#include "kapi/memory.hpp"

#include "kapi/boot.hpp"
#include "kapi/memory/buffered_allocator.hpp"
#include "kapi/system.hpp"

#include "arch/boot/boot.hpp"
#include "arch/boot/ld.hpp"
#include "arch/cpu/registers.hpp"
#include "arch/memory/kernel_mapper.hpp"
#include "arch/memory/mmu.hpp"
#include "arch/memory/page_table.hpp"
#include "arch/memory/page_utilities.hpp"
#include "arch/memory/paging_root.hpp"
#include "arch/memory/recursive_page_mapper.hpp"
#include "arch/memory/region_allocator.hpp"
#include "arch/memory/scoped_mapping.hpp"

#include <kstd/print>

#include <multiboot2/constants.hpp>
#include <multiboot2/information.hpp>

#include <algorithm>
#include <atomic>
#include <bit>
#include <cstddef>
#include <cstdint>
#include <memory>
#include <optional>
#include <ranges>
#include <span>
#include <utility>

namespace kapi::memory
{

  namespace
  {
    constexpr auto static unused_page_address = linear_address{0x0000'7fff'cafe'faceuz};
    constexpr auto static recursive_page_map_index = arch::memory::page_table::entry_count - 2;

    auto constinit region_based_allocator = std::optional<arch::memory::region_allocator>{};
    auto constinit allocation_buffer = std::optional<buffered_allocator<4>>{};
    auto constinit recursive_page_mapper = std::optional<arch::memory::recursive_page_mapper>{};

    //! Instantiate a basic, memory region based, early frame allocator for remapping.
    auto collect_memory_information()
    {
      auto memory_map = boot::bootstrap_information.mbi->maybe_memory_map();
      if (!memory_map)
      {
        system::panic("[x86_64] Failed to create early allocator, no memory map available.");
      }

      auto const & mbi = boot::bootstrap_information.mbi;
      auto mbi_span = std::span{std::bit_cast<std::byte *>(mbi), mbi->size_bytes()};
      auto image_span = std::span{&arch::boot::_start_physical, &arch::boot::_end_physical};

      return arch::memory::region_allocator::memory_information{
        .image_range = std::make_pair(physical_address{&image_span.front()}, physical_address{&image_span.back()}),
        .mbi_range = std::make_pair(physical_address{&mbi_span.front()}, physical_address{&mbi_span.back()}),
        .memory_map = *memory_map,
      };
    }

    //! Enable additional CPU protection features, required during later stages of the kernel.
    auto enable_cpu_protections() -> void
    {
      arch::cpu::cr0::set(arch::cpu::cr0::flags::write_protect);
      arch::cpu::i32_efer::set(arch::cpu::i32_efer::flags::execute_disable_bit_enable);
    }

    //! Inject, or graft, a faux recursive PML4 into the active page mapping structure.
    auto inject_faux_pml4(frame_allocator & allocator, page_mapper & mapper)
    {
      using arch::memory::page_table;
      using arch::memory::paging_root;
      using arch::memory::pml_index;
      using entry_flags = arch::memory::page_table::entry::flags;

      auto page = page::containing(unused_page_address);

      auto temporary_mapper = arch::memory::scoped_mapping{page, mapper};
      auto new_pml4_frame = allocator.allocate();

      auto pml4 = std::construct_at(temporary_mapper.map_as<page_table>(*new_pml4_frame, entry_flags::writable));
      (*pml4)[recursive_page_map_index].frame(new_pml4_frame.value(), entry_flags::present | entry_flags::writable);

      auto pml4_index = pml_index<4>(page);
      auto old_pml4 = paging_root::get();
      auto pml4_entry = (*old_pml4)[pml4_index];

      auto pml3_index = pml_index<3>(page);
      auto old_pml3 = old_pml4->next(pml4_index);
      auto pml3_entry = (**old_pml3)[pml3_index];

      auto pml2_index = pml_index<2>(page);
      auto old_pml2 = (**old_pml3).next(pml3_index);
      auto pml2_entry = (**old_pml2)[pml2_index];

      auto pml1_index = pml_index<1>(page);
      auto old_pml1 = (**old_pml2).next(pml2_index);
      auto pml1_entry = (**old_pml1)[pml1_index];

      (*paging_root::get())[recursive_page_map_index].frame(new_pml4_frame.value(),
                                                            entry_flags::present | entry_flags::writable);

      arch::memory::tlb_flush_all();

      auto new_pml4 = paging_root::get();
      (*new_pml4)[pml4_index] = pml4_entry;

      auto new_pml3 = new_pml4->next(pml4_index);
      (**new_pml3)[pml3_index] = pml3_entry;

      auto new_pml2 = (**new_pml3).next(pml3_index);
      (**new_pml2)[pml2_index] = pml2_entry;

      auto new_pml1 = (**new_pml2).next(pml2_index);
      (**new_pml1)[pml1_index] = pml1_entry;

      return *new_pml4_frame;
    }

    auto remap_kernel(page_mapper & mapper) -> void
    {
      auto kernel_mapper = arch::memory::kernel_mapper{boot::bootstrap_information.mbi};
      kernel_mapper.remap_kernel(mapper);
    }

    auto remap_vga_text_mode_buffer(page_mapper & mapper) -> void
    {
      constexpr auto vga_base = std::uintptr_t{0xb8000};
      auto vga_physical_start = physical_address{vga_base};
      auto vga_virtual_start = linear_address{vga_base + std::bit_cast<std::uintptr_t>(&arch::boot::TEACHOS_VMA)};

      auto page = page::containing(vga_virtual_start);
      auto frame = frame::containing(vga_physical_start);

      mapper.map(page, frame, page_mapper::flags::writable | page_mapper::flags::supervisor_only);
    }

    auto remap_multiboot_information(page_mapper & mapper) -> void
    {
      auto mbi_base = std::bit_cast<std::uintptr_t>(boot::bootstrap_information.mbi);
      auto mbi_size = boot::bootstrap_information.mbi->size_bytes();
      auto mbi_physical_start = physical_address{mbi_base & ~std::bit_cast<std::uintptr_t>(&arch::boot::TEACHOS_VMA)};
      auto mbi_virtual_start = linear_address{mbi_base};
      auto mbi_block_count = (mbi_size + PLATFORM_FRAME_SIZE - 1) / PLATFORM_FRAME_SIZE;

      for (auto i = 0uz; i < mbi_block_count; ++i)
      {
        auto page = page::containing(mbi_virtual_start) + i;
        auto frame = frame::containing(mbi_physical_start) + i;
        mapper.map(page, frame, page_mapper::flags::supervisor_only);
      }
    }

    auto handoff_to_kernel_pmm(frame_allocator & new_allocator) -> void
    {
      auto memory_map = boot::bootstrap_information.mbi->memory_map();

      for (auto const & region : memory_map.regions() | std::views::filter([](auto const & region) {
                                   return region.type == multiboot2::memory_type::available;
                                 }))
      {
        auto start = frame::containing(physical_address{region.base});
        auto count = region.size_in_B / page::size;
        new_allocator.release_many({start, count});
      }

      auto next_free_frame = region_based_allocator->next_free_frame();
      if (!next_free_frame)
      {
        system::panic("[x86_64:MEM] No more free memory!");
      }

      std::ranges::for_each(std::views::iota(kapi::memory::frame{}, *next_free_frame),
                            [&](auto frame) { new_allocator.mark_used(frame); });

      auto image_start = frame::containing(physical_address{&arch::boot::_start_physical});
      auto image_end = frame::containing(physical_address{&arch::boot::_end_physical}) + 1;

      std::ranges::for_each(std::views::iota(image_start, image_end),
                            [&](auto frame) { new_allocator.mark_used(frame); });

      auto mbi_base = std::bit_cast<std::uintptr_t>(boot::bootstrap_information.mbi);
      auto mbi_size = boot::bootstrap_information.mbi->size_bytes();
      auto mbi_address = physical_address{mbi_base & ~std::bit_cast<std::uintptr_t>(&arch::boot::TEACHOS_VMA)};
      auto mbi_start = frame::containing(mbi_address);
      auto mbi_end = frame::containing(mbi_address + mbi_size) + 1;

      // TODO BA-FS26: Protect MB2 boot modules

      std::ranges::for_each(std::views::iota(mbi_start, mbi_end), [&](auto frame) { new_allocator.mark_used(frame); });
    }

  }  // namespace

  auto init() -> void
  {
    auto static constinit is_initialized = std::atomic_flag{};

    if (is_initialized.test_and_set())
    {
      system::panic("[x86_64] Memory management has already been initialized.");
    }

    kstd::println("[x86_64:MEM] Enabling additional CPU protection features.");

    enable_cpu_protections();

    region_based_allocator.emplace(collect_memory_information());
    allocation_buffer.emplace(&*region_based_allocator);
    set_frame_allocator(*allocation_buffer);

    recursive_page_mapper.emplace();
    set_page_mapper(*recursive_page_mapper);

    kstd::println("[x86_64:MEM] Preparing new paging hierarchy.");

    auto new_pml4_frame = inject_faux_pml4(*allocation_buffer, *recursive_page_mapper);

    remap_kernel(*recursive_page_mapper);
    remap_vga_text_mode_buffer(*recursive_page_mapper);
    remap_multiboot_information(*recursive_page_mapper);

    kstd::println("[x86_64:MEM] Switching to new paging hierarchy.");

    auto cr3 = arch::cpu::cr3::read();
    cr3.frame(new_pml4_frame);
    arch::cpu::cr3::write(cr3);

    auto memory_map = boot::bootstrap_information.mbi->memory_map();
    auto highest_byte = std::ranges::max(std::views::transform(
        std::views::filter(memory_map.regions(),
                           [](auto const & region) { return region.type == multiboot2::memory_type::available; }),
        [](auto const & region) { return region.base + region.size_in_B; }));

    init_pmm(frame::containing(physical_address{highest_byte}).number() + 1, handoff_to_kernel_pmm);

    kstd::println("[x86_64:MEM] Releasing bootstrap memory allocators.");
    allocation_buffer.reset();
    region_based_allocator.reset();
  }

}  // namespace kapi::memory