// HACK: This is temporary implementation so we can have memory instructions working

mod paging;

use self::paging::{PageTable, Permission, PtEntry};
use alloc::boxed::Box;
use core::mem::MaybeUninit;
use {crate::vm::value::Value, ma_size::MemAccessSize};

#[derive(Clone, Debug)]
pub struct Memory {
    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 {
    // HACK: Just for allocation testing, will be removed when proper memory interfaces
    // implemented.
    pub fn insert_test_page(&mut self) {
        unsafe {
            let mut entry = PtEntry::new(
                {
                    let layout = alloc::alloc::Layout::from_size_align_unchecked(4096, 4096);
                    let ptr = alloc::alloc::alloc(layout);
                    if ptr.is_null() {
                        alloc::alloc::handle_alloc_error(layout);
                    }

                    core::ptr::write_bytes(ptr, 69, 10);
                    ptr.cast()
                },
                Permission::Write,
            );

            for _ in 0..4 {
                let mut pt = Box::<PageTable>::default();
                pt[0] = entry;
                entry = PtEntry::new(Box::into_raw(pt) as _, Permission::Node);
            }

            self.root_pt_mut()[0] = entry;
        }
    }

    /// Load value from an address
    pub fn load<S: MemAccessSize>(&self, addr: u64) -> Option<Value> {
        let lookup = self.page_lookup(addr)?;
        match lookup.perm {
            Permission::Empty | Permission::Node => None,
            Permission::Readonly | Permission::Write | Permission::Exec => {
                let mut value = MaybeUninit::<Value>::zeroed();
                unsafe {
                    core::ptr::copy_nonoverlapping::<u8>(lookup.ptr, value.as_mut_ptr().cast(), 1);
                    Some(value.assume_init())
                }
            }
        }
    }

    /// Store value to an address
    pub fn store<S: MemAccessSize>(&mut self, addr: u64, value: Value) -> Result<(), ()> {
        let lookup = self.page_lookup(addr).ok_or(())?;
        match lookup.perm {
            Permission::Write => {
                unsafe {
                    core::ptr::copy_nonoverlapping::<u8>(
                        (&value as *const Value).cast(),
                        lookup.ptr,
                        1,
                    )
                };
                Ok(())
            }
            _ => Err(()),
        }
    }

    #[inline]
    pub fn root_pt(&self) -> &PageTable {
        unsafe { &*self.root_pt }
    }

    #[inline]
    pub fn root_pt_mut(&mut self) -> &mut PageTable {
        unsafe { &mut *self.root_pt }
    }

    /// Resolve page and offset from the page
    fn page_lookup(&self, addr: u64) -> Option<PageLookupResult> {
        let mut current_pt = self.root_pt;
        for lvl in (0..5).rev() {
            unsafe {
                let entry = (*current_pt).get_unchecked(
                    usize::try_from((addr >> (lvl * 9 + 12)) & ((1 << 9) - 1))
                        .expect("?conradluget a better CPU"),
                );

                let ptr = entry.ptr();
                match entry.permission() {
                    Permission::Empty => return None,
                    Permission::Node => current_pt = ptr as _,
                    _ if lvl > 2 => return None,
                    perm => {
                        return Some(PageLookupResult {
                            perm,
                            ptr: ptr as _,
                            size: match lvl {
                                0 => 4096,
                                1 => 1024_usize.pow(2) * 2,
                                2 => 4096_usize.pow(3),
                                _ => unreachable!(),
                            },
                            offset: addr as usize & ((1 << 12) - 1),
                        })
                    }
                }
            }
        }
        None
    }
}

struct PageLookupResult {
    perm: Permission,
    ptr: *mut u8,
    size: usize,
    offset: usize,
}

macro_rules! size_markers {
    ($($name:ident = $size:expr),* $(,)?) => {
        pub mod ma_size {
            /// # Safety
            /// Implementor has to assure that [`MemAccessSize::BYTES`] won't be larger than
            /// size of [`Value`]
            pub unsafe trait MemAccessSize {
                const BYTES: usize;
            }

            $(
                pub struct $name;
                unsafe impl MemAccessSize for $name {
                    const BYTES: usize = $size;
                }
            )*
        }
    };
}

size_markers! {
    Byte    = 1,
    Doublet = 2,
    Quadlet = 4,
    Octlet  = 8,
}