diff --git a/fastfield_codecs/src/lib.rs b/fastfield_codecs/src/lib.rs
index 7a12af8bc..ae9cb34b7 100644
--- a/fastfield_codecs/src/lib.rs
+++ b/fastfield_codecs/src/lib.rs
@@ -37,12 +37,15 @@ mod line;
 mod linear;
 mod monotonic_mapping;
 mod monotonic_mapping_u128;
+mod null_index;
 mod null_index_footer;
 
 mod column;
 mod gcd;
 mod serialize;
 
+pub use null_index::*;
+
 use self::bitpacked::BitpackedCodec;
 use self::blockwise_linear::BlockwiseLinearCodec;
 pub use self::column::{monotonic_map_column, Column, IterColumn, VecColumn};
diff --git a/fastfield_codecs/src/null_index/dense.rs b/fastfield_codecs/src/null_index/dense.rs
new file mode 100644
index 000000000..294027d2d
--- /dev/null
+++ b/fastfield_codecs/src/null_index/dense.rs
@@ -0,0 +1,366 @@
+use std::io::{self, Write};
+
+use common::BinarySerializable;
+use itertools::Itertools;
+use ownedbytes::OwnedBytes;
+
+use super::{get_bit_at, set_bit_at};
+
+/// For the `DenseCodec`, `data` which contains the encoded blocks.
+/// Each block consists of [u8; 12]. The first 8 bytes is a bitvec for 64 elements.
+/// The last 4 bytes are the offset, the number of set bits so far.
+///
+/// When translating the original index to a dense index, the correct block can be computed
+/// directly `orig_idx/64`. Inside the block the position is `orig_idx%64`.
+///
+/// When translating a dense index to the original index, we can use the offset to find the correct
+/// block. Direct computation is not possible, but we can employ a linear or binary search.
+pub struct DenseCodec {
+    // data consists of blocks of 64 bits.
+    //
+    // The format is &[(u64, u32)]
+    // u64 is the bitvec
+    // u32 is the offset of the block, the number of set bits so far.
+    //
+    // At the end one block is appended, to store the number of values in the index in offset.
+    data: OwnedBytes,
+}
+const ELEMENTS_PER_BLOCK: u32 = 64;
+const BLOCK_BITVEC_SIZE: usize = 8;
+const BLOCK_OFFSET_SIZE: usize = 4;
+const SERIALIZED_BLOCK_SIZE: usize = BLOCK_BITVEC_SIZE + BLOCK_OFFSET_SIZE;
+
+fn count_ones(block: u64, pos_in_block: u32) -> u32 {
+    if pos_in_block == 63 {
+        block.count_ones()
+    } else {
+        let mask = (1u64 << (pos_in_block + 1)) - 1;
+        let masked_block = block & mask;
+        masked_block.count_ones()
+    }
+}
+
+impl DenseCodec {
+    /// Open the DenseCodec from OwnedBytes
+    pub fn open(data: OwnedBytes) -> Self {
+        Self { data }
+    }
+    #[inline]
+    /// Check if value at position is not null.
+    pub fn exists(&self, idx: u32) -> bool {
+        let block_pos = idx / ELEMENTS_PER_BLOCK;
+        let bitvec: u64 = self.block(block_pos as usize);
+
+        let pos_in_block = idx % ELEMENTS_PER_BLOCK;
+
+        get_bit_at(bitvec, pos_in_block)
+    }
+    #[inline]
+    pub(crate) fn block(&self, block_pos: usize) -> u64 {
+        let data = &mut &self.data[block_pos as usize * SERIALIZED_BLOCK_SIZE..];
+
+        let block: u64 =
+            BinarySerializable::deserialize(data).expect("could not read block in null index");
+        block
+    }
+
+    #[inline]
+    /// Returns (bitvec, offset)
+    ///
+    /// offset is the start offset of actual docids in the block.
+    pub(crate) fn block_and_offset(&self, block_pos: u32) -> (u64, u32) {
+        let data = &mut &self.data[block_pos as usize * SERIALIZED_BLOCK_SIZE..];
+
+        let block: u64 =
+            BinarySerializable::deserialize(data).expect("could not read block in null index");
+        let offset: u32 =
+            BinarySerializable::deserialize(data).expect("could not read block in null index");
+        (block, offset)
+    }
+
+    /// Return the number of non-null values in an index
+    pub fn num_non_null_vals(&self) -> u32 {
+        let last_block = (self.data.len() / SERIALIZED_BLOCK_SIZE) - 1;
+        self.block_and_offset(last_block as u32).1
+    }
+
+    #[inline]
+    /// Translate from the original index to the codec index.
+    pub fn translate_to_codec_idx(&self, idx: u32) -> Option<u32> {
+        let block_pos = idx / ELEMENTS_PER_BLOCK;
+        let (block, offset) = self.block_and_offset(block_pos);
+        let pos_in_block = idx % ELEMENTS_PER_BLOCK;
+        if get_bit_at(block, pos_in_block) {
+            let ones_in_block = count_ones(block, pos_in_block);
+            Some(offset + ones_in_block - 1) // -1 is ok, since idx does exist, so there's at least
+                                             // one
+        } else {
+            None
+        }
+    }
+
+    /// Translate positions from the codec index to the original index.
+    ///
+    /// # Panics
+    ///
+    /// May panic if any `idx` is greater than the column length.
+    pub fn translate_codec_idx_to_original_idx<'a>(
+        &'a self,
+        iter: impl Iterator<Item = u32> + 'a,
+    ) -> impl Iterator<Item = u32> + 'a {
+        let mut block_pos = 0u32;
+        iter.map(move |dense_idx| {
+            // update block_pos to limit search scope
+            block_pos = find_block(dense_idx, block_pos, &self.data);
+            let (bitvec, offset) = self.block_and_offset(block_pos);
+
+            // The next offset is higher than dense_idx and therefore:
+            // dense_idx <= offset + num_set_bits in block
+            let mut num_set_bits = 0;
+            for idx_in_block in 0..ELEMENTS_PER_BLOCK {
+                if get_bit_at(bitvec, idx_in_block) {
+                    num_set_bits += 1;
+                }
+                if num_set_bits == (dense_idx - offset + 1) {
+                    let orig_idx = block_pos * ELEMENTS_PER_BLOCK + idx_in_block as u32;
+                    return orig_idx;
+                }
+            }
+            panic!("Internal Error: Offset calculation in dense idx seems to be wrong.");
+        })
+    }
+}
+
+#[inline]
+/// Finds the block position containing the dense_idx.
+///
+/// # Correctness
+/// dense_idx needs to be smaller than the number of values in the index
+///
+/// The last offset number is equal to the number of values in the index.
+fn find_block(dense_idx: u32, mut block_pos: u32, data: &[u8]) -> u32 {
+    loop {
+        let data = &mut &data[BLOCK_BITVEC_SIZE + block_pos as usize * SERIALIZED_BLOCK_SIZE..];
+        let offset: u32 = BinarySerializable::deserialize(data)
+            .expect("could not read offset from block in null index");
+        if offset > dense_idx {
+            // offset
+            return block_pos - 1;
+        }
+        block_pos += 1;
+    }
+}
+
+/// Iterator over all values, true if set, otherwise false
+pub fn serialize_dense_codec(
+    iter: impl Iterator<Item = bool>,
+    mut out: impl Write,
+) -> io::Result<()> {
+    let mut offset: u32 = 0;
+
+    for chunk in &iter.chunks(ELEMENTS_PER_BLOCK as usize) {
+        let mut block: u64 = 0;
+        for (pos, is_bit_set) in chunk.enumerate() {
+            if is_bit_set {
+                set_bit_at(&mut block, pos as u64);
+            }
+        }
+
+        block.serialize(&mut out)?;
+        offset.serialize(&mut out)?;
+
+        offset += block.count_ones() as u32;
+    }
+    // Add sentinal block for the offset
+    let block: u64 = 0;
+    block.serialize(&mut out)?;
+    offset.serialize(&mut out)?;
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use proptest::prelude::{any, prop, *};
+    use proptest::strategy::Strategy;
+    use proptest::{prop_oneof, proptest};
+
+    use super::*;
+
+    fn random_bitvec() -> BoxedStrategy<Vec<bool>> {
+        prop_oneof![
+            1 => prop::collection::vec(proptest::bool::weighted(1.0), 0..100),
+            1 => prop::collection::vec(proptest::bool::weighted(1.0), 0..64),
+            1 => prop::collection::vec(proptest::bool::weighted(0.0), 0..100),
+            1 => prop::collection::vec(proptest::bool::weighted(0.0), 0..64),
+            8 => vec![any::<bool>()],
+            2 => prop::collection::vec(any::<bool>(), 0..50),
+        ]
+        .boxed()
+    }
+
+    proptest! {
+        #![proptest_config(ProptestConfig::with_cases(500))]
+        #[test]
+        fn test_with_random_bitvecs(bitvec1 in random_bitvec(), bitvec2 in random_bitvec(), bitvec3 in random_bitvec()) {
+            let mut bitvec = Vec::new();
+            bitvec.extend_from_slice(&bitvec1);
+            bitvec.extend_from_slice(&bitvec2);
+            bitvec.extend_from_slice(&bitvec3);
+            test_null_index(bitvec);
+        }
+    }
+
+    #[test]
+    fn dense_codec_test_one_block_false() {
+        let mut iter = vec![false; 64];
+        iter.push(true);
+        test_null_index(iter);
+    }
+
+    fn test_null_index(data: Vec<bool>) {
+        let mut out = vec![];
+
+        serialize_dense_codec(data.iter().cloned(), &mut out).unwrap();
+        let null_index = DenseCodec::open(OwnedBytes::new(out));
+
+        let orig_idx_with_value: Vec<u32> = data
+            .iter()
+            .enumerate()
+            .filter(|(_pos, val)| **val)
+            .map(|(pos, _val)| pos as u32)
+            .collect();
+
+        assert_eq!(
+            null_index
+                .translate_codec_idx_to_original_idx(0..orig_idx_with_value.len() as u32)
+                .collect_vec(),
+            orig_idx_with_value
+        );
+
+        for (dense_idx, orig_idx) in orig_idx_with_value.iter().enumerate() {
+            assert_eq!(
+                null_index.translate_to_codec_idx(*orig_idx),
+                Some(dense_idx as u32)
+            );
+        }
+
+        for (pos, value) in data.iter().enumerate() {
+            assert_eq!(null_index.exists(pos as u32), *value);
+        }
+    }
+
+    #[test]
+    fn dense_codec_test_translation() {
+        let mut out = vec![];
+
+        let iter = ([true, false, true, false]).iter().cloned();
+        serialize_dense_codec(iter, &mut out).unwrap();
+        let null_index = DenseCodec::open(OwnedBytes::new(out));
+
+        assert_eq!(
+            null_index
+                .translate_codec_idx_to_original_idx(0..2)
+                .collect_vec(),
+            vec![0, 2]
+        );
+    }
+
+    #[test]
+    fn dense_codec_translate() {
+        let mut out = vec![];
+
+        let iter = ([true, false, true, false]).iter().cloned();
+        serialize_dense_codec(iter, &mut out).unwrap();
+        let null_index = DenseCodec::open(OwnedBytes::new(out));
+        assert_eq!(null_index.translate_to_codec_idx(0), Some(0));
+        assert_eq!(null_index.translate_to_codec_idx(2), Some(1));
+    }
+
+    #[test]
+    fn dense_codec_test_small() {
+        let mut out = vec![];
+
+        let iter = ([true, false, true, false]).iter().cloned();
+        serialize_dense_codec(iter, &mut out).unwrap();
+        let null_index = DenseCodec::open(OwnedBytes::new(out));
+        assert!(null_index.exists(0));
+        assert!(!null_index.exists(1));
+        assert!(null_index.exists(2));
+        assert!(!null_index.exists(3));
+    }
+
+    #[test]
+    fn dense_codec_test_large() {
+        let mut docs = vec![];
+        docs.extend((0..1000).map(|_idx| false));
+        docs.extend((0..=1000).map(|_idx| true));
+
+        let iter = docs.iter().cloned();
+        let mut out = vec![];
+        serialize_dense_codec(iter, &mut out).unwrap();
+        let null_index = DenseCodec::open(OwnedBytes::new(out));
+        assert!(!null_index.exists(0));
+        assert!(!null_index.exists(100));
+        assert!(!null_index.exists(999));
+        assert!(null_index.exists(1000));
+        assert!(null_index.exists(1999));
+        assert!(null_index.exists(2000));
+        assert!(!null_index.exists(2001));
+    }
+
+    #[test]
+    fn test_count_ones() {
+        let mut block = 0;
+        set_bit_at(&mut block, 0);
+        set_bit_at(&mut block, 2);
+
+        assert_eq!(count_ones(block, 0), 1);
+        assert_eq!(count_ones(block, 1), 1);
+        assert_eq!(count_ones(block, 2), 2);
+    }
+}
+
+#[cfg(all(test, feature = "unstable"))]
+mod bench {
+
+    use rand::rngs::StdRng;
+    use rand::{Rng, SeedableRng};
+    use test::Bencher;
+
+    use super::*;
+
+    fn gen_bools() -> DenseCodec {
+        let mut out = Vec::new();
+        let mut rng: StdRng = StdRng::from_seed([1u8; 32]);
+        // 80% of values are set
+        let bools: Vec<_> = (0..100_000).map(|_| rng.gen_bool(8f64 / 10f64)).collect();
+        serialize_dense_codec(bools.into_iter(), &mut out).unwrap();
+
+        let codec = DenseCodec::open(OwnedBytes::new(out));
+        codec
+    }
+
+    #[bench]
+    fn bench_dense_codec_translate_orig_to_dense(bench: &mut Bencher) {
+        let codec = gen_bools();
+        bench.iter(|| {
+            let mut dense_idx: Option<u32> = None;
+            for idx in 0..100_000 {
+                dense_idx = codec.translate_to_codec_idx(idx);
+            }
+            dense_idx
+        });
+    }
+
+    #[bench]
+    fn bench_dense_codec_translate_dense_to_orig(bench: &mut Bencher) {
+        let codec = gen_bools();
+        let num_vals = codec.num_non_null_vals();
+        bench.iter(|| {
+            codec
+                .translate_codec_idx_to_original_idx(0..num_vals)
+                .last()
+        });
+    }
+}
diff --git a/fastfield_codecs/src/null_index/mod.rs b/fastfield_codecs/src/null_index/mod.rs
new file mode 100644
index 000000000..9a3e22bf0
--- /dev/null
+++ b/fastfield_codecs/src/null_index/mod.rs
@@ -0,0 +1,11 @@
+pub use dense::{serialize_dense_codec, DenseCodec};
+
+mod dense;
+
+fn get_bit_at(input: u64, n: u32) -> bool {
+    input & (1 << n) != 0
+}
+
+fn set_bit_at(input: &mut u64, n: u64) {
+    *input |= 1 << n;
+}