginationToken tokenObject = ((StringPaginationToken) paginationData.get());
+      tokenObject.verify(prefix);
+      paginationToken = tokenObject.getParsedToken(prefix);
     }
     return paginationToken;
   }
 
-
-  private int getTokenPrefixLength(String token) {
-    final ImmutableList<String> knownPrefixes =
-        ImmutableList.of(
-            PLAYLIST_TRACK_RELEASE_TOKEN_PREFIX,
-            PLAYLIST_TRACK_TOKEN_PREFIX,
-            PLAYLIST_RELEASE_TOKEN_PREFIX,
-            PLAYLIST_TOKEN_PREFIX,
-            TRACK_RELEASE_TOKEN_PREFIX,
-            TRACK_TOKEN_PREFIX,
-            RELEASE_TOKEN_PREFIX);
-
-    for (String prefix : knownPrefixes) {
-      if (token.startsWith(prefix)) {
-        return prefix.length();
-      }
-    }
-    return 0;
-  }
-
   private @Nullable List<MusicGroup> createMusicGroups(GoogleArtist[] artists) {
     if (artists == null) {
       return null;

portability-types-common/src/main/java/org/datatransferproject/types/common/StringPaginationToken.java

@@ -3,7 +3,8 @@
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.annotation.JsonTypeName;
-import org.datatransferproject.types.common.PaginationData;
+import com.google.common.base.Preconditions;
+import java.util.Optional;
 
 /**
  * String pagination data.
@@ -25,4 +26,18 @@ public StringPaginationToken(@JsonProperty("token") String token) {
   public String getToken() {
     return token;
   }
+
+  public void verify(String prefix){
+    Preconditions.checkArgument(
+        getToken().startsWith(prefix), "Invalid pagination token %s",
+        getToken());
+  }
+
+  public Optional<String> getParsedToken(String prefix){
+      Optional<String> parsedToken = Optional.empty();
+      if (prefix.length() < token.length()) {
+        parsedToken = Optional.of(token.substring(prefix.length()));
+      }
+    return parsedToken;
+  }
 }