feat: configurable OPDS download folder and filename format (#2571)

src/util/OpdsFilename.cpp

@@ -0,0 +1,23 @@
+#include "OpdsFilename.h"
+
+#include "StringUtils.h"
+
+std::string opdsBookFilename(const std::string& author, const std::string& title, OpdsFilenameFormat format) {
+  std::string base;
+  switch (format) {
+    case OpdsFilenameFormat::TitleAuthor:
+      base = author.empty() ? title : title + " - " + author;
+      break;
+    case OpdsFilenameFormat::TitleOnly:
+      base = title;
+      break;
+    case OpdsFilenameFormat::AuthorTitle:
+    default:
+      base = author.empty() ? title : author + " - " + title;
+      break;
+  }
+  // sanitizeFilename caps at 100 bytes and never returns empty (falls back to
+  // "book"); ".epub" is appended after so the extension is never truncated —
+  // identical treatment to the previous inline construction.
+  return StringUtils::sanitizeFilename(base) + ".epub";
+}

src/util/OpdsFilename.h

@@ -0,0 +1,18 @@
+#pragma once
+#include <cstdint>
+#include <string>
+
+// On-disk filename format for books downloaded from an OPDS server. Stored as a
+// uint8_t in CrossPointSettings; cast to this enum at the call sites. `Count` is
+// the number of selectable formats (used to cycle the setting in the UI).
+enum class OpdsFilenameFormat : uint8_t {
+  AuthorTitle = 0,  // "Author - Title.epub" (default; matches legacy behaviour)
+  TitleAuthor = 1,  // "Title - Author.epub"
+  TitleOnly = 2,    // "Title.epub"
+  Count = 3,
+};
+
+// Composes and sanitizes the on-disk filename (including the ".epub" extension)
+// for a downloaded OPDS book, according to `format`. When the author is empty,
+// every format collapses to just the sanitized title. Pure: no I/O, no globals.
+std::string opdsBookFilename(const std::string& author, const std::string& title, OpdsFilenameFormat format);