Resume chapter at specific page

- open from outside reader
 - is read
  - yes: 1st page
  - no : last read page
- inside reader via
 - chapter selection: first page
 - previous chapter : last page
 - next chapter     : first page

src/modules/reader/utils/Reader.utils.ts

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+import { ReaderResumeMode } from '@/modules/reader/types/Reader.types.ts';
+
+export const getInitialReaderPageIndex = (
+    resumeMode: ReaderResumeMode,
+    lastReadPageIndex: number,
+    lastPageIndex: number,
+): number => {
+    if (resumeMode === ReaderResumeMode.START) {
+        return 0;
+    }
+
+    if (resumeMode === ReaderResumeMode.END) {
+        return lastPageIndex;
+    }
+
+    return Math.max(0, Math.min(lastPageIndex, lastReadPageIndex));
+};