diff --git a/src/available_documents_ids.rs b/src/available_documents_ids.rs
new file mode 100644
index 000000000..34ff743f0
--- /dev/null
+++ b/src/available_documents_ids.rs
@@ -0,0 +1,67 @@
+use std::iter::{Chain, FromIterator};
+use std::ops::RangeInclusive;
+use roaring::bitmap::{RoaringBitmap, IntoIter};
+
+pub struct AvailableDocumentsIds {
+    iter: Chain<IntoIter, RangeInclusive<u32>>,
+}
+
+impl AvailableDocumentsIds {
+    pub fn from_documents_ids(docids: &RoaringBitmap) -> AvailableDocumentsIds {
+        match docids.max() {
+            Some(last_id) => {
+                let mut available = RoaringBitmap::from_iter(0..last_id);
+                available.difference_with(&docids);
+
+                let iter = match last_id.checked_add(1) {
+                    Some(id) => id..=u32::max_value(),
+                    None => 1..=0, // empty range iterator
+                };
+
+                AvailableDocumentsIds {
+                    iter: available.into_iter().chain(iter),
+                }
+            },
+            None => {
+                let empty = RoaringBitmap::new().into_iter();
+                AvailableDocumentsIds {
+                    iter: empty.chain(0..=u32::max_value()),
+                }
+            },
+        }
+    }
+}
+
+impl Iterator for AvailableDocumentsIds {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter.next()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty() {
+        let base = RoaringBitmap::new();
+        let left = AvailableDocumentsIds::from_documents_ids(&base);
+        let right = 0..=u32::max_value();
+        left.zip(right).take(500).for_each(|(l, r)| assert_eq!(l, r));
+    }
+
+    #[test]
+    fn scattered() {
+        let mut base = RoaringBitmap::new();
+        base.insert(0);
+        base.insert(10);
+        base.insert(100);
+        base.insert(405);
+
+        let left = AvailableDocumentsIds::from_documents_ids(&base);
+        let right = (0..=u32::max_value()).filter(|&n| n != 0 && n != 10 && n != 100 && n != 405);
+        left.zip(right).take(500).for_each(|(l, r)| assert_eq!(l, r));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 2d03b4ddf..366bf5428 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,4 @@
+mod available_documents_ids;
 mod criterion;
 mod fields_ids_map;
 mod index;
@@ -34,4 +35,3 @@ pub type BEU64 = heed::zerocopy::U64<heed::byteorder::BE>;
 pub type DocumentId = u32;
 pub type Attribute = u32;
 pub type Position = u32;
-