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//! Data structures representing a memory region

use bitflags::bitflags;
use core::mem::size_of;
use core::ptr::NonNull;

bitflags! {
    /// Flags for a given memory region
    pub struct RegionFlags: u16 {
        /// Signifies that the memory region is in use
        const IN_USE = 1;
    }
}

#[repr(C)]
#[derive(Debug)]
/// Header of a memory region.
pub struct Region {
    /// Available size of the memory region (excluding the size of the header).
    pub size: usize,
    /// Starting address of this memory region's data
    pub data: usize,
    /// Number of bytes of padding to place at the start of the region
    pub padding_start: usize,
    /// Number of bytes of padding to place at the end of the region
    pub padding_end: usize,
    /// Flags for this memory region
    pub flags: RegionFlags,
    /// Address of the header of the memory regoin preceding this one
    pub prev: usize,
    /// Address of the header of the memory region following this one
    pub next: usize,
}

impl Region {
    /// Creates a new Region with the given `addr` and `size`.
    pub fn new(addr: usize, size: usize) -> Region {
        assert!(size >= Self::min_size());

        Region {
            size: size - size_of::<Self>(),
            data: addr + size_of::<Self>(),
            padding_start: 0,
            padding_end: 0,
            flags: RegionFlags::empty(),
            prev: 0,
            next: 0,
        }
    }

    /// Creates a new Region with the given `addr` and `size`, and also sets
    /// the start and end padding of the region.
    pub fn new_with_padding(
        addr: usize,
        size: usize,
        padding_start: usize,
        padding_end: usize,
    ) -> Region {
        let mut region = Region::new(addr, size);
        region.padding_start = padding_start;
        region.padding_end = padding_end;

        region
    }

    /// The minimum size of a memory region.
    pub fn min_size() -> usize {
        size_of::<Self>() + (size_of::<usize>() * 2)
    }

    /// Returns whether or not this memory region is in use
    pub fn get_used(&self) -> bool {
        self.flags.contains(RegionFlags::IN_USE)
    }

    /// Sets the in use flag of this memory region to the value given in
    /// `used`.
    pub fn set_used(&mut self, used: bool) {
        self.flags.set(RegionFlags::IN_USE, used);
    }

    /// Returns the total size, including padding, of this memory region
    pub fn size(&self) -> usize {
        self.padding_start
            .wrapping_add(self.size)
            .wrapping_add(self.padding_end)
    }

    /// Returns a raw pointer to this memory region's header
    pub unsafe fn header_ptr(&self) -> Option<NonNull<u8>> {
        Some(NonNull::new_unchecked(
            (self.data - size_of::<Self>()) as *mut u8,
        ))
    }

    /// Returns a raw pointer to the data of this memory region
    pub unsafe fn data_ptr(&self) -> Option<NonNull<u8>> {
        if self.data == 0 {
            None
        } else {
            Some(NonNull::new_unchecked(
                (self.data + self.padding_start) as *mut u8,
            ))
        }
    }
}