diff --git a/common/Cargo.toml b/common/Cargo.toml
index d262da05e..b9f09b950 100644
--- a/common/Cargo.toml
+++ b/common/Cargo.toml
@@ -10,3 +10,6 @@ description = "common traits and utility functions used by multiple tantivy subc
 
 [dependencies]
 byteorder = "1.4.3"
+
+[dev-dependencies]
+rand = "0.8.4"
diff --git a/src/common/bitset.rs b/common/src/bitset.rs
similarity index 89%
rename from src/common/bitset.rs
rename to common/src/bitset.rs
index 0eb4d8da6..942a94269 100644
--- a/src/common/bitset.rs
+++ b/common/src/bitset.rs
@@ -2,7 +2,7 @@ use std::fmt;
 use std::u64;
 
 #[derive(Clone, Copy, Eq, PartialEq)]
-pub(crate) struct TinySet(u64);
+pub struct TinySet(u64);
 
 impl fmt::Debug for TinySet {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -178,7 +178,7 @@ impl BitSet {
     ///
     /// Reminder: the tiny set with the bucket `bucket`, represents the
     /// elements from `bucket * 64` to `(bucket+1) * 64`.
-    pub(crate) fn first_non_empty_bucket(&self, bucket: u32) -> Option<u32> {
+    pub fn first_non_empty_bucket(&self, bucket: u32) -> Option<u32> {
         self.tinysets[bucket as usize..]
             .iter()
             .cloned()
@@ -193,7 +193,7 @@ impl BitSet {
     /// Returns the tiny bitset representing the
     /// the set restricted to the number range from
     /// `bucket * 64` to `(bucket + 1) * 64`.
-    pub(crate) fn tinyset(&self, bucket: u32) -> TinySet {
+    pub fn tinyset(&self, bucket: u32) -> TinySet {
         self.tinysets[bucket as usize]
     }
 }
@@ -203,11 +203,9 @@ mod tests {
 
     use super::BitSet;
     use super::TinySet;
-    use crate::docset::{DocSet, TERMINATED};
-    use crate::query::BitSetDocSet;
-    use crate::tests;
-    use crate::tests::generate_nonunique_unsorted;
-    use std::collections::BTreeSet;
+    use rand::distributions::Bernoulli;
+    use rand::rngs::StdRng;
+    use rand::{Rng, SeedableRng};
     use std::collections::HashSet;
 
     #[test]
@@ -263,29 +261,6 @@ mod tests {
         test_against_hashset(&[62u32, 63u32], 64);
     }
 
-    #[test]
-    fn test_bitset_large() {
-        let arr = generate_nonunique_unsorted(100_000, 5_000);
-        let mut btreeset: BTreeSet<u32> = BTreeSet::new();
-        let mut bitset = BitSet::with_max_value(100_000);
-        for el in arr {
-            btreeset.insert(el);
-            bitset.insert(el);
-        }
-        for i in 0..100_000 {
-            assert_eq!(btreeset.contains(&i), bitset.contains(i));
-        }
-        assert_eq!(btreeset.len(), bitset.len());
-        let mut bitset_docset = BitSetDocSet::from(bitset);
-        let mut remaining = true;
-        for el in btreeset.into_iter() {
-            assert!(remaining);
-            assert_eq!(bitset_docset.doc(), el);
-            remaining = bitset_docset.advance() != TERMINATED;
-        }
-        assert!(!remaining);
-    }
-
     #[test]
     fn test_bitset_num_buckets() {
         use super::num_buckets;
@@ -340,10 +315,23 @@ mod tests {
         assert_eq!(bitset.len(), 3);
     }
 
+    pub fn sample_with_seed(n: u32, ratio: f64, seed_val: u8) -> Vec<u32> {
+        StdRng::from_seed([seed_val; 32])
+            .sample_iter(&Bernoulli::new(ratio).unwrap())
+            .take(n as usize)
+            .enumerate()
+            .filter_map(|(val, keep)| if keep { Some(val as u32) } else { None })
+            .collect()
+    }
+
+    pub fn sample(n: u32, ratio: f64) -> Vec<u32> {
+        sample_with_seed(n, ratio, 4)
+    }
+
     #[test]
     fn test_bitset_clear() {
         let mut bitset = BitSet::with_max_value(1_000);
-        let els = tests::sample(1_000, 0.01f64);
+        let els = sample(1_000, 0.01f64);
         for &el in &els {
             bitset.insert(el);
         }
diff --git a/common/src/lib.rs b/common/src/lib.rs
index b3c24163b..46b2e0222 100644
--- a/common/src/lib.rs
+++ b/common/src/lib.rs
@@ -1,9 +1,11 @@
 pub use byteorder::LittleEndian as Endianness;
 
+mod bitset;
 mod serialize;
 mod vint;
 mod writer;
 
+pub use bitset::*;
 pub use serialize::{BinarySerializable, DeserializeFrom, FixedSize};
 pub use vint::{read_u32_vint, read_u32_vint_no_advance, serialize_vint_u32, write_u32_vint, VInt};
 pub use writer::{AntiCallToken, CountingWriter, TerminatingWrite};
diff --git a/src/common/mod.rs b/src/common/mod.rs
index b82b352f8..cb1f6949b 100644
--- a/src/common/mod.rs
+++ b/src/common/mod.rs
@@ -1,9 +1,3 @@
-mod bitset;
-mod composite_file;
-
-pub use self::bitset::BitSet;
-pub(crate) use self::bitset::TinySet;
-pub(crate) use self::composite_file::{CompositeFile, CompositeWrite};
 pub use byteorder::LittleEndian as Endianness;
 pub use common::CountingWriter;
 pub use common::{
diff --git a/src/core/segment_reader.rs b/src/core/segment_reader.rs
index 27110ce31..73de5fb4c 100644
--- a/src/core/segment_reader.rs
+++ b/src/core/segment_reader.rs
@@ -2,7 +2,9 @@ use crate::core::InvertedIndexReader;
 use crate::core::Segment;
 use crate::core::SegmentComponent;
 use crate::core::SegmentId;
+use crate::directory::CompositeFile;
 use crate::directory::FileSlice;
+use crate::error::DataCorruption;
 use crate::fastfield::DeleteBitSet;
 use crate::fastfield::FacetReader;
 use crate::fastfield::FastFieldReaders;
@@ -14,7 +16,6 @@ use crate::space_usage::SegmentSpaceUsage;
 use crate::store::StoreReader;
 use crate::termdict::TermDictionary;
 use crate::DocId;
-use crate::{common::CompositeFile, error::DataCorruption};
 use fail::fail_point;
 use std::fmt;
 use std::sync::Arc;
diff --git a/src/common/composite_file.rs b/src/directory/composite_file.rs
similarity index 99%
rename from src/common/composite_file.rs
rename to src/directory/composite_file.rs
index babee87d1..5619f803c 100644
--- a/src/common/composite_file.rs
+++ b/src/directory/composite_file.rs
@@ -1,5 +1,6 @@
 use crate::common::BinarySerializable;
 use crate::common::CountingWriter;
+use crate::common::HasLen;
 use crate::common::VInt;
 use crate::directory::FileSlice;
 use crate::directory::{TerminatingWrite, WritePtr};
@@ -11,8 +12,6 @@ use std::io::{self, Read, Write};
 use std::iter::ExactSizeIterator;
 use std::ops::Range;
 
-use super::HasLen;
-
 #[derive(Eq, PartialEq, Hash, Copy, Ord, PartialOrd, Clone, Debug)]
 pub struct FileAddr {
     field: Field,
diff --git a/src/directory/mod.rs b/src/directory/mod.rs
index fcfe90342..fcf6fdd35 100644
--- a/src/directory/mod.rs
+++ b/src/directory/mod.rs
@@ -20,6 +20,9 @@ mod watch_event_router;
 /// Errors specific to the directory module.
 pub mod error;
 
+mod composite_file;
+
+pub(crate) use self::composite_file::{CompositeFile, CompositeWrite};
 pub use self::directory::DirectoryLock;
 pub use self::directory::{Directory, DirectoryClone};
 pub use self::directory_lock::{Lock, INDEX_WRITER_LOCK, META_LOCK};
diff --git a/src/fastfield/delete.rs b/src/fastfield/delete.rs
index a2f14aa7c..fbf3c33dd 100644
--- a/src/fastfield/delete.rs
+++ b/src/fastfield/delete.rs
@@ -1,4 +1,5 @@
-use crate::common::{BitSet, HasLen};
+use crate::common::HasLen;
+use common::BitSet;
 use crate::directory::FileSlice;
 use crate::directory::OwnedBytes;
 use crate::directory::WritePtr;
diff --git a/src/fastfield/mod.rs b/src/fastfield/mod.rs
index bc46e5bec..24cf4872b 100644
--- a/src/fastfield/mod.rs
+++ b/src/fastfield/mod.rs
@@ -213,7 +213,7 @@ fn value_to_u64(value: &Value) -> u64 {
 mod tests {
 
     use super::*;
-    use crate::common::CompositeFile;
+    use crate::directory::CompositeFile;
     use crate::common::HasLen;
     use crate::directory::{Directory, RamDirectory, WritePtr};
     use crate::merge_policy::NoMergePolicy;
@@ -588,7 +588,7 @@ mod bench {
     use super::tests::FIELD;
     use super::tests::{generate_permutation, SCHEMA};
     use super::*;
-    use crate::common::CompositeFile;
+    use crate::directory::CompositeFile;
     use crate::directory::{Directory, RamDirectory, WritePtr};
     use crate::fastfield::FastFieldReader;
     use std::collections::HashMap;
diff --git a/src/fastfield/reader.rs b/src/fastfield/reader.rs
index 47adcddfb..b5902796e 100644
--- a/src/fastfield/reader.rs
+++ b/src/fastfield/reader.rs
@@ -1,6 +1,6 @@
 use super::FastValue;
 use crate::common::BinarySerializable;
-use crate::common::CompositeFile;
+use crate::directory::CompositeFile;
 use crate::directory::FileSlice;
 use crate::directory::OwnedBytes;
 use crate::directory::{Directory, RamDirectory, WritePtr};
diff --git a/src/fastfield/readers.rs b/src/fastfield/readers.rs
index f202bbb35..b85754641 100644
--- a/src/fastfield/readers.rs
+++ b/src/fastfield/readers.rs
@@ -1,4 +1,4 @@
-use crate::common::CompositeFile;
+use crate::directory::CompositeFile;
 use crate::directory::FileSlice;
 use crate::fastfield::MultiValuedFastFieldReader;
 use crate::fastfield::{BitpackedFastFieldReader, FastFieldNotAvailableError};
diff --git a/src/fastfield/serializer/mod.rs b/src/fastfield/serializer/mod.rs
index 2279ce1b5..26a9edbbe 100644
--- a/src/fastfield/serializer/mod.rs
+++ b/src/fastfield/serializer/mod.rs
@@ -1,5 +1,5 @@
 use crate::common::BinarySerializable;
-use crate::common::CompositeWrite;
+use crate::directory::CompositeWrite;
 use crate::common::CountingWriter;
 use crate::directory::WritePtr;
 use crate::schema::Field;
diff --git a/src/fieldnorm/reader.rs b/src/fieldnorm/reader.rs
index 71535e3f9..ea264b2ff 100644
--- a/src/fieldnorm/reader.rs
+++ b/src/fieldnorm/reader.rs
@@ -1,5 +1,5 @@
 use super::{fieldnorm_to_id, id_to_fieldnorm};
-use crate::common::CompositeFile;
+use crate::directory::CompositeFile;
 use crate::directory::FileSlice;
 use crate::directory::OwnedBytes;
 use crate::schema::Field;
diff --git a/src/fieldnorm/serializer.rs b/src/fieldnorm/serializer.rs
index 057626fcc..54043b1e9 100644
--- a/src/fieldnorm/serializer.rs
+++ b/src/fieldnorm/serializer.rs
@@ -1,4 +1,4 @@
-use crate::common::CompositeWrite;
+use crate::directory::CompositeWrite;
 use crate::directory::WritePtr;
 use crate::schema::Field;
 use std::io;
diff --git a/src/indexer/index_writer.rs b/src/indexer/index_writer.rs
index 183741a20..e644a1e61 100644
--- a/src/indexer/index_writer.rs
+++ b/src/indexer/index_writer.rs
@@ -1,7 +1,7 @@
 use super::operation::{AddOperation, UserOperation};
 use super::segment_updater::SegmentUpdater;
 use super::PreparedCommit;
-use crate::common::BitSet;
+use common::BitSet;
 use crate::core::Index;
 use crate::core::Segment;
 use crate::core::SegmentComponent;
diff --git a/src/indexer/segment_entry.rs b/src/indexer/segment_entry.rs
index 1808fd1da..4ac352e50 100644
--- a/src/indexer/segment_entry.rs
+++ b/src/indexer/segment_entry.rs
@@ -1,7 +1,7 @@
-use crate::common::BitSet;
 use crate::core::SegmentId;
 use crate::core::SegmentMeta;
 use crate::indexer::delete_queue::DeleteCursor;
+use common::BitSet;
 use std::fmt;
 
 /// A segment entry describes the state of
diff --git a/src/postings/serializer.rs b/src/postings/serializer.rs
index 8631586c2..eca28a7c8 100644
--- a/src/postings/serializer.rs
+++ b/src/postings/serializer.rs
@@ -1,7 +1,8 @@
 use super::TermInfo;
+use crate::common::CountingWriter;
 use crate::common::{BinarySerializable, VInt};
-use crate::common::{CompositeWrite, CountingWriter};
 use crate::core::Segment;
+use crate::directory::CompositeWrite;
 use crate::directory::WritePtr;
 use crate::fieldnorm::FieldNormReader;
 use crate::positions::PositionSerializer;
diff --git a/src/query/automaton_weight.rs b/src/query/automaton_weight.rs
index 2ffa4309a..5f9f646ce 100644
--- a/src/query/automaton_weight.rs
+++ b/src/query/automaton_weight.rs
@@ -1,4 +1,4 @@
-use crate::common::BitSet;
+use common::BitSet;
 use crate::core::SegmentReader;
 use crate::query::ConstScorer;
 use crate::query::{BitSetDocSet, Explanation};
diff --git a/src/query/bitset/mod.rs b/src/query/bitset/mod.rs
index b74ccd2e8..030fdeae7 100644
--- a/src/query/bitset/mod.rs
+++ b/src/query/bitset/mod.rs
@@ -1,6 +1,6 @@
-use crate::common::{BitSet, TinySet};
 use crate::docset::{DocSet, TERMINATED};
 use crate::DocId;
+use common::{BitSet, TinySet};
 
 /// A `BitSetDocSet` makes it possible to iterate through a bitset as if it was a `DocSet`.
 ///
@@ -96,10 +96,13 @@ impl DocSet for BitSetDocSet {
 
 #[cfg(test)]
 mod tests {
+    use std::collections::BTreeSet;
+
     use super::BitSetDocSet;
-    use crate::common::BitSet;
     use crate::docset::{DocSet, TERMINATED};
+    use crate::tests::generate_nonunique_unsorted;
     use crate::DocId;
+    use common::BitSet;
 
     fn create_docbitset(docs: &[DocId], max_doc: DocId) -> BitSetDocSet {
         let mut docset = BitSet::with_max_value(max_doc);
@@ -109,6 +112,29 @@ mod tests {
         BitSetDocSet::from(docset)
     }
 
+    #[test]
+    fn test_bitset_large() {
+        let arr = generate_nonunique_unsorted(100_000, 5_000);
+        let mut btreeset: BTreeSet<u32> = BTreeSet::new();
+        let mut bitset = BitSet::with_max_value(100_000);
+        for el in arr {
+            btreeset.insert(el);
+            bitset.insert(el);
+        }
+        for i in 0..100_000 {
+            assert_eq!(btreeset.contains(&i), bitset.contains(i));
+        }
+        assert_eq!(btreeset.len(), bitset.len());
+        let mut bitset_docset = BitSetDocSet::from(bitset);
+        let mut remaining = true;
+        for el in btreeset.into_iter() {
+            assert!(remaining);
+            assert_eq!(bitset_docset.doc(), el);
+            remaining = bitset_docset.advance() != TERMINATED;
+        }
+        assert!(!remaining);
+    }
+
     #[test]
     fn test_empty() {
         let bitset = BitSet::with_max_value(1000);
diff --git a/src/query/range_query.rs b/src/query/range_query.rs
index fa230d015..a625a3354 100644
--- a/src/query/range_query.rs
+++ b/src/query/range_query.rs
@@ -1,4 +1,3 @@
-use crate::common::BitSet;
 use crate::core::Searcher;
 use crate::core::SegmentReader;
 use crate::error::TantivyError;
@@ -10,6 +9,7 @@ use crate::schema::Type;
 use crate::schema::{Field, IndexRecordOption, Term};
 use crate::termdict::{TermDictionary, TermStreamer};
 use crate::{DocId, Score};
+use common::BitSet;
 use std::io;
 use std::ops::{Bound, Range};
 
diff --git a/src/query/union.rs b/src/query/union.rs
index 8185f7c6c..cf7b4d956 100644
--- a/src/query/union.rs
+++ b/src/query/union.rs
@@ -1,9 +1,9 @@
-use crate::common::TinySet;
 use crate::docset::{DocSet, TERMINATED};
 use crate::query::score_combiner::{DoNothingCombiner, ScoreCombiner};
 use crate::query::Scorer;
 use crate::DocId;
 use crate::Score;
+use common::TinySet;
 
 const HORIZON_NUM_TINYBITSETS: usize = 64;
 const HORIZON: u32 = 64u32 * HORIZON_NUM_TINYBITSETS as u32;