undo changes and fix root cause

src/bitmap/immutable.rs

@@ -239,7 +239,12 @@ impl Bitmap {
     pub fn make_mut(self) -> MutableBitmap {
         match self.into_mut() {
             Either::Left(data) => {
-                MutableBitmap::from_vec(data.bytes.as_ref().to_vec(), data.length)
+                if data.offset > 0 {
+                    // we have to recreate the bytes because a MutableBitmap does not have an `offset`.
+                    data.iter().collect()
+                } else {
+                    MutableBitmap::from_vec(data.bytes.as_ref().to_vec(), data.length)
+                }
             }
             Either::Right(data) => data,
         }

src/bitmap/mutable.rs

@@ -313,7 +313,7 @@ impl MutableBitmap {
 
     /// Returns an iterator over mutable slices, [`BitChunksExactMut`]
     pub(crate) fn bitchunks_exact_mut<T: BitChunk>(&mut self) -> BitChunksExactMut<T> {
-        BitChunksExactMut::new(&mut self.buffer)
+        BitChunksExactMut::new(&mut self.buffer, self.length)
     }
 }
 

src/bitmap/utils/chunks_exact_mut.rs

@@ -16,13 +16,12 @@ pub struct BitChunksExactMut<'a, T: BitChunk> {
 impl<'a, T: BitChunk> BitChunksExactMut<'a, T> {
     /// Returns a new [`BitChunksExactMut`]
     #[inline]
-    pub fn new(bitmap: &'a mut [u8]) -> Self {
+    pub fn new(bitmap: &'a mut [u8], length: usize) -> Self {
         let size_of = std::mem::size_of::<T>();
 
-        let length = bitmap.len();
-        let split = (length / size_of) * size_of;
+        let split = (length / 8 / size_of) * size_of;
         let (chunks, remainder) = bitmap.split_at_mut(split);
-        let remainder_len = (length - chunks.len()) * 8;
+        let remainder_len = length - chunks.len() * 8;
 
         let chunks = chunks.chunks_exact_mut(size_of);
         Self {