feat: Update the number of documents in the KV

meilidb-data/src/database/index/main_index.rs

@@ -1,4 +1,5 @@
 use std::sync::Arc;
+use std::convert::TryInto;
 
 use meilidb_schema::Schema;
 
@@ -9,6 +10,7 @@ const SCHEMA_KEY:     &str = "schema";
 const WORDS_KEY:               &str = "words";
 const SYNONYMS_KEY:            &str = "synonyms";
 const RANKED_MAP_KEY:          &str = "ranked-map";
+const NUMBER_OF_DOCUMENTS_KEY: &str = "number-of-documents";
 
 #[derive(Clone)]
 pub struct MainIndex(pub(crate) crate::CfTree);
@@ -79,4 +81,22 @@ impl MainIndex {
         self.0.insert(RANKED_MAP_KEY, bytes)?;
         Ok(())
     }
+
+    pub fn number_of_documents(&self) -> Result<u64, Error> {
+        match self.0.get(NUMBER_OF_DOCUMENTS_KEY)? {
+            Some(bytes) => {
+                let array = (*bytes).try_into().unwrap();
+                Ok(u64::from_be_bytes(array))
+            },
+            None => Ok(0),
+        }
+    }
+
+    pub fn set_number_of_documents<F>(&self, f: F) -> Result<u64, Error>
+    where F: FnOnce(u64) -> u64,
+    {
+        let new = self.number_of_documents().map(f)?;
+        self.0.insert(NUMBER_OF_DOCUMENTS_KEY, new.to_be_bytes())?;
+        Ok(new)
+    }
 }

meilidb-data/src/database/update/documents_addition.rs

@@ -123,6 +123,9 @@ pub fn apply_documents_addition(
     main.set_words_set(&words)?;
     main.set_ranked_map(&ranked_map)?;
 
+    let inserted_documents_len = document_ids.len() as u64;
+    let number_of_documents = main.set_number_of_documents(|old| old + inserted_documents_len)?;
+
     // update the "consistent" view of the Index
     let cache = ref_index.cache;
     let words = Arc::new(words);

meilidb-data/src/database/update/documents_deletion.rs

@@ -134,6 +134,9 @@ pub fn apply_documents_deletion(
     main.set_words_set(&words)?;
     main.set_ranked_map(&ranked_map)?;
 
+    let deleted_documents_len = deleted_documents.len() as u64;
+    let number_of_documents = main.set_number_of_documents(|old| old - deleted_documents_len)?;
+
     // update the "consistent" view of the Index
     let cache = ref_index.cache;
     let words = Arc::new(words);