diff --git a/src/celcompat/CMakeLists.txt b/src/celcompat/CMakeLists.txt
new file mode 100644
index 00000000..c7815dd1
--- /dev/null
+++ b/src/celcompat/CMakeLists.txt
@@ -0,0 +1,6 @@
+set(CELCOMPAT_SOURCES
+  fs.cpp
+  fs.h
+)
+
+add_library(celcompat OBJECT ${CELCOMPAT_SOURCES})
diff --git a/src/celcompat/filesystem.h b/src/celcompat/filesystem.h
new file mode 100644
index 00000000..abc41e99
--- /dev/null
+++ b/src/celcompat/filesystem.h
@@ -0,0 +1,14 @@
+#pragma once
+
+#include <config.h>
+
+#if __cplusplus >= 201703L
+#include <filesystem>
+namespace fs = std::filesystem;
+#elif defined(HAVE_EXPERIMENTAL_FILESYSTEM)
+#include <experimental/filesystem>
+namespace fs = std::experimental::filesystem;
+#else
+#include "fs.h"
+namespace fs = celestia::filesystem;
+#endif
diff --git a/src/celcompat/fs.cpp b/src/celcompat/fs.cpp
new file mode 100644
index 00000000..0f853dba
--- /dev/null
+++ b/src/celcompat/fs.cpp
@@ -0,0 +1,415 @@
+#include "fs.h"
+#include <vector>
+#include <memory>
+#ifdef _WIN32
+#include <celutil/winutil.h>
+#else
+#include <sys/stat.h>
+#endif
+
+
+namespace celestia
+{
+namespace filesystem
+{
+
+// we should skip "." and ".."
+template<typename CharT> static bool is_special_dir(const CharT s[])
+{
+    return s[0] == '.' && (s[1] == '\0' || (s[1] == '.' && s[2]  == '\0'));
+}
+
+path operator/(const path& lhs, const path& rhs)
+{
+    return path(lhs) /= rhs;
+}
+
+std::ostream& operator<<(std::ostream& os, const path& p)
+{
+    os << '"' << p.string() << '"';
+    return os;
+}
+
+path u8path(const std::string& source)
+{
+#ifdef _WIN32
+    return UTF8ToWide(source);
+#else
+    return source; // FIXME
+#endif
+}
+
+#ifdef _WIN32
+void path::fixup_separators()
+{
+    if (m_fmt == native_format)
+        return;
+
+    string_type::size_type pos = 0;
+    for(;;)
+    {
+        pos = m_path.find(L'/', pos);
+        if (pos == path::string_type::npos)
+            break;
+        m_path[pos] = preferred_separator;
+        pos++;
+    }
+}
+#endif
+
+path path::filename() const
+{
+    auto pos = m_path.rfind(preferred_separator);
+    if (pos == string_type::npos)
+        pos = 0;
+    else
+        pos++;
+
+    auto fn = m_path.substr(pos);
+    if (is_special_dir(fn.c_str()))
+        return path();
+
+    path p(fn);
+    return p;
+}
+
+path path::stem() const
+{
+    auto fn = filename().native();
+    auto pos = fn.rfind('.');
+
+    if (pos == 0 || pos == string_type::npos)
+        return fn;
+
+    return fn.substr(0, pos);
+}
+
+path path::extension() const
+{
+    auto fn = filename().native();
+    auto pos = fn.rfind('.');
+
+    if (pos == 0 || pos == string_type::npos)
+        return path();
+
+    return fn.substr(pos);
+}
+
+path path::parent_path() const
+{
+    auto pos = m_path.rfind(preferred_separator);
+    if (pos == 0)
+        return path(m_path.substr(0, 1));
+
+    if (pos == string_type::npos)
+        return path();
+
+    return path(m_path.substr(0, pos));
+}
+
+
+bool path::is_absolute() const
+{
+#ifdef _WIN32
+    return m_path[0] == preferred_separator || m_path[1] == L':';
+#else
+    return m_path[0] == preferred_separator;
+#endif
+}
+
+struct directory_iterator::SearchImpl
+{
+    path    m_path      {};
+#ifdef _WIN32
+    HANDLE  m_handle    { INVALID_HANDLE_VALUE };
+#else
+    DIR    *m_dir       { nullptr };
+#endif
+    explicit SearchImpl(const path& p) :
+        m_path(p)
+    {
+    };
+    ~SearchImpl();
+
+    bool advance(directory_entry&);
+};
+
+directory_iterator::SearchImpl::~SearchImpl()
+{
+#ifdef _WIN32
+    if (m_handle != INVALID_HANDLE_VALUE)
+    {
+        FindClose(m_handle);
+        m_handle = INVALID_HANDLE_VALUE;
+    }
+#else
+    if (m_dir != nullptr)
+    {
+        closedir(m_dir);
+        m_dir = nullptr;
+    }
+#endif
+}
+
+bool directory_iterator::SearchImpl::advance(directory_entry& entry)
+{
+#ifdef _WIN32
+    WIN32_FIND_DATAW findData;
+
+    if (m_handle == INVALID_HANDLE_VALUE)
+    {
+        m_handle = FindFirstFileW((m_path / L"*").c_str(), &findData);
+        if (m_handle == INVALID_HANDLE_VALUE)
+            return false;
+    }
+    else
+    {
+        if (!FindNextFileW(m_handle, &findData))
+            return false;
+    }
+
+    while (is_special_dir(findData.cFileName))
+    {
+        if (!FindNextFileW(m_handle, &findData))
+            return false;
+    }
+
+    entry = directory_entry(std::move(m_path / findData.cFileName));
+    return true;
+#else
+    if (m_dir == nullptr)
+    {
+        m_dir = opendir(m_path.c_str());
+        if (m_dir == nullptr)
+            return false;
+    }
+
+    struct dirent* ent = nullptr;
+    do
+    {
+        ent = readdir(m_dir);
+        if (ent == nullptr)
+            return false;
+    }
+    while (is_special_dir(ent->d_name));
+
+    entry = directory_entry(std::move(m_path / ent->d_name));
+    return true;
+#endif
+}
+
+directory_iterator::directory_iterator(const path& p) :
+    directory_iterator::directory_iterator(p, m_ec)
+{
+}
+
+directory_iterator::directory_iterator(const path& p, std::error_code& ec) :
+    m_path(p),
+    m_ec(ec),
+    m_search(std::make_shared<SearchImpl>(p))
+{
+    if (!m_search->advance(m_entry))
+        reset();
+}
+
+void directory_iterator::reset()
+{
+    *this = directory_iterator();
+}
+
+bool directory_iterator::operator==(const directory_iterator& other) const noexcept
+{
+    return m_search == other.m_search && m_entry == other.m_entry;
+}
+
+directory_iterator& directory_iterator::operator++()
+{
+    //  *this != end(*this) ...
+    if (m_search != nullptr && !m_search->advance(m_entry))
+        reset();
+    return *this;
+}
+
+
+struct recursive_directory_iterator::DirStack
+{
+    std::vector<directory_iterator> m_dirIters;
+};
+
+recursive_directory_iterator::recursive_directory_iterator(const path& p)
+{
+    if (m_dirs == nullptr)
+        m_dirs = std::make_shared<DirStack>();
+
+    m_iter = directory_iterator(p);
+}
+
+bool recursive_directory_iterator::operator==(const recursive_directory_iterator& other) const noexcept
+{
+    return m_depth   == other.m_depth   &&
+           m_pending == other.m_pending &&
+           m_dirs    == other.m_dirs    &&
+           m_iter    == other.m_iter;
+}
+
+
+recursive_directory_iterator& recursive_directory_iterator::operator++()
+{
+    // *this == end(*this)
+    if (m_dirs == nullptr)
+        return *this;
+
+    auto& p = m_iter->path();
+    if (m_pending && is_directory(p))
+    {
+        m_dirs->m_dirIters.emplace_back(m_iter);
+        m_iter = directory_iterator(p);
+    }
+    else
+    {
+        ++m_iter;
+    }
+
+    while (m_iter == end(m_iter))
+    {
+        if (m_dirs->m_dirIters.empty())
+        {
+            reset();
+            return *this;
+        }
+        m_iter = std::move(m_dirs->m_dirIters.back());
+        ++m_iter;
+        m_dirs->m_dirIters.pop_back();
+    }
+
+    return *this;
+}
+
+void recursive_directory_iterator::pop()
+{
+    std::error_code ec;
+    pop(ec);
+}
+
+void recursive_directory_iterator::pop(std::error_code& ec)
+{
+    m_dirs->m_dirIters.pop_back();
+}
+
+void recursive_directory_iterator::reset()
+{
+    *this = recursive_directory_iterator();
+}
+
+
+uintmax_t file_size(const path& p, std::error_code& ec) noexcept
+{
+#ifdef _WIN32
+    WIN32_FILE_ATTRIBUTE_DATA attr;
+    LARGE_INTEGER fileSize = { 0 };
+    if (GetFileAttributesExW(p.c_str(), GetFileExInfoStandard, &attr))
+    {
+        fileSize.LowPart  = attr.nFileSizeLow;
+        fileSize.HighPart = attr.nFileSizeHigh;
+        return static_cast<uintmax_t>(fileSize.QuadPart);
+    }
+    else
+    {
+        ec = std::error_code(errno, std::system_category());
+        return static_cast<uintmax_t>(-1);
+    }
+#else
+    struct stat stat_buf;
+    int rc = stat(p.c_str(), &stat_buf);
+    if (rc == -1)
+    {
+        ec = std::error_code(errno, std::system_category());
+        return static_cast<uintmax_t>(-1);
+    }
+
+    return static_cast<uintmax_t>(stat_buf.st_size);
+#endif
+}
+
+uintmax_t file_size(const path& p)
+{
+    std::error_code ec;
+    uintmax_t s = file_size(p, ec);
+    if (ec)
+        throw filesystem_error(ec, "celfs::file_size error");
+    return s;
+}
+
+
+bool exists(const path& p, std::error_code& ec) noexcept
+{
+#ifdef _WIN32
+    DWORD attr = GetFileAttributesW(&p.native()[0]);
+    if (attr != INVALID_FILE_ATTRIBUTES)
+        return true;
+
+    switch (GetLastError())
+    {
+    // behave like boost::filesystem
+    case ERROR_FILE_NOT_FOUND:
+    case ERROR_PATH_NOT_FOUND:
+    case ERROR_INVALID_NAME:
+    case ERROR_INVALID_DRIVE:
+    case ERROR_NOT_READY:
+    case ERROR_INVALID_PARAMETER:
+    case ERROR_BAD_PATHNAME:
+    case ERROR_BAD_NETPATH:
+        return false;
+    default:
+        break;
+    }
+#else
+    struct stat buf;
+    if (stat(p.c_str(), &buf) == 0)
+        return true;
+
+    if (errno == ENOENT)
+        return false;
+#endif
+    ec = std::error_code(errno, std::system_category());
+    return false;
+}
+
+bool exists(const path& p)
+{
+    std::error_code ec;
+    bool r = exists(p, ec);
+    if (ec)
+        throw filesystem_error(ec, "celfs::exists error");
+    return r;
+}
+
+
+bool is_directory(const path& p, std::error_code& ec) noexcept
+{
+#ifdef _WIN32
+    DWORD attr = GetFileAttributesW(&p.native()[0]);
+    if (attr == INVALID_FILE_ATTRIBUTES)
+    {
+        ec = std::error_code(errno, std::system_category());
+        return false;
+    }
+
+    return (attr & FILE_ATTRIBUTE_DIRECTORY) != 0;
+#else
+    struct stat buf;
+    return (stat(p.c_str(), &buf) == 0) && S_ISDIR(buf.st_mode);
+#endif
+}
+
+bool is_directory(const path& p)
+{
+    std::error_code ec;
+    bool r = is_directory(p, ec);
+    if (ec)
+        throw filesystem_error(ec, "celfs::is_directory error");
+    return r;
+}
+
+}
+}
diff --git a/src/celcompat/fs.h b/src/celcompat/fs.h
new file mode 100644
index 00000000..65473a8e
--- /dev/null
+++ b/src/celcompat/fs.h
@@ -0,0 +1,412 @@
+#pragma once
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <dirent.h>
+#endif
+#include <system_error>
+#include <string>
+#include <iostream>
+#include <memory>
+#ifdef _WIN32
+#include <celutil/winutil.h>
+#endif
+
+
+namespace celestia
+{
+namespace filesystem
+{
+class filesystem_error : std::system_error
+{
+ public:
+    filesystem_error(std::error_code ec, const char* msg) :
+        std::system_error(ec, msg)
+    {
+    }
+}; // filesystem_error
+
+
+class path
+{
+ public:
+#ifdef _WIN32
+    using string_type = std::wstring;
+#else
+    using string_type = std::string;
+#endif
+    using value_type = string_type::value_type;
+
+#ifdef _WIN32
+    static constexpr value_type preferred_separator = L'\\';
+#else
+    static constexpr value_type preferred_separator = '/';
+#endif
+
+    enum format
+    {
+        native_format,
+        generic_format,
+        auto_format
+    };
+
+    path() noexcept
+    {
+    };
+    path(const path&) = default;
+    path(path&& p) noexcept :
+        m_path(std::move(p.m_path)),
+        m_fmt(p.m_fmt)
+    {
+    };
+    path(string_type&& p, format fmt = auto_format) :
+        m_path(std::move(p)),
+        m_fmt(fmt)
+    {
+#ifdef _WIN32
+        fixup_separators();
+#endif
+    };
+    template<typename T> path(const T& p, format fmt = auto_format ) :
+        m_path(encconv(p)),
+        m_fmt(fmt)
+    {
+#ifdef _WIN32
+        fixup_separators();
+#endif
+    };
+    ~path() = default;
+    path& operator=(const path& p) = default;
+    path& operator=(path&&) = default;
+
+    template<typename T> path& append(const T& p)
+    {
+        if (empty())
+            m_path = p;
+        else
+            m_path.append(1, preferred_separator).append(p);
+        return *this;
+    }
+
+    template<typename T> path& operator/=(const T& p)
+    {
+        return append(p);
+    }
+
+    template<typename T> path& concat(const T& p)
+    {
+        m_path += p;
+        return *this;
+    }
+
+    template<typename T> path& operator+=(const T& p)
+    {
+        return concat(p);
+    }
+
+    bool operator==(const path& other) const noexcept
+    {
+        return m_path == other.m_path && m_fmt == other.m_fmt;
+    }
+
+    bool operator!=(const path& other) const noexcept
+    {
+        return !(*this == other);
+    }
+
+    bool operator<(const path& other) const noexcept
+    {
+        return m_path < other.m_path;
+    }
+
+    bool operator>(const path& other) const noexcept
+    {
+        return m_path > other.m_path;
+    }
+
+    bool operator<=(const path& other) const noexcept
+    {
+        return m_path <= other.m_path;
+    }
+
+    bool operator>=(const path& other) const noexcept
+    {
+        return m_path >= other.m_path;
+    }
+
+    std::string string() const noexcept
+    {
+#ifdef _WIN32
+        return WideToCurrentCP(m_path);
+#else
+        return m_path;
+#endif
+    }
+
+    std::wstring wstring() const noexcept
+    {
+#ifdef _WIN32
+        return m_path;
+#else
+        return L""; // FIXME
+#endif
+    }
+
+    std::string u8string() const noexcept
+    {
+#ifdef _WIN32
+        return WideToUTF8(m_path);
+#else
+        return m_path; // FIXME
+#endif
+    }
+
+    const value_type* c_str() const noexcept
+    {
+        return m_path.c_str();
+    }
+
+    const string_type& native() const noexcept
+    {
+       return m_path;
+    }
+
+    operator string_type() const noexcept
+    {
+        return m_path;
+    }
+
+    bool empty() const noexcept
+    {
+        return m_path.empty();
+    }
+
+    path filename() const;
+    path stem() const;
+    path extension() const;
+    path parent_path() const;
+
+    bool is_relative() const
+    {
+        return !is_absolute();
+    }
+    bool is_absolute() const;
+
+ private:
+    inline string_type encconv(const std::string& p) const
+    {
+#ifdef _WIN32
+        return CurrentCPToWide(p);
+#else
+        return p;
+#endif
+    }
+    inline string_type encconv(const std::wstring& p) const
+    {
+#ifdef _WIN32
+        return p;
+#else
+        return ""; // FIXME
+#endif
+    }
+#ifdef _WIN32
+    void fixup_separators();
+#endif
+
+    string_type m_path;
+    format      m_fmt       { auto_format };
+}; // path
+
+path operator/(const path& lhs, const path& rhs);
+
+std::ostream& operator<<(std::ostream& os, const path& p);
+
+path u8path(const std::string& source);
+
+
+class directory_iterator;
+class recursive_directory_iterator;
+class directory_entry
+{
+ public:
+    directory_entry() = default;
+    directory_entry(const directory_entry&) = default;
+    directory_entry(directory_entry&&) = default;
+    explicit directory_entry(const path& p) :
+        m_path(p)
+    {
+    };
+    directory_entry& operator=(const directory_entry&) = default;
+    directory_entry& operator=(directory_entry&&) = default;
+
+    const class path& path() const noexcept
+    {
+        return m_path;
+    }
+    operator const class path& () const noexcept
+    {
+        return m_path;
+    };
+
+    bool operator==(const directory_entry& other) const noexcept
+    {
+        return m_path == other.m_path;
+    }
+    bool operator!=(const directory_entry& other) const noexcept
+    {
+        return !(*this == other);
+    }
+
+ private:
+    friend class directory_iterator;
+    friend class recursive_directory_iterator;
+
+    class path m_path;
+}; // directory_entry
+
+
+class directory_iterator
+{
+ public:
+    using value_type        = directory_entry;
+    using difference_type   = std::ptrdiff_t;
+    using pointer           = const directory_entry*;
+    using reference         = const directory_entry&;
+    using iterator_category = std::input_iterator_tag;
+
+    directory_iterator() = default;
+    explicit directory_iterator(const path& p);
+    directory_iterator(const path& p, std::error_code& ec);
+
+    directory_iterator(const directory_iterator&) = default;
+    directory_iterator(directory_iterator&&) = default;
+    ~directory_iterator() = default;
+
+    directory_iterator& operator=(const directory_iterator&) = default;
+    directory_iterator& operator=(directory_iterator&&) = default;
+
+    directory_iterator& operator++();
+
+    const directory_entry& operator*() const noexcept
+    {
+        return m_entry;
+    }
+
+    const directory_entry* operator->() const noexcept
+    {
+        return &m_entry;
+    }
+
+    bool operator==(const directory_iterator& other) const noexcept;
+    bool operator!=(const directory_iterator& other) const noexcept
+    {
+        return !(*this == other);
+    }
+
+ private:
+    struct SearchImpl;
+    void reset();
+
+    path                        m_path      {};
+    directory_entry             m_entry     {};
+    std::error_code             m_ec        {};
+    std::shared_ptr<SearchImpl> m_search    { nullptr };
+}; // directory_iterator
+
+inline directory_iterator begin(directory_iterator iter) noexcept
+{
+    return iter;
+}
+
+inline directory_iterator end(directory_iterator) noexcept
+{
+    return directory_iterator();
+}
+
+
+class recursive_directory_iterator
+{
+ public:
+    using value_type        = directory_entry;
+    using difference_type   = std::ptrdiff_t;
+    using pointer           = const directory_entry*;
+    using reference         = const directory_entry&;
+    using iterator_category = std::input_iterator_tag;
+
+    recursive_directory_iterator() = default;
+    recursive_directory_iterator(const recursive_directory_iterator&) = default;
+    recursive_directory_iterator(recursive_directory_iterator&&) noexcept = default;
+    explicit recursive_directory_iterator(const path& p);
+//    recursive_directory_iterator(const path&, directory_options);
+//    recursive_directory_iterator(const path&, directory_options, std::error_code&);
+    recursive_directory_iterator(const path& p, std::error_code& ec);
+    ~recursive_directory_iterator() = default;
+
+    recursive_directory_iterator& operator=(const recursive_directory_iterator&) = default;
+    recursive_directory_iterator& operator=(recursive_directory_iterator&&) = default;
+
+    bool recursion_pending() const
+    {
+        return m_pending;
+    };
+
+    void disable_recursion_pending()
+    {
+        m_pending = false;
+    };
+
+    void pop();
+    void pop(std::error_code& ec);
+
+    recursive_directory_iterator& operator++();
+
+    const directory_entry& operator*() const noexcept
+    {
+        return *m_iter;
+    }
+
+    const directory_entry* operator->() const noexcept
+    {
+        return &*m_iter;
+    }
+
+    bool operator==(const recursive_directory_iterator& other) const noexcept;
+    bool operator!=(const recursive_directory_iterator& other) const noexcept
+    {
+        return !(*this == other);
+    }
+
+ private:
+    void reset();
+
+    std::error_code m_ec                {};
+    int             m_depth             { 0 };
+    bool            m_pending           { true };
+
+    struct DirStack;
+    std::shared_ptr<DirStack>   m_dirs  { nullptr };
+    directory_iterator          m_iter  {};
+}; // recursive_directory_iterator
+
+inline recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept
+{
+    return iter;
+}
+
+inline recursive_directory_iterator end(recursive_directory_iterator) noexcept
+{
+    return recursive_directory_iterator();
+}
+
+
+uintmax_t file_size(const path& p, std::error_code& ec) noexcept;
+uintmax_t file_size(const path& p);
+
+bool exists(const path& p);
+bool exists(const path& p, std::error_code& ec) noexcept;
+
+bool is_directory(const path& p);
+bool is_directory(const path& p, std::error_code& ec) noexcept;
+};
+};
diff --git a/src/celcompat/testfs.cc b/src/celcompat/testfs.cc
new file mode 100644
index 00000000..17fab4d4
--- /dev/null
+++ b/src/celcompat/testfs.cc
@@ -0,0 +1,30 @@
+#include <iostream>
+#include "fs.h"
+namespace fs = celestia::filesystem;
+
+int main()
+{
+    std::cout << fs::path("/foo/bar.txt").extension() << '\n'
+              << fs::path("/foo/bar.").extension() << '\n'
+              << fs::path("/foo/bar").extension() << '\n'
+              << fs::path("/foo/bar.txt/bar.cc").extension() << '\n'
+              << fs::path("/foo/bar.txt/bar.").extension() << '\n'
+              << fs::path("/foo/bar.txt/bar").extension() << '\n'
+              << fs::path("/foo/.").extension() << '\n'
+              << fs::path("/foo/..").extension() << '\n'
+              << fs::path("/foo/.hidden").extension() << '\n'
+              << fs::path("/foo/..bar").extension() << '\n';
+
+    std::cout << "----------\n";
+    std::cout << fs::path("/foo/bar.txt").stem() << '\n'
+              << fs::path("/foo/.bar").stem() << '\n';
+
+    for (fs::path p = "foo.bar.baz.tar"; !p.extension().empty(); p = p.stem())
+        std::cout << p.extension() << '\n';
+
+    std::cout << "----------\n";
+    std::cout << fs::path("/foo/bar.txt") << '\n';
+    std::cout << fs::path("baz/foo/bar.txt") << '\n';
+    std::cout << fs::path("c:\\foo\\bar.txt") << '\n';
+    std::cout << fs::path(L"c:\\foo\\bar.txt") << '\n';
+}