[YouTube] Add utility methods to get images from InfoItems and thumbnails arrays

Unmodifiable lists of Images are returned, parsed from a given YouTube
"thumbnails" JSON array.

These methods will be used in all YouTube extractors and InfoItems, as the
structures between content types (videos, channels, playlists, ...) are common.

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeParsingHelper.java

@@ -33,6 +33,9 @@ import com.grack.nanojson.JsonParser;
 import com.grack.nanojson.JsonParserException;
 import com.grack.nanojson.JsonWriter;
 import org.jsoup.nodes.Entities;
+
+import org.schabi.newpipe.extractor.Image;
+import org.schabi.newpipe.extractor.Image.ResolutionLevel;
 import org.schabi.newpipe.extractor.MetaInfo;
 import org.schabi.newpipe.extractor.downloader.Response;
 import org.schabi.newpipe.extractor.exceptions.AccountTerminatedException;
@@ -69,6 +72,7 @@ import java.util.Optional;
 import java.util.Random;
 import java.util.Set;
 import java.util.regex.Pattern;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import javax.annotation.Nonnull;
@@ -1133,17 +1137,61 @@ public final class YoutubeParsingHelper {
         return result;
     }
 
-    public static String getThumbnailUrlFromInfoItem(final JsonObject infoItem)
+    /**
+     * Get thumbnails from a {@link JsonObject} representing a YouTube
+     * {@link org.schabi.newpipe.extractor.InfoItem InfoItem}.
+     *
+     * <p>
+     * Thumbnails are got from the {@code thumbnails} {@link JsonArray} inside the {@code thumbnail}
+     * {@link JsonObject} of the YouTube {@link org.schabi.newpipe.extractor.InfoItem InfoItem},
+     * using {@link #getImagesFromThumbnailsArray(JsonArray)}.
+     * </p>
+     *
+     * @param infoItem a YouTube {@link org.schabi.newpipe.extractor.InfoItem InfoItem}
+     * @return an unmodifiable list of {@link Image}s found in the {@code thumbnails}
+     * {@link JsonArray}
+     * @throws ParsingException if an exception occurs when
+     *                          {@link #getImagesFromThumbnailsArray(JsonArray)} is executed
+     */
+    @Nonnull
+    public static List<Image> getThumbnailsFromInfoItem(@Nonnull final JsonObject infoItem)
             throws ParsingException {
-        // TODO: Don't simply get the first item, but look at all thumbnails and their resolution
         try {
-            return fixThumbnailUrl(infoItem.getObject("thumbnail").getArray("thumbnails")
-                    .getObject(0).getString("url"));
+            return getImagesFromThumbnailsArray(infoItem.getObject("thumbnail")
+                    .getArray("thumbnails"));
         } catch (final Exception e) {
-            throw new ParsingException("Could not get thumbnail url", e);
+            throw new ParsingException("Could not get thumbnails from InfoItem", e);
         }
     }
 
+    /**
+     * Get images from a YouTube {@code thumbnails} {@link JsonArray}.
+     *
+     * <p>
+     * The properties of the {@link Image}s created will be set using the corresponding ones of
+     * thumbnail items.
+     * </p>
+     *
+     * @param thumbnails a YouTube {@code thumbnails} {@link JsonArray}
+     * @return an unmodifiable list of {@link Image}s extracted from the given {@link JsonArray}
+     */
+    @Nonnull
+    public static List<Image> getImagesFromThumbnailsArray(
+            @Nonnull final JsonArray thumbnails) {
+        return thumbnails.stream()
+                .filter(JsonObject.class::isInstance)
+                .map(JsonObject.class::cast)
+                .filter(thumbnail -> !isNullOrEmpty(thumbnail.getString("url")))
+                .map(thumbnail -> {
+                    final int height = thumbnail.getInt("height", Image.HEIGHT_UNKNOWN);
+                    return new Image(fixThumbnailUrl(thumbnail.getString("url")),
+                            height,
+                            thumbnail.getInt("width", Image.WIDTH_UNKNOWN),
+                            ResolutionLevel.fromHeight(height));
+                })
+                .collect(Collectors.toUnmodifiableList());
+    }
+
     @Nonnull
     public static String getValidJsonResponseBody(@Nonnull final Response response)
             throws ParsingException, MalformedURLException {