Move OPF file path detection into own function.

src/book.cpp

@@ -189,28 +189,28 @@ string page(const document &doc, const size_t pos)
     return string(last);
 }
 
+fs::path get_opf_file_path(const fs::path &zipfile)
+{
+    pugi::xml_document xml;
+    const std::string container{
+        zip::read_file(zipfile, "META-INF/container.xml")};
+    const auto result{xml.load_buffer(&container[0], container.size())};
+    if (result)
+    {
+        return fs::path{xml.child("container")
+                            .child("rootfiles")
+                            .first_child()
+                            .attribute("full-path")
+                            .value()};
+    }
+    LOG(log::sev::error) << result.description() << '\n';
+
+    return fs::path{};
+};
+
 std::vector<string> list_spine(const fs::path &filepath)
 {
-    const auto opf_file_path{
+    auto opf_file_path{get_opf_file_path(filepath)};
-        [&filepath]
-        {
-            pugi::xml_document xml;
-            const std::string container{
-                zip::read_file(filepath, "META-INF/container.xml")};
-            const auto result{xml.load_buffer(&container[0], container.size())};
-            if (result)
-            {
-                return fs::path{xml.child("container")
-                                    .child("rootfiles")
-                                    .first_child()
-                                    .attribute("full-path")
-                                    .value()};
-            }
-            LOG(log::sev::error) << result.description() << '\n';
-
-            return fs::path{};
-        }()};
-
     std::vector<std::string> spine_filepaths;
     if (!opf_file_path.empty())
     {

src/book.hpp

@@ -62,6 +62,9 @@ struct book
 //! Return current page if possible.
 [[nodiscard]] std::string page(const document &doc, size_t pos);
 
+//! Returns the file path of the OPF file in the EPUB.
+[[nodiscard]] fs::path get_opf_file_path(const fs::path &zipfile);
+
 //! Returns the files in the EPUB “spine” (all pages that are actually text).
 [[nodiscard]] std::vector<string> list_spine(const fs::path &filepath);