[#3210] refactor file utilities

- Unify filename.h and file_utilities.h under filesystem.h.
- Rename Filename class to Path since it more accurately represents
  it.
- Make its interface as close as possible to that of std::filesystem::path.
- Remove unused method expandWithDefault.
- Rename useAsDefault to replaceExtension since it was only used in the
  purpose of replacing extension.
- Unroll the split method in the constructor.

src/lib/util/filesystem.h

@@ -0,0 +1,132 @@
+// Copyright (C) 2021-2024 Internet Systems Consortium, Inc. ("ISC")
+//
+// 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 http://mozilla.org/MPL/2.0/.
+
+#ifndef KEA_UTIL_FILESYSTEM_H
+#define KEA_UTIL_FILESYSTEM_H
+
+#include <string>
+
+namespace isc {
+namespace util {
+namespace file {
+
+/// \brief Get the content of a regular file.
+///
+/// \param file_name The file name.
+///
+/// \return The content of the file.
+/// \throw BadValue when the file can't be opened or is not a regular one.
+std::string
+getContent(const std::string& file_name);
+
+/// \brief Check if there is a file or directory at the given path.
+///
+/// \param path The path being checked.
+///
+/// \return True if the path points to a file or a directory, false otherwise.
+bool
+exists(const std::string& path);
+
+/// \brief Check if there is a directory at the given path.
+///
+/// \param path The path being checked.
+///
+/// \return True if the path points to a directory, false otherwise including
+/// if the pointed location does not exist.
+bool
+isDir(const std::string& path);
+
+/// \brief Check if there is a file at the given path.
+///
+/// \param path The path being checked.
+///
+/// \return True if the path points to a file, false otherwise including
+/// if the pointed location does not exist.
+bool
+isFile(const std::string& path);
+
+/// \brief Paths on a filesystem
+struct Path {
+    /// \brief Constructor
+    ///
+    /// Splits the full name into components.
+    Path(std::string const& path);
+
+    /// \brief Get the path in textual format.
+    ///
+    /// Counterpart for std::filesystem::path::string.
+    ///
+    /// \return stored filename.
+    std::string str() const;
+
+    /// \brief Get the parent path.
+    ///
+    /// Counterpart for std::filesystem::path::parent_path.
+    ///
+    /// \return parent path of current path.
+    std::string parentPath() const;
+
+    /// \brief Get the base name of the file without the extension.
+    ///
+    /// Counterpart for std::filesystem::path::stem.
+    ///
+    /// \return the base name of the file without the extension.
+    std::string stem() const;
+
+    /// \brief Get the extension of the file.
+    ///
+    /// Counterpart for std::filesystem::path::extension.
+    ///
+    /// \return extension of current path.
+    std::string extension() const;
+
+    /// \brief Get the extension of the file.
+    ///
+    /// Counterpart for std::filesystem::path::extension.
+    ///
+    /// \return name + extension of current path.
+    std::string filename() const;
+
+    /// \brief Identifies the extension in {replacement}, trims it, and
+    /// replaces this instance's extension with it.
+    ///
+    /// Counterpart for std::filesystem::path::replace_extension.
+    ///
+    /// The change is done in the members and {this} is returned to allow call
+    /// chaining.
+    ///
+    /// \param replacement The extension to replace with.
+    ///
+    /// \return The current instance after the replacement was done.
+    Path& replaceExtension(std::string const& replacement = std::string());
+
+    /// \brief Trims {replacement} and replaces this instance's parent path with
+    /// it.
+    ///
+    /// The change is done in the members and {this} is returned to allow call
+    /// chaining.
+    ///
+    /// \param replacement The parent path to replace with.
+    ///
+    /// \return The current instance after the replacement was done.
+    Path& replaceParentPath(std::string const& replacement = std::string());
+
+private:
+    /// \brief Parent path.
+    std::string parent_path_;
+
+    /// \brief Stem.
+    std::string stem_;
+
+    /// \brief File name extension.
+    std::string extension_;
+};
+
+}  // namespace file
+}  // namespace util
+}  // namespace isc
+
+#endif  // KEA_UTIL_FILESYSTEM_H