Introduce the word documents count method on the index

milli/src/index.rs

@@ -352,6 +352,17 @@ impl Index {
         }
     }
 
+    /* word documents count */
+
+    /// Returns the number of documents ids associated with the given word,
+    /// it is much faster than deserializing the bitmap and getting the length of it.
+    pub fn word_documents_count(&self, rtxn: &RoTxn, word: &str) -> anyhow::Result<Option<u64>> {
+        self.word_docids
+            .remap_data_type::<RoaringBitmapLenCodec>()
+            .get(rtxn, word)
+            .map_err(Into::into)
+    }
+
     /* documents */
 
     /// Returns a [`Vec`] of the requested documents. Returns an error if a document is missing.