diff --git a/src/dask_ngs/_index.py b/src/dask_ngs/_index.py
index b9e374e..b1e5f2f 100644
--- a/src/dask_ngs/_index.py
+++ b/src/dask_ngs/_index.py
@@ -8,7 +8,7 @@
 BAI_MIN_SHIFT = 14
 BAI_DEPTH = 5
 COMPRESSED_POSITION_SHIFT = 16
-UNCOMPRESSED_POSITION_MASK = 0xffff
+UNCOMPRESSED_POSITION_MASK = 0xFFFF
 BLOCKSIZE = 65536
 
 
@@ -16,7 +16,7 @@ def read_bai(path: str):
     """
     https://samtools.github.io/hts-specs/SAMv1.pdf
     """
-    int_kwargs = {'byteorder': 'little', 'signed': False}
+    int_kwargs = {"byteorder": "little", "signed": False}
     with Path(path).open("rb") as f:
         # read the 4-byte magic number
         f.read(4)
@@ -27,7 +27,7 @@ def read_bai(path: str):
         # read the reference sequence indices
         references = []
         for i in range(n_ref):
-            ref = {'ref_id': i}
+            ref = {"ref_id": i}
 
             # The "Bin Index"
             chunks = []
@@ -66,13 +66,15 @@ def read_bai(path: str):
                     chunk_end_cpos = vpos >> COMPRESSED_POSITION_SHIFT
                     chunk_end_upos = vpos & UNCOMPRESSED_POSITION_MASK
 
-                    chunks.append((
-                        bin_id,
-                        chunk_beg_cpos,
-                        chunk_beg_upos,
-                        chunk_end_cpos,
-                        chunk_end_upos
-                    ))
+                    chunks.append(
+                        (
+                            bin_id,
+                            chunk_beg_cpos,
+                            chunk_beg_upos,
+                            chunk_end_cpos,
+                            chunk_end_upos,
+                        )
+                    )
 
                 ref["bins"] = chunks
 
@@ -94,17 +96,21 @@ def read_bai(path: str):
                 n_no_coor = None
 
             for ref in references:
-                if 'bins' not in ref:
+                if "bins" not in ref:
                     continue
 
                 ref["bins"] = pd.DataFrame(
                     ref["bins"],
-                    columns=["bin_id", "chunk_beg.cpos", "chunk_beg.upos",
-                            "chunk_end.cpos", "chunk_end.upos"]
+                    columns=[
+                        "bin_id",
+                        "chunk_beg.cpos",
+                        "chunk_beg.upos",
+                        "chunk_end.cpos",
+                        "chunk_end.upos",
+                    ],
                 )
                 ref["ioffsets"] = pd.DataFrame(
-                    ref["ioffsets"],
-                    columns=["ioffset.cpos", "ioffset.upos"]
+                    ref["ioffsets"], columns=["ioffset.cpos", "ioffset.upos"]
                 )
 
     return references, n_no_coor
@@ -146,21 +152,20 @@ def _cumsum_assign_chunks(arr: np.array, thresh: int) -> tuple[np.array, np.arra
 
 def map_offsets_to_chunks(offsets: pd.DataFrame, chunksize_bytes: int) -> pd.DataFrame:
     """Given a dataframe of offset positions, calculate the difference
-       between each byte offset.
-       Group those differences into chunks of size `chunksize_bytes`.
-
-       Returns:
-           A Pandas dataframe with additional columns:
-           chunk_id : int
-               The chunk index that row was assigned
-           size : int
-               The cumulative size of that chunk
+    between each byte offset.
+    Group those differences into chunks of size `chunksize_bytes`.
+
+    Returns:
+        A Pandas dataframe with additional columns:
+        chunk_id : int
+            The chunk index that row was assigned
+        size : int
+            The cumulative size of that chunk
     """
 
     # calculate the difference in byte positions from the prior row
     # i.e. current row - previous
-    offsets["ioffset.cpos.diff"] = offsets['ioffset.cpos'].diff().fillna(
-        0).astype(int)
+    offsets["ioffset.cpos.diff"] = offsets["ioffset.cpos"].diff().fillna(0).astype(int)
 
     # group the offsets so
     # this produces a dataframe that looks like this:
@@ -170,13 +175,15 @@ def map_offsets_to_chunks(offsets: pd.DataFrame, chunksize_bytes: int) -> pd.Dat
     #       456717 |	    19251 |            299074
     # this represents how far to read each compressed array
     # e.g. 38660 + 118983 = 157643
-    offsets_uniq = offsets.groupby("ioffset.cpos").agg({
-        "ioffset.upos": "first",
-        "ioffset.cpos.diff": "first"
-    }).reset_index()
+    offsets_uniq = (
+        offsets.groupby("ioffset.cpos")
+        .agg({"ioffset.upos": "first", "ioffset.cpos.diff": "first"})
+        .reset_index()
+    )
 
     cumsums, chunk_ids = _cumsum_assign_chunks(
-        offsets_uniq["ioffset.cpos.diff"].to_numpy(), chunksize_bytes)
+        offsets_uniq["ioffset.cpos.diff"].to_numpy(), chunksize_bytes
+    )
     offsets_uniq["chunk_id"] = chunk_ids
     offsets_uniq["size"] = cumsums
 
@@ -185,17 +192,15 @@ def map_offsets_to_chunks(offsets: pd.DataFrame, chunksize_bytes: int) -> pd.Dat
 
 def consolidate_chunks(offsets_uniq: pd.DataFrame) -> pd.DataFrame:
     """Group the data by `chunk_id`,
-       keeping the first compressed byte value (`ioffset.cpos`)
-       and the first uncompressed byte value of that stream (`ioffset.upos`).
-       Take the last `size` value which tells you how many compressed bytes to read.
-
-       Returns:
-           A Pandas dataframe grouped by `chunk_id`
-           Now you can decompress the data starting from `ioffset.cpos` and read `size` bytes.
-           `ioffsets.upos` tells you which byte to read first from the uncompressed data.
+    keeping the first compressed byte value (`ioffset.cpos`)
+    and the first uncompressed byte value of that stream (`ioffset.upos`).
+    Take the last `size` value which tells you how many compressed bytes to read.
+
+    Returns:
+        A Pandas dataframe grouped by `chunk_id`
+        Now you can decompress the data starting from `ioffset.cpos` and read `size` bytes.
+        `ioffsets.upos` tells you which byte to read first from the uncompressed data.
     """
-    return offsets_uniq.groupby("chunk_id").agg({
-        "ioffset.cpos": "first",
-        "ioffset.upos": "first",
-        "size": "last"
-    })
+    return offsets_uniq.groupby("chunk_id").agg(
+        {"ioffset.cpos": "first", "ioffset.upos": "first", "size": "last"}
+    )