Use mdb_stat for db.len()

Closes #56

heed/src/db/polymorph.rs

@@ -596,24 +596,22 @@ impl PolyDatabase {
     /// assert_eq!(ret, 3);
     ///
     /// wtxn.commit()?;
+    ///
     /// # Ok(()) }
     /// ```
-    pub fn len<'txn>(&self, txn: &'txn RoTxn) -> Result<usize> {
+    pub fn len<'txn>(&self, txn: &'txn RoTxn) -> Result<u64> {
         assert_eq!(self.env_ident, txn.env.env_mut_ptr() as usize);
 
-        let mut cursor = RoCursor::new(txn, self.dbi)?;
-        let mut count = 0;
+        let mut db_stat = mem::MaybeUninit::uninit();
+        let result = unsafe { mdb_result(ffi::mdb_stat(txn.txn, self.dbi, db_stat.as_mut_ptr())) };
 
-        match cursor.move_on_first()? {
-            Some(_) => count += 1,
-            None => return Ok(0),
-        }
-
-        while let Some(_) = cursor.move_on_next()? {
-            count += 1;
+        match result {
+            Ok(()) => {
+                let stats = unsafe { db_stat.assume_init() };
+                Ok(stats.ms_entries as u64)
+            }
+            Err(e) => Err(e.into()),
         }
-
-        Ok(count)
     }
 
     /// Returns `true` if and only if this database is empty.

heed/src/db/uniform.rs

@@ -509,7 +509,7 @@ impl<KC, DC> Database<KC, DC> {
     /// wtxn.commit()?;
     /// # Ok(()) }
     /// ```
-    pub fn len<'txn>(&self, txn: &'txn RoTxn) -> Result<usize> {
+    pub fn len<'txn>(&self, txn: &'txn RoTxn) -> Result<u64> {
         self.dyndb.len(txn)
     }
 

heed/src/mdb/lmdb_ffi.rs

@@ -2,9 +2,9 @@ pub use ffi::{
     mdb_cursor_close, mdb_cursor_del, mdb_cursor_get, mdb_cursor_open, mdb_cursor_put,
     mdb_dbi_open, mdb_del, mdb_drop, mdb_env_close, mdb_env_copyfd2 as mdb_env_copy2fd,
     mdb_env_create, mdb_env_open, mdb_env_set_mapsize, mdb_env_set_maxdbs, mdb_env_set_maxreaders,
-    mdb_env_sync, mdb_filehandle_t, mdb_get, mdb_put, mdb_txn_abort, mdb_txn_begin, mdb_txn_commit,
-    MDB_cursor, MDB_dbi, MDB_env, MDB_txn, MDB_APPEND, MDB_CP_COMPACT, MDB_CREATE, MDB_CURRENT,
-    MDB_RDONLY,
+    mdb_env_stat, mdb_env_sync, mdb_filehandle_t, mdb_get, mdb_put, mdb_stat, mdb_txn_abort,
+    mdb_txn_begin, mdb_txn_commit, MDB_cursor, MDB_dbi, MDB_env, MDB_stat as MDB_Stat, MDB_txn,
+    MDB_APPEND, MDB_CP_COMPACT, MDB_CREATE, MDB_CURRENT, MDB_RDONLY,
 };
 use lmdb_master3_sys as ffi;