1 files changed, 77 insertions, 0 deletions
diff --git a/crates/alloc_buddy/src/bitvec.rs b/crates/alloc_buddy/src/bitvec.rs
new file mode 100644
index 0000000..5cabc5e
--- /dev/null
+++ b/crates/alloc_buddy/src/bitvec.rs
@@ -0,0 +1,77 @@
+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,
+}