Imported implementation for updating library by next expected update from Neko (#5436)

* Imported implementation for updating library by next expected update from Neko. This sort uses the last 4 updates for a manga to compute an average time between updates and then extrapolates when the next update should occur.

Currently seems to work perfectly. However, I may have silently messed something up along the way.

All code and algorithms are credited to kyjibo on GitHub. The original commit adding this functionality is here: https://github.com/CarlosEsco/Neko/commit/681003926ae1e07b925155d4e1f43972bbe2b843

* Imported implementation for updating library by next expected update from Neko. This sort uses the last 4 updates for a manga to compute an average time between updates and then extrapolates when the next update should occur.

Currently seems to work perfectly. However, I may have silently messed something up along the way.

All code and algorithms are credited to kyjibo on GitHub. The original commit adding this functionality is here: https://github.com/CarlosEsco/Neko/commit/681003926ae1e07b925155d4e1f43972bbe2b843

* Remove commented-out line from LibraryUpdateRanker

I missed removing this when first committing. The removed line is a holdover from Neko, which requires 7+, but I removed the function that requires this.

app/src/main/java/eu/kanade/tachiyomi/data/library/LibraryUpdateRanker.kt

@@ -1,6 +1,9 @@
 package eu.kanade.tachiyomi.data.library
 
 import eu.kanade.tachiyomi.data.database.models.Manga
+import java.util.Collections
+import kotlin.Comparator
+import kotlin.math.abs
 
 /**
  * This class will provide various functions to rank manga to efficiently schedule manga to update.
@@ -9,9 +12,26 @@ object LibraryUpdateRanker {
 
     val rankingScheme = listOf(
         (this::lexicographicRanking)(),
-        (this::latestFirstRanking)()
+        (this::latestFirstRanking)(),
+        (this::nextFirstRanking)()
     )
 
+    /**
+     * Provides a total ordering over all the Mangas.
+     *
+     * Orders the manga based on the distance between the next expected update and now.
+     * The comparator is reversed, placing the smallest (and thus closest to updating now) first.
+     */
+    fun nextFirstRanking(): Comparator<Manga> {
+        val time = System.currentTimeMillis()
+        return Collections.reverseOrder(
+            Comparator { mangaFirst: Manga,
+                mangaSecond: Manga ->
+                compareValues(abs(mangaSecond.next_update - time), abs(mangaFirst.next_update - time))
+            }
+        )
+    }
+
     /**
      * Provides a total ordering over all the [Manga]s.
      *