use std::{marker::PhantomData, mem::MaybeUninit};

use tracing::warn;

use crate::MemoryAccessError;

use super::MemoryView;

/// Underlying buffer for a memory.
#[derive(Debug, Copy, Clone)]
pub(crate) struct MemoryBuffer<'a> {
    pub(crate) base: *mut u8,
    pub(crate) len: usize,
    pub(crate) marker: PhantomData<&'a MemoryView<'a>>,
}

impl<'a> MemoryBuffer<'a> {
    pub(crate) fn read(&self, offset: u64, buf: &mut [u8]) -> Result<(), MemoryAccessError> {
        let end = offset
            .checked_add(buf.len() as u64)
            .ok_or(MemoryAccessError::Overflow)?;
        if end > self.len.try_into().unwrap() {
            warn!(
                "attempted to read ({} bytes) beyond the bounds of the memory view ({} > {})",
                buf.len(),
                end,
                self.len
            );
            return Err(MemoryAccessError::HeapOutOfBounds);
        }
        unsafe {
            volatile_memcpy_read(self.base.add(offset as usize), buf.as_mut_ptr(), buf.len());
        }
        Ok(())
    }

    pub(crate) fn read_uninit<'b>(
        &self,
        offset: u64,
        buf: &'b mut [MaybeUninit<u8>],
    ) -> Result<&'b mut [u8], MemoryAccessError> {
        let end = offset
            .checked_add(buf.len() as u64)
            .ok_or(MemoryAccessError::Overflow)?;
        if end > self.len.try_into().unwrap() {
            warn!(
                "attempted to read ({} bytes) beyond the bounds of the memory view ({} > {})",
                buf.len(),
                end,
                self.len
            );
            return Err(MemoryAccessError::HeapOutOfBounds);
        }
        let buf_ptr = buf.as_mut_ptr() as *mut u8;
        unsafe {
            volatile_memcpy_read(self.base.add(offset as usize), buf_ptr, buf.len());
        }

        Ok(unsafe { std::slice::from_raw_parts_mut(buf_ptr, buf.len()) })
    }

    pub(crate) fn write(&self, offset: u64, data: &[u8]) -> Result<(), MemoryAccessError> {
        let end = offset
            .checked_add(data.len() as u64)
            .ok_or(MemoryAccessError::Overflow)?;
        if end > self.len.try_into().unwrap() {
            warn!(
                "attempted to write ({} bytes) beyond the bounds of the memory view ({} > {})",
                data.len(),
                end,
                self.len
            );
            return Err(MemoryAccessError::HeapOutOfBounds);
        }
        unsafe {
            volatile_memcpy_write(data.as_ptr(), self.base.add(offset as usize), data.len());
        }
        Ok(())
    }
}
// We can't use a normal memcpy here because it has undefined behavior if the
// memory is being concurrently modified. So we need to write our own memcpy
// implementation which uses volatile operations.
//
// The implementation of these functions can optimize very well when inlined
// with a fixed length: they should compile down to a single load/store
// instruction for small (8/16/32/64-bit) copies.
#[inline]
unsafe fn volatile_memcpy_read(mut src: *const u8, mut dst: *mut u8, mut len: usize) {
    #[inline]
    unsafe fn copy_one<T>(src: &mut *const u8, dst: &mut *mut u8, len: &mut usize) {
        #[repr(packed)]
        struct Unaligned<T>(T);
        let val = (*src as *const Unaligned<T>).read_volatile();
        (*dst as *mut Unaligned<T>).write(val);
        *src = src.add(std::mem::size_of::<T>());
        *dst = dst.add(std::mem::size_of::<T>());
        *len -= std::mem::size_of::<T>();
    }

    while len >= 8 {
        copy_one::<u64>(&mut src, &mut dst, &mut len);
    }
    if len >= 4 {
        copy_one::<u32>(&mut src, &mut dst, &mut len);
    }
    if len >= 2 {
        copy_one::<u16>(&mut src, &mut dst, &mut len);
    }
    if len >= 1 {
        copy_one::<u8>(&mut src, &mut dst, &mut len);
    }
}
#[inline]
unsafe fn volatile_memcpy_write(mut src: *const u8, mut dst: *mut u8, mut len: usize) {
    #[inline]
    unsafe fn copy_one<T>(src: &mut *const u8, dst: &mut *mut u8, len: &mut usize) {
        #[repr(packed)]
        struct Unaligned<T>(T);
        let val = (*src as *const Unaligned<T>).read();
        (*dst as *mut Unaligned<T>).write_volatile(val);
        *src = src.add(std::mem::size_of::<T>());
        *dst = dst.add(std::mem::size_of::<T>());
        *len -= std::mem::size_of::<T>();
    }

    while len >= 8 {
        copy_one::<u64>(&mut src, &mut dst, &mut len);
    }
    if len >= 4 {
        copy_one::<u32>(&mut src, &mut dst, &mut len);
    }
    if len >= 2 {
        copy_one::<u16>(&mut src, &mut dst, &mut len);
    }
    if len >= 1 {
        copy_one::<u8>(&mut src, &mut dst, &mut len);
    }
}
