Use auto dtype for positions

Author: benjeffery

bio2zarr/tskit.py

@@ -153,16 +153,17 @@ def generate_schema(
         )
 
         # Check if positions will fit in i4 (max ~2.1 billion)
+        min_position = 0
         max_position = 0
         if self.ts.num_sites > 0:
+            min_position = np.min(self.ts.sites_position)
             max_position = np.max(self.ts.sites_position)
-        position_dtype = "i4" if max_position <= np.iinfo(np.int32).max else "i8"
 
         array_specs = [
             vcz.ZarrArraySpec(
                 source="position",
                 name="variant_position",
-                dtype=position_dtype,
+                dtype=core.min_int_dtype(min_position, max_position),
                 dimensions=["variants"],
                 description="Position of each variant",
             ),