const std = @import("std");
const easyboot = @cImport(@cInclude("easyboot.h"));
const mmap = @import("../../mmap.zig");
const pmm = @import("../../pmm.zig");

const USER_ADDRESS_RANGE_END = 0x0000_7fff_ffff_ffff;
const PHYSICAL_MAPPING_BASE = 0xffff_8000_0000_0000;
const HUGE_PAGE_SIZE = 0x200000; // 2 MiB

pub const PageTableEntry = packed struct {
    present: u1,
    read_write: u1,
    user: u1,
    write_through: u1,
    cache_disabled: u1,
    accessed: u1,
    ignore0: u1,
    larger_pages: u1,
    global: u1,
    available: u3,
    address: u48,
    available2: u3,
    no_execute: u1,

    pub fn set_address(self: *PageTableEntry, address: u64) void {
        self.address = @intCast(address >> 12);
    }

    pub fn get_address(self: *PageTableEntry) u64 {
        return self.address << 12;
    }

    pub fn clear(self: *PageTableEntry) void {
        self = std.mem.zeroes(@TypeOf(self));
    }
};

pub const PageDirectory = struct {
    entries: [512]PageTableEntry,
};

const Flags = enum(u32) {
    None = 0,
    ReadWrite = 1,
    User = 2,
    NoExecute = 4,
    WriteThrough = 8,
    CacheDisable = 16,
    Global = 32,
};

const PageTableIndexes = struct {
    level4: u24,
    level3: u24,
    level2: u24,
    level1: u24,
};

fn calculatePageTableIndexes(address: usize) PageTableIndexes {
    return .{ .level4 = @intCast((address >> 39) & 0o777), .level3 = @intCast((address >> 30) & 0o777), .level2 = @intCast((address >> 21) & 0o777), .level1 = @intCast((address >> 12) & 0o777) };
}

fn hasFlag(flags: u32, flag: Flags) u1 {
    return switch ((flags & @intFromEnum(flag)) > 0) {
        true => 1,
        false => 0,
    };
}

fn updatePageTableEntry(entry: *PageTableEntry, phys: pmm.PhysFrame, flags: u32) void {
    entry.present = 1;
    entry.read_write = hasFlag(flags, Flags.ReadWrite);
    entry.user = hasFlag(flags, Flags.User);
    entry.write_through = hasFlag(flags, Flags.WriteThrough);
    entry.cache_disabled = hasFlag(flags, Flags.CacheDisable);
    entry.no_execute = hasFlag(flags, Flags.NoExecute);
    entry.global = hasFlag(flags, Flags.Global);
    entry.set_address(phys.address);
}

fn setUpParentPageTableEntry(allocator: *pmm.FrameAllocator, pte: *PageTableEntry, flags: u32, base: usize) !void {
    if (pte.present == 0) {
        const frame = try pmm.allocFrame(allocator);
        pte.present = 1;
        pte.set_address(frame.address);
        getTable(pte, base).* = std.mem.zeroes(PageDirectory);
    }
    if (hasFlag(flags, Flags.ReadWrite) == 1) pte.read_write = 1;
    if (hasFlag(flags, Flags.User) == 1) pte.user = 1;
}

fn getTable(pte: *PageTableEntry, base: usize) *allowzero PageDirectory {
    const frame = pmm.PhysFrame{ .address = pte.get_address() };
    return @ptrFromInt(frame.virtualAddress(base));
}

pub fn map(allocator: *pmm.FrameAllocator, directory: *PageDirectory, base: usize, virt_address: u64, phys: pmm.PhysFrame, flags: u32, use_huge_pages: bool) !void {
    const indexes = calculatePageTableIndexes(virt_address);
    const l4 = &directory.entries[indexes.level4];
    try setUpParentPageTableEntry(allocator, l4, flags, base);

    const l3 = &getTable(l4, base).entries[indexes.level3];
    if (l3.larger_pages == 1) return error.MemoryAlreadyInUse;
    try setUpParentPageTableEntry(allocator, l3, flags, base);

    const l2 = &getTable(l3, base).entries[indexes.level2];
    if (l2.larger_pages == 1) return error.MemoryAlreadyInUse;

    if (use_huge_pages) {
        l2.larger_pages = 1;
        updatePageTableEntry(l2, phys, flags);
        return;
    }

    try setUpParentPageTableEntry(allocator, l2, flags, base);

    const l1 = &getTable(l2, base).entries[indexes.level1];
    if (l1.present == 1) return error.MemoryAlreadyInUse;
    updatePageTableEntry(l1, phys, flags);
}

fn mapPhysicalMemory(allocator: *pmm.FrameAllocator, tag: *easyboot.multiboot_tag_mmap_t, directory: *PageDirectory, base: usize, flags: u32) !void {
    const address_space_size = mmap.getAddressSpaceSize(tag) orelse return error.InvalidMemoryMap;
    const address_space_pages = address_space_size / HUGE_PAGE_SIZE;

    var index: usize = 0;
    while (index < address_space_pages) : (index += 1) {
        try map(allocator, directory, 0, base + index * HUGE_PAGE_SIZE, pmm.PhysFrame{ .address = index * HUGE_PAGE_SIZE }, flags, true);
    }
}

fn lockPageDirectoryFrames(allocator: *pmm.FrameAllocator, directory: *PageDirectory, index: u8) !void {
    if (index > 1) {
        var i: u64 = 0;
        while (i < 512) : (i += 1) {
            const pte = &directory.entries[i];
            if (pte.present == 0) continue;
            if ((index < 4) and (pte.larger_pages == 1)) continue;

            try pmm.lockFrame(allocator, pte.get_address());

            const child_table: *PageDirectory = @ptrFromInt(pte.get_address());

            try lockPageDirectoryFrames(allocator, child_table, index - 1);
        }
    }
}

fn lockPageDirectory(allocator: *pmm.FrameAllocator, directory: *PageDirectory) !void {
    try pmm.lockFrame(allocator, @intFromPtr(directory));
    try lockPageDirectoryFrames(allocator, directory, 4);
}

fn setUpKernelPageDirectory(allocator: *pmm.FrameAllocator, tag: *easyboot.multiboot_tag_mmap_t) !*PageDirectory {
    const directory = readPageDirectory();

    try lockPageDirectory(allocator, directory);
    try mapPhysicalMemory(allocator, tag, directory, PHYSICAL_MAPPING_BASE, @intFromEnum(Flags.ReadWrite) | @intFromEnum(Flags.NoExecute) | @intFromEnum(Flags.Global));

    return directory;
}

fn setUpInitialUserPageDirectory(allocator: *pmm.FrameAllocator, tag: *easyboot.multiboot_tag_mmap_t, kernel_directory: *PageDirectory, user_directory: *PageDirectory) !usize {
    const physical_address_space_size = mmap.getAddressSpaceSize(tag) orelse return error.InvalidMemoryMap;

    user_directory.* = std.mem.zeroes(PageDirectory);

    const directory_upper_half: *[256]PageTableEntry = kernel_directory.entries[256..];
    const user_directory_upper_half: *[256]PageTableEntry = user_directory.entries[256..];
    @memcpy(user_directory_upper_half, directory_upper_half);

    const user_physical_address_base = (USER_ADDRESS_RANGE_END + 1) - physical_address_space_size;

    try mapPhysicalMemory(allocator, tag, user_directory, user_physical_address_base, @intFromEnum(Flags.ReadWrite) | @intFromEnum(Flags.NoExecute) | @intFromEnum(Flags.User));

    return user_physical_address_base;
}

pub fn createInitialMappings(allocator: *pmm.FrameAllocator, tag: *easyboot.multiboot_tag_mmap_t, user_directory: *PageDirectory) !usize {
    const directory = try setUpKernelPageDirectory(allocator, tag);
    const base = try setUpInitialUserPageDirectory(allocator, tag, directory, user_directory);

    setPageDirectory(directory);

    allocator.bitmap.location = @ptrFromInt(@as(usize, PHYSICAL_MAPPING_BASE) + @intFromPtr(allocator.bitmap.location));

    return base;
}

pub fn readPageDirectory() *PageDirectory {
    var directory: *PageDirectory = undefined;
    asm volatile ("mov %%cr3, %[dir]"
        : [dir] "=r" (directory),
    );
    return directory;
}

pub fn setPageDirectory(directory: *PageDirectory) void {
    asm volatile ("mov %[dir], %%cr3"
        :
        : [dir] "{rdi}" (directory),
    );
}