use contracts::requires;
use core::{fmt, mem::transmute};

/// A fixed-length vector of bits used for determining whether a subregion is in the free lists or
/// not.
pub struct Bitset([usize]);

impl Bitset {
    fn from_mut(words: &mut [usize]) -> &mut Bitset {
        // SAFETY: The types have a newtype relationship.
        unsafe { transmute(words) }
    }

    fn from_ref(words: &[usize]) -> &Bitset {
        // SAFETY: The types have a newtype relationship.
        unsafe { transmute(words) }
    }

    /// Retrieves the value of a bit from the Bitset.
    #[requires(i < self.len())]
    pub fn get(&self, i: usize) -> SubregionStatus {
        let word_index = i / usize::BITS as usize;
        let subword_index = i % usize::BITS as usize;
        let one_hot = 1 << subword_index;
        if (self.0[word_index] & one_hot) == 0 {
            SubregionStatus::NotInFreeList
        } else {
            SubregionStatus::InFreeList
        }
    }

    /// Returns whether the Bitset is empty.
    pub fn is_empty(&self) -> bool {
        self.0.is_empty()
    }

    /// Returns the number of bits in the Bitset.
    pub fn len(&self) -> usize {
        self.0.len() * usize::BITS as usize
    }

    /// Sets the value of a bit in the Bitset.
    #[requires(i < self.len())]
    pub fn set(&mut self, i: usize, status: SubregionStatus) {
        let word_index = i / usize::BITS as usize;
        let subword_index = i % usize::BITS as usize;
        let one_hot = 1 << subword_index;
        match status {
            SubregionStatus::NotInFreeList => {
                self.0[word_index] &= !one_hot;
            }
            SubregionStatus::InFreeList => {
                self.0[word_index] |= one_hot;
            }
        }
    }
}

impl fmt::Debug for Bitset {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        write!(fmt, "Bitset(")?;
        for word in &self.0 {
            write!(fmt, "{word:064b}")?;
        }
        write!(fmt, ")")
    }
}

/// The status of a subregion, as tracked by `Bitset`.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub enum SubregionStatus {
    /// The region is not in the free list.
    NotInFreeList = 0,

    /// The region is in the free list.
    InFreeList = 1,
}