//! Program memory implementation

pub mod paging;

mod pfhandler;

pub use pfhandler::HandlePageFault;

use {
    self::paging::{PageTable, Permission, PtEntry},
    super::VmRunError,
    alloc::boxed::Box,
    core::mem::MaybeUninit,
    derive_more::Display,
};

/// HoleyBytes virtual memory
#[derive(Clone, Debug)]
pub struct Memory {
    /// Root page table
    root_pt: *mut PageTable,
}

impl Default for Memory {
    fn default() -> Self {
        Self {
            root_pt: Box::into_raw(Box::default()),
        }
    }
}

impl Drop for Memory {
    fn drop(&mut self) {
        let _ = unsafe { Box::from_raw(self.root_pt) };
    }
}

impl Memory {
    /// Maps host's memory into VM's memory
    ///
    /// # Safety
    /// Who knows.
    pub unsafe fn map(
        &mut self,
        host: *mut u8,
        target: u64,
        perm: Permission,
        pagesize: PageSize,
    ) -> Result<(), MapError> {
        let mut current_pt = self.root_pt;

        let lookup_depth = match pagesize {
            PageSize::Size4K => 4,
            PageSize::Size2M => 3,
            PageSize::Size1G => 2,
        };

        // Lookup pagetable above
        for lvl in (0..lookup_depth).rev() {
            let entry = (*current_pt)
                .table
                .get_unchecked_mut(addr_extract_index(target, lvl));

            let ptr = entry.ptr();
            match entry.permission() {
                Permission::Empty => {
                    (*current_pt).childen += 1;
                    let table = Box::into_raw(Box::new(paging::PtPointedData {
                        pt: PageTable::default(),
                    }));

                    core::ptr::write(entry, PtEntry::new(table, Permission::Node));
                    current_pt = table as _;
                }
                Permission::Node => current_pt = ptr as _,
                _ => return Err(MapError::AlreadyMapped),
            }
        }

        // Write entry
        (*current_pt).childen += 1;
        core::ptr::write(
            (*current_pt)
                .table
                .get_unchecked_mut(addr_extract_index(target, 4 - lookup_depth)),
            PtEntry::new(host.cast(), perm),
        );

        Ok(())
    }

    /// Unmaps pages from VM's memory
    pub fn unmap(&mut self, addr: u64) -> Result<(), NothingToUnmap> {
        let mut current_pt = self.root_pt;
        let mut page_tables = [core::ptr::null_mut(); 5];

        for lvl in (0..5).rev() {
            let entry = unsafe {
                (*current_pt)
                    .table
                    .get_unchecked_mut(addr_extract_index(addr, lvl))
            };

            let ptr = entry.ptr();
            match entry.permission() {
                Permission::Empty => return Err(NothingToUnmap),
                Permission::Node => {
                    page_tables[lvl as usize] = entry;
                    current_pt = ptr as _
                }
                _ => unsafe {
                    core::ptr::write(entry, Default::default());
                },
            }
        }

        for entry in page_tables.into_iter() {
            if entry.is_null() {
                continue;
            }

            unsafe {
                let children = &mut (*(*entry).ptr()).pt.childen;
                *children -= 1;
                if *children == 0 {
                    core::mem::drop(Box::from_raw((*entry).ptr() as *mut PageTable));
                }

                core::ptr::write(entry, Default::default());
            }
        }

        Ok(())
    }

    /// Load value from an address
    ///
    /// # Safety
    /// Applies same conditions as for [`core::ptr::copy_nonoverlapping`]
    pub unsafe fn load(
        &mut self,
        addr: u64,
        target: *mut u8,
        count: usize,
        traph: &mut impl HandlePageFault,
    ) -> Result<(), LoadError> {
        self.memory_access(
            MemoryAccessReason::Load,
            addr,
            target,
            count,
            |perm| {
                matches!(
                    perm,
                    Permission::Readonly | Permission::Write | Permission::Exec
                )
            },
            |src, dst, count| core::ptr::copy_nonoverlapping(src, dst, count),
            traph,
        )
        .map_err(LoadError)
    }

    /// Store value to an address
    ///
    /// # Safety
    /// Applies same conditions as for [`core::ptr::copy_nonoverlapping`]
    pub unsafe fn store(
        &mut self,
        addr: u64,
        source: *const u8,
        count: usize,
        traph: &mut impl HandlePageFault,
    ) -> Result<(), StoreError> {
        self.memory_access(
            MemoryAccessReason::Store,
            addr,
            source.cast_mut(),
            count,
            |perm| perm == Permission::Write,
            |dst, src, count| core::ptr::copy_nonoverlapping(src, dst, count),
            traph,
        )
        .map_err(StoreError)
    }

    /// Copy a block of memory
    ///
    /// # Safety
    /// - Same as for [`Self::load`] and [`Self::store`]
    /// - Your faith in the gods of UB
    ///     - Addr-san claims it's fine but who knows is she isn't lying :ferrisSus:
    pub unsafe fn block_copy(
        &mut self,
        mut src: u64,
        mut dst: u64,
        count: usize,
        traph: &mut impl HandlePageFault,
    ) -> Result<(), BlkCopyError> {
        // Yea, i know it is possible to do this more efficiently, but I am too lazy.

        impl Memory {
            #[inline]
            unsafe fn act(
                &mut self,
                src: u64,
                dst: u64,
                buf: *mut u8,
                count: usize,
                traph: &mut impl HandlePageFault,
            ) -> Result<(), BlkCopyError> {
                self.memory_access(
                    MemoryAccessReason::Load,
                    src,
                    buf,
                    STACK_BUFFER_SIZE,
                    |perm| {
                        matches!(
                            perm,
                            Permission::Readonly | Permission::Write | Permission::Exec
                        )
                    },
                    |src, dst, count| core::ptr::copy(src, dst, count),
                    traph,
                )
                .map_err(|addr| BlkCopyError {
                    access_reason: MemoryAccessReason::Load,
                    addr,
                })?;

                self.memory_access(
                    MemoryAccessReason::Store,
                    dst,
                    buf,
                    count,
                    |perm| perm == Permission::Write,
                    |dst, src, count| core::ptr::copy(src, dst, count),
                    traph,
                )
                .map_err(|addr| BlkCopyError {
                    access_reason: MemoryAccessReason::Store,
                    addr,
                })?;

                Ok(())
            }
        }

        const STACK_BUFFER_SIZE: usize = 4096;

        // Decide if to use stack-allocated buffer or to heap allocate
        // Deallocation is again decided on size at the end of the function
        let mut buf = MaybeUninit::<[u8; STACK_BUFFER_SIZE]>::uninit();

        let n_buffers = count / STACK_BUFFER_SIZE;
        let rem = count % STACK_BUFFER_SIZE;

        for _ in 0..n_buffers {
            self.act(src, dst, buf.as_mut_ptr().cast(), STACK_BUFFER_SIZE, traph)?;
            src += STACK_BUFFER_SIZE as u64;
            dst += STACK_BUFFER_SIZE as u64;
        }

        self.act(src, dst, buf.as_mut_ptr().cast(), rem, traph)
    }

    /// Split address to pages, check their permissions and feed pointers with offset
    /// to a specified function.
    ///
    /// If page is not found, execute page fault trap handler.
    #[allow(clippy::too_many_arguments)] // Silence peasant
    fn memory_access(
        &mut self,
        reason: MemoryAccessReason,
        src: u64,
        mut dst: *mut u8,
        len: usize,
        permission_check: fn(Permission) -> bool,
        action: fn(*mut u8, *mut u8, usize),
        traph: &mut impl HandlePageFault,
    ) -> Result<(), u64> {
        let mut pspl = AddrPageLookuper::new(src, len, self.root_pt);
        loop {
            match pspl.next() {
                // Page found
                Some(Ok(AddrPageLookupOk {
                    vaddr,
                    ptr,
                    size,
                    perm,
                })) => {
                    if !permission_check(perm) {
                        return Err(vaddr);
                    }

                    // Perform memory action and bump dst pointer
                    action(ptr, dst, size);
                    dst = unsafe { dst.add(size) };
                }
                Some(Err(AddrPageLookupError { addr, size })) => {
                    // Execute page fault handler
                    if traph.page_fault(reason, self, addr, size, dst) {
                        // Shift the splitter address
                        pspl.bump(size);

                        // Bump dst pointer
                        dst = unsafe { dst.add(size as _) };
                    } else {
                        return Err(addr); // Unhandleable
                    }
                }
                None => return Ok(()),
            }
        }
    }
}

/// Result from address split
struct AddrPageLookupOk {
    /// Virtual address
    vaddr: u64,

    /// Pointer to the start for perform operation
    ptr: *mut u8,

    /// Size to the end of page / end of desired size
    size: usize,

    /// Page permission
    perm: Permission,
}

struct AddrPageLookupError {
    /// Address of failure
    addr: u64,

    /// Requested page size
    size: PageSize,
}

/// Address splitter into pages
struct AddrPageLookuper {
    /// Current address
    addr: u64,

    /// Size left
    size: usize,

    /// Page table
    pagetable: *const PageTable,
}

impl AddrPageLookuper {
    /// Create a new page splitter
    pub const fn new(addr: u64, size: usize, pagetable: *const PageTable) -> Self {
        Self {
            addr,
            size,
            pagetable,
        }
    }

    /// Bump address by size X
    fn bump(&mut self, page_size: PageSize) {
        self.addr += page_size as u64;
        self.size = self.size.saturating_sub(page_size as _);
    }
}

impl Iterator for AddrPageLookuper {
    type Item = Result<AddrPageLookupOk, AddrPageLookupError>;

    fn next(&mut self) -> Option<Self::Item> {
        // The end, everything is fine
        if self.size == 0 {
            return None;
        }

        let (base, perm, size, offset) = 'a: {
            let mut current_pt = self.pagetable;

            // Walk the page table
            for lvl in (0..5).rev() {
                // Get an entry
                unsafe {
                    let entry = (*current_pt)
                        .table
                        .get_unchecked(addr_extract_index(self.addr, lvl));

                    let ptr = entry.ptr();
                    match entry.permission() {
                        // No page → page fault
                        Permission::Empty => {
                            return Some(Err(AddrPageLookupError {
                                addr: self.addr,
                                size: PageSize::from_lvl(lvl)?,
                            }))
                        }

                        // Node → proceed waking
                        Permission::Node => current_pt = ptr as _,

                        // Leaf → return relevant data
                        perm => {
                            break 'a (
                                // Pointer in host memory
                                ptr as *mut u8,
                                perm,
                                PageSize::from_lvl(lvl)?,
                                // In-page offset
                                self.addr as usize & ((1 << (lvl * 9 + 12)) - 1),
                            );
                        }
                    }
                }
            }
            return None; // Reached the end (should not happen)
        };

        // Get available byte count in the selected page with offset
        let avail = (size as usize - offset).clamp(0, self.size);
        self.bump(size);

        Some(Ok(AddrPageLookupOk {
            vaddr: self.addr,
            ptr: unsafe { base.add(offset) }, // Return pointer to the start of region
            size: avail,
            perm,
        }))
    }
}

fn addr_extract_index(addr: u64, lvl: u8) -> usize {
    debug_assert!(lvl <= 4);
    usize::try_from((addr >> (lvl * 9 + 12)) & ((1 << 9) - 1)).expect("?conradluget a better CPU")
}

/// Page size
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum PageSize {
    /// 4 KiB page (on level 0)
    Size4K = 4096,

    /// 2 MiB page (on level 1)
    Size2M = 1024 * 1024 * 2,

    /// 1 GiB page (on level 2)
    Size1G = 1024 * 1024 * 1024,
}

impl PageSize {
    /// Convert page table level to size of page
    const fn from_lvl(lvl: u8) -> Option<Self> {
        match lvl {
            0 => Some(PageSize::Size4K),
            1 => Some(PageSize::Size2M),
            2 => Some(PageSize::Size1G),
            _ => None,
        }
    }
}

/// Unhandled load access trap
#[derive(Clone, Copy, Display, Debug, PartialEq, Eq)]
pub struct LoadError(u64);

/// Unhandled store access trap
#[derive(Clone, Copy, Display, Debug, PartialEq, Eq)]
pub struct StoreError(u64);

#[derive(Clone, Copy, Display, Debug)]
pub struct NothingToUnmap;

#[derive(Clone, Copy, Display, Debug, PartialEq, Eq)]
pub enum MemoryAccessReason {
    Load,
    Store,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct BlkCopyError {
    access_reason: MemoryAccessReason,
    addr: u64,
}

impl From<BlkCopyError> for VmRunError {
    fn from(value: BlkCopyError) -> Self {
        match value.access_reason {
            MemoryAccessReason::Load => Self::LoadAccessEx(value.addr),
            MemoryAccessReason::Store => Self::StoreAccessEx(value.addr),
        }
    }
}

impl From<LoadError> for VmRunError {
    fn from(value: LoadError) -> Self {
        Self::LoadAccessEx(value.0)
    }
}

impl From<StoreError> for VmRunError {
    fn from(value: StoreError) -> Self {
        Self::StoreAccessEx(value.0)
    }
}

#[derive(Clone, Copy, Display, Debug, PartialEq, Eq)]
pub enum MapError {
    AlreadyMapped,
}