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 #ifndef PODIO_READER_H
 #define PODIO_READER_H
 
 #include "podio/Frame.h"
 #include "podio/podioVersion.h"
 
 namespace podio {
 
 /// Generic (type erased) reader class that can handle different I/O backends
 /// transparently
 ///
 /// Offers some more high level functionality compared to the lower level
 /// backend specific readers that this class wraps. In contrast to the lower
 /// level readers that usually return arbitrary FrameData, this interface class
 /// will return fully constructed Frames. In addition, it provides convenience
 /// methods to deal specifically with the "events" frame category.
 ///
 /// @note The recommended way to construct is to use the makeReader() functions
 /// since they handle the instantiation of the correct low level readers
 class Reader {
 private:
   struct ReaderConcept {
     virtual ~ReaderConcept() = default;
 
     virtual podio::Frame readNextFrame(const std::string& name) = 0;
     virtual podio::Frame readFrame(const std::string& name, size_t index) = 0;
     virtual size_t getEntries(const std::string& name) const = 0;
     virtual podio::version::Version currentFileVersion() const = 0;
     virtual std::optional<podio::version::Version> currentFileVersion(const std::string& name) const = 0;
     virtual std::vector<std::string_view> getAvailableCategories() const = 0;
     virtual const std::string_view getDatamodelDefinition(const std::string& name) const = 0;
     virtual std::vector<std::string> getAvailableDatamodels() const = 0;
   };
 
 private:
   template <typename T>
   struct ReaderModel final : ReaderConcept {
     ReaderModel(std::unique_ptr<T> reader) : m_reader(std::move(reader)) {
     }
     ReaderModel(const ReaderModel&) = delete;
     ReaderModel& operator=(const ReaderModel&) = delete;
     ReaderModel(ReaderModel&&) = default;
     ReaderModel& operator=(ReaderModel&&) = default;
 
     ~ReaderModel() = default;
 
     podio::Frame readNextFrame(const std::string& name) override {
       auto maybeFrame = m_reader->readNextEntry(name);
       if (maybeFrame) {
         return maybeFrame;
       }
       throw std::runtime_error("Failed reading category " + name + " (reading beyond bounds?)");
     }
 
     podio::Frame readFrame(const std::string& name, size_t index) override {
       auto maybeFrame = m_reader->readEntry(name, index);
       if (maybeFrame) {
         return maybeFrame;
       }
       throw std::runtime_error("Failed reading category " + name + " at frame " + std::to_string(index) +
                                " (reading beyond bounds?)");
     }
     size_t getEntries(const std::string& name) const override {
       return m_reader->getEntries(name);
     }
     podio::version::Version currentFileVersion() const override {
       return m_reader->currentFileVersion();
     }
 
     std::optional<podio::version::Version> currentFileVersion(const std::string& name) const override {
       return m_reader->currentFileVersion(name);
     }
 
     std::vector<std::string_view> getAvailableCategories() const override {
       return m_reader->getAvailableCategories();
     }
 
     const std::string_view getDatamodelDefinition(const std::string& name) const override {
       return m_reader->getDatamodelDefinition(name);
     }
 
     std::vector<std::string> getAvailableDatamodels() const override {
       return m_reader->getAvailableDatamodels();
     }
 
     std::unique_ptr<T> m_reader;
   };
 
   std::unique_ptr<ReaderConcept> m_self{nullptr};
 
 public:
   /// Create a reader from a low level reader
   ///
   /// @tparam T The type of the low level reader (will be deduced)
   /// @param actualReader a low level reader that provides access to FrameDataT
   template <typename T>
   Reader(std::unique_ptr<T> actualReader);
 
   Reader(const Reader&) = delete;
   Reader& operator=(const Reader&) = delete;
   Reader(Reader&&) = default;
   Reader& operator=(Reader&&) = default;
   ~Reader() = default;
 
   /// Read the next frame of a given category
   ///
   /// @param name The category name for which to read the next frame
   ///
   /// @returns A fully constructed Frame with the contents read from file
   ///
   /// @throws std::invalid_argument in case the category is not available or in
   ///         case no more entries are available
   podio::Frame readNextFrame(const std::string& name) {
     return m_self->readNextFrame(name);
   }
 
   /// Read the next frame of the "events" category
   ///
   /// @returns A fully constructed Frame with the contents read from file
   ///
   /// @throws std::invalid_argument in case no (more) events are available
   podio::Frame readNextEvent() {
     return readNextFrame(podio::Category::Event);
   }
 
   /// Read a specific frame for a given category
   ///
   /// @param name  The category name for which to read the next entry
   /// @param index The entry number to read
   ///
   /// @returns A fully constructed Frame with the contents read from file
   ///
   /// @throws std::invalid_argument in case the category is not available or in
   ///         case the specified entry is not available
   podio::Frame readFrame(const std::string& name, size_t index) {
     return m_self->readFrame(name, index);
   }
 
   /// Read a specific frame of the "events" category
   ///
   /// @param index The event number to read
   ///
   /// @returns A fully constructed Frame with the contents read from file
   ///
   /// @throws std::invalid_argument in case the desired event is not available
   podio::Frame readEvent(size_t index) {
     return readFrame(podio::Category::Event, index);
   }
 
   /// Get the number of entries for the given name
   ///
   /// @param name The name of the category
   ///
   /// @returns The number of entries that are available for the category
   size_t getEntries(const std::string& name) const {
     return m_self->getEntries(name);
   }
 
   /// Get the number of events
   ///
   /// @returns The number of entries that are available for the category
   size_t getEvents() const {
     return getEntries(podio::Category::Event);
   }
 
   /// Get the build version of podio that has been used to write the current
   /// file
   ///
   /// @returns The podio build version
   podio::version::Version currentFileVersion() const {
     return m_self->currentFileVersion();
   }
 
   /// Get the (build) version of a datamodel that has been used to write the
   /// current file
   ///
   /// @param name The name of the datamodel
   ///
   /// @returns The (build) version of the datamodel if available or an empty
   ///          optional
   std::optional<podio::version::Version> currentFileVersion(const std::string& name) const {
     return m_self->currentFileVersion(name);
   }
 
   /// Get the names of all the available Frame categories in the current file(s).
   ///
   /// @returns The names of the available categories from the file
   std::vector<std::string_view> getAvailableCategories() const {
     return m_self->getAvailableCategories();
   }
 
   /// Get the datamodel definition for the given name
   ///
   /// @param name The name of the datamodel
   ///
   /// @returns The high level definition of the datamodel in JSON format
   const std::string_view getDatamodelDefinition(const std::string& name) const {
     return m_self->getDatamodelDefinition(name);
   }
 
   /// Get all names of the datamodels that are available from this reader
   ///
   /// @returns The names of the datamodels
   std::vector<std::string> getAvailableDatamodels() const {
     return m_self->getAvailableDatamodels();
   }
 };
 
 /// Create a Reader is able to read the file
 ///
 /// This will inspect the filename as well as peek at the file contents to
 /// instantiate the correct low level reader to open and read the file
 ///
 /// @param filename The (path to the) file to read from
 ///
 /// @returns A Reader that has been initialized and that can be used for reading
 ///          data from the passed file
 Reader makeReader(const std::string& filename);
 
 /// Create a Reader that is able to read the files
 ///
 /// This will inspect the filenames as well as peek into the **first file only**
 /// to decide based on the contents which low level reader to instantiate for
 /// reading. All files are assumed to be of the same I/O format, no switching
 /// between formats is possible.
 ///
 /// @note For SIO files this will only work with exactly one file!
 ///
 /// @param filenames The (paths to the) files to read from
 ///
 /// @returns A Reader that has been initialized and that can be used for reading
 ///          data from the passed files
 ///
 /// @throws std::runtime_error in case the file extensions differ or in case
 ///         support for the necessary I/O backend has not been built or in case
 ///         multiple files for the SIO backend are passed
 Reader makeReader(const std::vector<std::string>& filenames);
 
 } // namespace podio
 
 #endif // PODIO_READER_H

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