Add filesystem directory enumeration.

Borrow API from std::filesystem -- Boost and C++17.

firmware/application/file.hpp

@@ -27,6 +27,8 @@
 #include <cstddef>
 #include <string>
 #include <array>
+#include <memory>
+#include <iterator>
 
 class File {
 public:
@@ -55,4 +57,61 @@ private:
 	FIL f;
 };
 
+namespace std {
+namespace filesystem {
+
+using file_status = BYTE;
+
+struct directory_entry : public FILINFO {
+	file_status status() const {
+		return fattrib;
+	}
+
+	const std::string path() const noexcept { return fname; };
+};
+
+class directory_iterator {
+	struct Impl {
+		DIR dir;
+		directory_entry filinfo;
+
+		~Impl() {
+			f_closedir(&dir);
+		}
+	};
+
+	std::shared_ptr<Impl> impl;
+
+	friend bool operator!=(const directory_iterator& lhs, const directory_iterator& rhs);
+
+public:
+	using difference_type = std::ptrdiff_t;
+	using value_type = directory_entry;
+	using pointer = const directory_entry*;
+	using reference = const directory_entry&;
+	using iterator_category = std::input_iterator_tag;
+
+	directory_iterator() noexcept { };
+	directory_iterator(const char* path, const char* wild);
+
+	~directory_iterator() { }
+
+	directory_iterator& operator++();
+
+	reference operator*() const {
+		// TODO: Exception or assert if impl == nullptr.
+		return impl->filinfo;
+	}
+};
+
+inline const directory_iterator& begin(const directory_iterator& iter) noexcept { return iter; };
+inline directory_iterator end(const directory_iterator&) noexcept { return { }; };
+
+inline bool operator!=(const directory_iterator& lhs, const directory_iterator& rhs) { return lhs.impl != rhs.impl; };
+
+bool is_regular_file(const file_status s);
+
+} /* namespace filesystem */
+} /* namespace std */
+
 #endif/*__FILE_H__*/