chapter deletion logic fixed (#3320)

(cherry picked from commit f642f23366ef352c7d107c8fd90a049033391504)

app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderPresenter.kt

@@ -362,7 +362,7 @@ class ReaderPresenter(
         if (selectedChapter.pages?.lastIndex == page.index) {
             selectedChapter.chapter.read = true
             updateTrackChapterRead(selectedChapter)
-            enqueueDeleteReadChapters(selectedChapter)
+            deleteChapterIfNeeded(selectedChapter)
         }
 
         if (selectedChapter != currentChapters.currChapter) {
@@ -372,6 +372,22 @@ class ReaderPresenter(
         }
     }
 
+    /**
+     * Determines if deleting option is enabled and nth to last chapter actually exists.
+     * If both conditions are satisfied enqueues chapter for delete
+     * @param currentChapter current chapter, which is going to be marked as read.
+     */
+    private fun deleteChapterIfNeeded(currentChapter: ReaderChapter) {
+        // Determine which chapter should be deleted and enqueue
+        val currentChapterPosition = chapterList.indexOf(currentChapter)
+        val removeAfterReadSlots = preferences.removeAfterReadSlots()
+        val chapterToDelete = chapterList.getOrNull(currentChapterPosition - removeAfterReadSlots)
+        // Check if deleting option is enabled and chapter exists
+        if (removeAfterReadSlots != -1 && chapterToDelete != null) {
+            enqueueDeleteReadChapters(chapterToDelete)
+        }
+    }
+
     /**
      * Called when a chapter changed from [fromChapter] to [toChapter]. It updates [fromChapter]
      * on the database.
@@ -660,19 +676,8 @@ class ReaderPresenter(
         if (!chapter.chapter.read || chapter.pageLoader !is DownloadPageLoader) return
         val manga = manga ?: return
 
-        // Return if the setting is disabled
-        val removeAfterReadSlots = preferences.removeAfterReadSlots()
-        if (removeAfterReadSlots == -1) return
-
         launchIO {
-            // Position of the read chapter
-            val position = chapterList.indexOf(chapter)
-
-            // Retrieve chapter to delete according to preference
-            val chapterToDelete = chapterList.getOrNull(position - removeAfterReadSlots)
-            if (chapterToDelete != null) {
-                downloadManager.enqueueDeleteChapters(listOf(chapterToDelete.chapter), manga)
-            }
+            downloadManager.enqueueDeleteChapters(listOf(chapter.chapter), manga)
         }
     }