9 files changed, 213 insertions, 0 deletions

diff --git a/kernel/CMakeLists.txt b/kernel/CMakeLists.txt
index c3c9698..830c527 100644
--- a/kernel/CMakeLists.txt
+++ b/kernel/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_subdirectory("devices")
+add_subdirectory("filesystem")
 
 add_executable("kernel"
     # Platform-independent KAPI implementation
@@ -27,6 +28,7 @@ target_link_libraries("kernel" PRIVATE
     "os::arch"
     "os::kapi"
     "kernel::devices"
+    "kernel::filesystem"
 )
 
 target_link_options("kernel" PRIVATE
diff --git a/kernel/filesystem/CMakeLists.txt b/kernel/filesystem/CMakeLists.txt
new file mode 100644
index 0000000..2a93c62
--- /dev/null
+++ b/kernel/filesystem/CMakeLists.txt
@@ -0,0 +1,25 @@
+add_library("kernel_filesystem" STATIC)
+add_library("kernel::filesystem" ALIAS "kernel_filesystem")
+
+target_sources("kernel_filesystem" PRIVATE
+    "src/file.cpp"
+    "src/open_file_description.cpp"
+    "src/file_descriptor_table.cpp"
+)
+
+target_sources("kernel_filesystem" PUBLIC
+    FILE_SET HEADERS
+    BASE_DIRS "include"
+    FILES
+    "include/filesystem/file.hpp"
+    "include/filesystem/open_file_description.hpp"
+    "include/filesystem/file_descriptor_table.hpp"
+)
+
+target_include_directories("kernel_filesystem" PUBLIC
+    "include"
+)
+
+target_link_libraries("kernel_filesystem" PRIVATE
+    "os::kapi"
+)
diff --git a/kernel/filesystem/include/filesystem/file.hpp b/kernel/filesystem/include/filesystem/file.hpp
new file mode 100644
index 0000000..e7e1b12
--- /dev/null
+++ b/kernel/filesystem/include/filesystem/file.hpp
@@ -0,0 +1,19 @@
+#ifndef TEACH_OS_KERNEL_FILESYSTEM_FILE_HPP
+#define TEACH_OS_KERNEL_FILESYSTEM_FILE_HPP
+
+#include <cstddef>
+
+namespace filesystem
+{
+  struct file
+  {
+    virtual ~file() = default;
+
+    virtual auto open() -> void = 0;
+
+    virtual auto read(void * buffer, size_t offset, size_t size) const -> size_t = 0;
+    virtual auto write(void const * buffer, size_t offset, size_t size) -> size_t = 0;
+  };
+}  // namespace filesystem
+
+#endif
\ No newline at end of file
diff --git a/kernel/filesystem/include/filesystem/file_descriptor_table.hpp b/kernel/filesystem/include/filesystem/file_descriptor_table.hpp
new file mode 100644
index 0000000..a865d32
--- /dev/null
+++ b/kernel/filesystem/include/filesystem/file_descriptor_table.hpp
@@ -0,0 +1,28 @@
+#ifndef TEACH_OS_KERNEL_FILESYSTEM_FILE_DESCRIPTOR_TABLE_HPP
+#define TEACH_OS_KERNEL_FILESYSTEM_FILE_DESCRIPTOR_TABLE_HPP
+
+#include "open_file_description.hpp"
+
+#include <array>
+
+namespace filesystem
+{
+  struct file_descriptor_table
+  {
+    auto static init() -> void;
+    auto static get() -> file_descriptor_table &;
+
+    ~file_descriptor_table() = default;
+
+    auto add_file(open_file_description & f) -> int;
+    auto get_file(int fd) -> open_file_description &;
+    auto remove_file(int fd) -> void;
+
+  private:
+    file_descriptor_table() = default;
+
+    std::array<open_file_description *, 32> m_open_files{};  // TODO BA-FS26 use kstd::vector when available
+  };
+}  // namespace filesystem
+
+#endif
\ No newline at end of file
diff --git a/kernel/filesystem/include/filesystem/open_file_description.hpp b/kernel/filesystem/include/filesystem/open_file_description.hpp
new file mode 100644
index 0000000..e4febbd
--- /dev/null
+++ b/kernel/filesystem/include/filesystem/open_file_description.hpp
@@ -0,0 +1,24 @@
+#ifndef TEACH_OS_KERNEL_FILESYSTEM_OPEN_FILE_DESCRIPTION_HPP
+#define TEACH_OS_KERNEL_FILESYSTEM_OPEN_FILE_DESCRIPTION_HPP
+
+#include "file.hpp"
+
+#include <cstddef>
+
+namespace filesystem
+{
+  struct open_file_description
+  {
+    ~open_file_description() = default;
+
+    auto read(void * buffer, size_t size) -> size_t;
+    auto write(void const * buffer, size_t size) -> size_t;
+
+  private:
+    file * m_file{};
+    size_t m_offset{0};
+  };
+
+}  // namespace filesystem
+
+#endif
\ No newline at end of file
diff --git a/kernel/filesystem/src/file.cpp b/kernel/filesystem/src/file.cpp
new file mode 100644
index 0000000..c67ad1a
--- /dev/null
+++ b/kernel/filesystem/src/file.cpp
@@ -0,0 +1,6 @@
+#include "filesystem/file.hpp"
+
+namespace filesystem
+{
+  // TODO BA-FS26
+}
\ No newline at end of file
diff --git a/kernel/filesystem/src/file_descriptor_table.cpp b/kernel/filesystem/src/file_descriptor_table.cpp
new file mode 100644
index 0000000..0c79431
--- /dev/null
+++ b/kernel/filesystem/src/file_descriptor_table.cpp
@@ -0,0 +1,81 @@
+#include "filesystem/file_descriptor_table.hpp"
+
+#include "kapi/system.hpp"
+
+#include "filesystem/open_file_description.hpp"
+
+#include <algorithm>
+#include <cstddef>
+#include <optional>
+
+namespace filesystem
+{
+  namespace
+  {
+    constinit auto static global_file_descriptor_table = std::optional<file_descriptor_table>{};
+  }  // namespace
+
+  auto file_descriptor_table::init() -> void
+  {
+    if (global_file_descriptor_table)
+    {
+      kapi::system::panic("[FILESYSTEM] File descriptor table has already been initialized.");
+    }
+
+    global_file_descriptor_table.emplace(file_descriptor_table{});
+  }
+
+  auto file_descriptor_table::get() -> file_descriptor_table &
+  {
+    if (!global_file_descriptor_table)
+    {
+      kapi::system::panic("[FILESYSTEM] File descriptor table has not been initialized.");
+    }
+
+    return *global_file_descriptor_table;
+  }
+
+  auto file_descriptor_table::add_file(open_file_description & f) -> int
+  {
+    auto it = std::ranges::find(m_open_files, nullptr);
+    if (it != m_open_files.end())
+    {
+      *it = &f;
+      return static_cast<int>(it - m_open_files.begin());
+    }
+
+    return -1;
+  }
+
+  auto file_descriptor_table::get_file(int fd) -> open_file_description &
+  {
+    if (fd < 0)
+    {
+      kapi::system::panic("[FILESYSTEM] get_file called with negative descriptor.");
+    }
+
+    auto const index = static_cast<size_t>(fd);
+    if (index >= m_open_files.size() || m_open_files[index] == nullptr)
+    {
+      kapi::system::panic("[FILESYSTEM] get_file called with invalid descriptor.");
+    }
+
+    return *m_open_files[index];
+  }
+
+  auto file_descriptor_table::remove_file(int fd) -> void
+  {
+    if (fd < 0)
+    {
+      kapi::system::panic("[FILESYSTEM] remove_file called with negative descriptor.");
+    }
+
+    auto const index = static_cast<size_t>(fd);
+    if (index >= m_open_files.size())
+    {
+      kapi::system::panic("[FILESYSTEM] remove_file called with out-of-range descriptor.");
+    }
+
+    m_open_files[index] = nullptr;
+  }
+}  // namespace filesystem
\ No newline at end of file
diff --git a/kernel/filesystem/src/open_file_description.cpp b/kernel/filesystem/src/open_file_description.cpp
new file mode 100644
index 0000000..8c20397
--- /dev/null
+++ b/kernel/filesystem/src/open_file_description.cpp
@@ -0,0 +1,24 @@
+#include "filesystem/open_file_description.hpp"
+
+#include "filesystem/file.hpp"
+
+#include <cstddef>
+
+namespace filesystem
+{
+  auto open_file_description::read(void * buffer, size_t size) -> size_t
+  {
+    // TODO BA-FS26 nullptr check
+    auto read_bytes = m_file->read(buffer, m_offset, size);
+    m_offset += read_bytes;
+    return read_bytes;
+  }
+
+  auto open_file_description::write(void const * buffer, size_t size) -> size_t
+  {
+    // TODO BA-FS26 nullptr check
+    auto written_bytes = m_file->write(buffer, m_offset, size);
+    m_offset += written_bytes;
+    return written_bytes;
+  }
+}  // namespace filesystem
\ No newline at end of file
diff --git a/kernel/src/main.cpp b/kernel/src/main.cpp
index 07aa8ae..4b74bd4 100644
--- a/kernel/src/main.cpp
+++ b/kernel/src/main.cpp
@@ -4,6 +4,7 @@
 #include "kapi/system.hpp"
 
 #include "devices/storage/storage_management.hpp"
+#include "filesystem/file_descriptor_table.hpp"
 
 #include "kernel/memory.hpp"
 
@@ -25,5 +26,8 @@ auto main() -> int
   devices::storage::storage_management::init();
   kstd::println("[OS] Storage management initialized.");
 
+  filesystem::file_descriptor_table::init();
+  kstd::println("[OS] Global file descriptor table initialized.");
+
   kapi::system::panic("Returning from kernel main!");
 }