#include "kapi/memory.hpp"

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

#include "arch/boot/boot.hpp"
#include "arch/boot/ld.hpp"
#include "arch/cpu/registers.hpp"
#include "arch/memory/higher_half_mapper.hpp"
#include "arch/memory/kernel_mapper.hpp"
#include "arch/memory/page_table.hpp"
#include "arch/memory/page_utilities.hpp"
#include "arch/memory/region_allocator.hpp"

#include <kstd/print>
#include <kstd/units>

#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>

using namespace kstd::units_literals;

namespace kapi::memory
{

  namespace
  {
    auto constinit region_based_allocator = std::optional<arch::memory::region_allocator>{};
    auto constinit higher_half_mapper = std::optional<arch::memory::higher_half_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), static_cast<std::size_t>(mbi->size())};
      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,
        .mbi = mbi,
      };
    }

    auto establish_higher_half_direct_mapping() -> void
    {
      auto pml3_frame = kapi::memory::allocate_frame();
      auto pml3 = static_cast<arch::memory::page_table *>(pml3_frame->start_address());
      pml3->clear();

      std::ranges::for_each(std::views::iota(0uz, 512uz), [&](auto index) {
        auto frame = kapi::memory::frame{(1024uz * 1024uz * 1024uz * index)};
        auto & entry = (*pml3)[index];
        entry.frame(frame, arch::memory::page_table::entry::flags::present |
                               arch::memory::page_table::entry::flags::writable |
                               arch::memory::page_table::entry::flags::huge_page);
      });

      auto current_cr3 = arch::cpu::cr3::read();
      auto pml4 = static_cast<arch::memory::page_table *>(current_cr3.address());
      (*pml4)[256].frame(*pml3_frame, arch::memory::page_table::entry::flags::present |
                                          arch::memory::page_table::entry::flags::writable |
                                          arch::memory::page_table::entry::flags::global);
    }

    //! 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);
    }

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

    [[maybe_unused]] 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);
    }

    [[maybe_unused]] 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();
      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 + frame::size - 1_B) / 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);
      }
    }

    [[maybe_unused]] auto remap_bootloader_modules(page_mapper & mapper) -> void
    {
      std::ranges::for_each(boot::bootstrap_information.mbi->modules(), [&mapper](auto const & module) {
        auto module_physical_start = physical_address{module.start_address};
        auto module_virtual_start =
            linear_address{module.start_address + std::bit_cast<std::uintptr_t>(&arch::boot::TEACHOS_VMA)};
        auto module_size = static_cast<kstd::units::bytes>(module.end_address - module.start_address);
        auto module_block_count = (module_size + frame::size - 1_B) / frame::size;

        for (auto i = 0uz; i < module_block_count; ++i)
        {
          auto page = page::containing(module_virtual_start) + i;
          auto frame = frame::containing(module_physical_start) + i;
          mapper.map(page, frame, page_mapper::flags::writable | page_mapper::flags::supervisor_only);
        }
      });
    }

    [[maybe_unused]] 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 = kstd::units::bytes{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();
      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;

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

      std::ranges::for_each(boot::bootstrap_information.mbi->modules(), [&](auto const & module) {
        auto module_physical_start = physical_address{module.start_address};
        auto module_size = module.end_address - module.start_address;
        auto module_start = frame::containing(module_physical_start);
        auto module_end = frame::containing(module_physical_start + module_size) + 1;

        std::ranges::for_each(std::views::iota(module_start, module_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());
    set_frame_allocator(*region_based_allocator);

    kstd::println("[x86_64:MEM] Establishing higher-half direct mapping.");

    establish_higher_half_direct_mapping();

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

    auto new_pml4_frame = kapi::memory::allocate_frame();
    if (!new_pml4_frame)
    {
      system::panic("[x86_64:MEM] Failed to allocate new PML4!");
    }
    auto new_pml4 = arch::memory::to_higher_half_pointer<arch::memory::page_table>(new_pml4_frame->start_address());
    std::construct_at(new_pml4);

    higher_half_mapper.emplace(new_pml4);
    set_page_mapper(*higher_half_mapper);

    remap_kernel(*higher_half_mapper);
    remap_vga_text_mode_buffer(*higher_half_mapper);
    remap_multiboot_information(*higher_half_mapper);
    remap_bootloader_modules(*higher_half_mapper);

    auto current_cr3 = arch::cpu::cr3::read();
    auto old_pml4 = static_cast<arch::memory::page_table *>(current_cr3.address());
    (*new_pml4)[256] = (*old_pml4)[256];

    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.");
    region_based_allocator.reset();
  }

}  // namespace kapi::memory