use libc::{
    c_uint,
    c_void,
    size_t,
};
use std::marker::PhantomData;
use std::{
    fmt,
    mem,
    ptr,
    result,
    slice,
};

use ffi;

use cursor::{
    RoCursor,
    RwCursor,
};
use database::Database;
use environment::{
    Environment,
    Stat,
};
use error::{
    lmdb_result,
    Error,
    Result,
};
use flags::{
    DatabaseFlags,
    EnvironmentFlags,
    WriteFlags,
};

/// An LMDB transaction.
///
/// All database operations require a transaction.
pub trait Transaction: Sized {
    /// Returns a raw pointer to the underlying LMDB transaction.
    ///
    /// The caller **must** ensure that the pointer is not used after the
    /// lifetime of the transaction.
    fn txn(&self) -> *mut ffi::MDB_txn;

    /// Commits the transaction.
    ///
    /// Any pending operations will be saved.
    fn commit(self) -> Result<()> {
        unsafe {
            let result = lmdb_result(ffi::mdb_txn_commit(self.txn()));
            mem::forget(self);
            result
        }
    }

    /// Aborts the transaction.
    ///
    /// Any pending operations will not be saved.
    fn abort(self) {
        // Abort should be performed in transaction destructors.
    }

    /// Opens a database in the transaction.
    ///
    /// If `name` is `None`, then the default database will be opened, otherwise
    /// a named database will be opened. The database handle will be private to
    /// the transaction until the transaction is successfully committed. If the
    /// transaction is aborted the returned database handle should no longer be
    /// used.
    ///
    /// Prefer using `Environment::open_db`.
    ///
    /// ## Safety
    ///
    /// This function (as well as `Environment::open_db`,
    /// `Environment::create_db`, and `Database::create`) **must not** be called
    /// from multiple concurrent transactions in the same environment. A
    /// transaction which uses this function must finish (either commit or
    /// abort) before any other transaction may use this function.
    unsafe fn open_db(&self, name: Option<&str>) -> Result<Database> {
        Database::new(self.txn(), name, 0)
    }

    /// Gets an item from a database.
    ///
    /// This function retrieves the data associated with the given key in the
    /// database. If the database supports duplicate keys
    /// (`DatabaseFlags::DUP_SORT`) then the first data item for the key will be
    /// returned. Retrieval of other items requires the use of
    /// `Transaction::cursor_get`. If the item is not in the database, then
    /// `Error::NotFound` will be returned.
    fn get<'txn, K>(&'txn self, database: Database, key: &K) -> Result<&'txn [u8]>
    where
        K: AsRef<[u8]>,
    {
        let key = key.as_ref();
        let mut key_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: key.len() as size_t,
            mv_data: key.as_ptr() as *mut c_void,
        };
        let mut data_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: 0,
            mv_data: ptr::null_mut(),
        };
        unsafe {
            match ffi::mdb_get(self.txn(), database.dbi(), &mut key_val, &mut data_val) {
                ffi::MDB_SUCCESS => Ok(slice::from_raw_parts(data_val.mv_data as *const u8, data_val.mv_size as usize)),
                err_code => Err(Error::from_err_code(err_code)),
            }
        }
    }

    /// Open a new read-only cursor on the given database.
    fn open_ro_cursor<'txn>(&'txn self, db: Database) -> Result<RoCursor<'txn>> {
        RoCursor::new(self, db)
    }

    /// Gets the option flags for the given database in the transaction.
    fn db_flags(&self, db: Database) -> Result<DatabaseFlags> {
        let mut flags: c_uint = 0;
        unsafe {
            lmdb_result(ffi::mdb_dbi_flags(self.txn(), db.dbi(), &mut flags))?;
        }
        Ok(DatabaseFlags::from_bits_truncate(flags))
    }

    /// Retrieves database statistics.
    fn stat(&self, db: Database) -> Result<Stat> {
        unsafe {
            let mut stat = Stat::new();
            lmdb_try!(ffi::mdb_stat(self.txn(), db.dbi(), stat.mdb_stat()));
            Ok(stat)
        }
    }
}

/// An LMDB read-only transaction.
pub struct RoTransaction<'env> {
    txn: *mut ffi::MDB_txn,
    _marker: PhantomData<&'env ()>,
}

impl<'env> fmt::Debug for RoTransaction<'env> {
    fn fmt(&self, f: &mut fmt::Formatter) -> result::Result<(), fmt::Error> {
        f.debug_struct("RoTransaction").finish()
    }
}

impl<'env> Drop for RoTransaction<'env> {
    fn drop(&mut self) {
        unsafe { ffi::mdb_txn_abort(self.txn) }
    }
}

impl<'env> RoTransaction<'env> {
    /// Creates a new read-only transaction in the given environment. Prefer
    /// using `Environment::begin_ro_txn`.
    pub(crate) fn new(env: &'env Environment) -> Result<RoTransaction<'env>> {
        let mut txn: *mut ffi::MDB_txn = ptr::null_mut();
        unsafe {
            lmdb_result(ffi::mdb_txn_begin(env.env(), ptr::null_mut(), ffi::MDB_RDONLY, &mut txn))?;
            Ok(RoTransaction {
                txn: txn,
                _marker: PhantomData,
            })
        }
    }

    /// Resets the read-only transaction.
    ///
    /// Abort the transaction like `Transaction::abort`, but keep the
    /// transaction handle.  `InactiveTransaction::renew` may reuse the handle.
    /// This saves allocation overhead if the process will start a new read-only
    /// transaction soon, and also locking overhead if
    /// `EnvironmentFlags::NO_TLS` is in use. The reader table lock is released,
    /// but the table slot stays tied to its thread or transaction. Reader locks
    /// generally don't interfere with writers, but they keep old versions of
    /// database pages allocated. Thus they prevent the old pages from being
    /// reused when writers commit new data, and so under heavy load the
    /// database size may grow much more rapidly than otherwise.
    pub fn reset(self) -> InactiveTransaction<'env> {
        let txn = self.txn;
        unsafe {
            mem::forget(self);
            ffi::mdb_txn_reset(txn)
        };
        InactiveTransaction {
            txn: txn,
            _marker: PhantomData,
        }
    }
}

impl<'env> Transaction for RoTransaction<'env> {
    fn txn(&self) -> *mut ffi::MDB_txn {
        self.txn
    }
}

/// An inactive read-only transaction.
pub struct InactiveTransaction<'env> {
    txn: *mut ffi::MDB_txn,
    _marker: PhantomData<&'env ()>,
}

impl<'env> fmt::Debug for InactiveTransaction<'env> {
    fn fmt(&self, f: &mut fmt::Formatter) -> result::Result<(), fmt::Error> {
        f.debug_struct("InactiveTransaction").finish()
    }
}

impl<'env> Drop for InactiveTransaction<'env> {
    fn drop(&mut self) {
        unsafe { ffi::mdb_txn_abort(self.txn) }
    }
}

impl<'env> InactiveTransaction<'env> {
    /// Renews the inactive transaction, returning an active read-only
    /// transaction.
    ///
    /// This acquires a new reader lock for a transaction handle that had been
    /// released by `RoTransaction::reset`.
    pub fn renew(self) -> Result<RoTransaction<'env>> {
        let txn = self.txn;
        unsafe {
            mem::forget(self);
            lmdb_result(ffi::mdb_txn_renew(txn))?
        };
        Ok(RoTransaction {
            txn: txn,
            _marker: PhantomData,
        })
    }
}

/// An LMDB read-write transaction.
pub struct RwTransaction<'env> {
    txn: *mut ffi::MDB_txn,
    _marker: PhantomData<&'env ()>,
}

impl<'env> fmt::Debug for RwTransaction<'env> {
    fn fmt(&self, f: &mut fmt::Formatter) -> result::Result<(), fmt::Error> {
        f.debug_struct("RwTransaction").finish()
    }
}

impl<'env> Drop for RwTransaction<'env> {
    fn drop(&mut self) {
        unsafe { ffi::mdb_txn_abort(self.txn) }
    }
}

impl<'env> RwTransaction<'env> {
    /// Creates a new read-write transaction in the given environment. Prefer
    /// using `Environment::begin_ro_txn`.
    pub(crate) fn new(env: &'env Environment) -> Result<RwTransaction<'env>> {
        let mut txn: *mut ffi::MDB_txn = ptr::null_mut();
        unsafe {
            lmdb_result(ffi::mdb_txn_begin(env.env(), ptr::null_mut(), EnvironmentFlags::empty().bits(), &mut txn))?;
            Ok(RwTransaction {
                txn: txn,
                _marker: PhantomData,
            })
        }
    }

    /// Opens a database in the provided transaction, creating it if necessary.
    ///
    /// If `name` is `None`, then the default database will be opened, otherwise
    /// a named database will be opened. The database handle will be private to
    /// the transaction until the transaction is successfully committed. If the
    /// transaction is aborted the returned database handle should no longer be
    /// used.
    ///
    /// Prefer using `Environment::create_db`.
    ///
    /// ## Safety
    ///
    /// This function (as well as `Environment::open_db`,
    /// `Environment::create_db`, and `Database::open`) **must not** be called
    /// from multiple concurrent transactions in the same environment. A
    /// transaction which uses this function must finish (either commit or
    /// abort) before any other transaction may use this function.
    pub unsafe fn create_db(&self, name: Option<&str>, flags: DatabaseFlags) -> Result<Database> {
        Database::new(self.txn(), name, flags.bits() | ffi::MDB_CREATE)
    }

    /// Opens a new read-write cursor on the given database and transaction.
    pub fn open_rw_cursor<'txn>(&'txn mut self, db: Database) -> Result<RwCursor<'txn>> {
        RwCursor::new(self, db)
    }

    /// Stores an item into a database.
    ///
    /// This function stores key/data pairs in the database. The default
    /// behavior is to enter the new key/data pair, replacing any previously
    /// existing key if duplicates are disallowed, or adding a duplicate data
    /// item if duplicates are allowed (`DatabaseFlags::DUP_SORT`).
    pub fn put<K, D>(&mut self, database: Database, key: &K, data: &D, flags: WriteFlags) -> Result<()>
    where
        K: AsRef<[u8]>,
        D: AsRef<[u8]>,
    {
        let key = key.as_ref();
        let data = data.as_ref();
        let mut key_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: key.len() as size_t,
            mv_data: key.as_ptr() as *mut c_void,
        };
        let mut data_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: data.len() as size_t,
            mv_data: data.as_ptr() as *mut c_void,
        };
        unsafe { lmdb_result(ffi::mdb_put(self.txn(), database.dbi(), &mut key_val, &mut data_val, flags.bits())) }
    }

    /// Returns a buffer which can be used to write a value into the item at the
    /// given key and with the given length. The buffer must be completely
    /// filled by the caller.
    pub fn reserve<'txn, K>(
        &'txn mut self,
        database: Database,
        key: &K,
        len: size_t,
        flags: WriteFlags,
    ) -> Result<&'txn mut [u8]>
    where
        K: AsRef<[u8]>,
    {
        let key = key.as_ref();
        let mut key_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: key.len() as size_t,
            mv_data: key.as_ptr() as *mut c_void,
        };
        let mut data_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: len,
            mv_data: ptr::null_mut::<c_void>(),
        };
        unsafe {
            lmdb_result(ffi::mdb_put(
                self.txn(),
                database.dbi(),
                &mut key_val,
                &mut data_val,
                flags.bits() | ffi::MDB_RESERVE,
            ))?;
            Ok(slice::from_raw_parts_mut(data_val.mv_data as *mut u8, data_val.mv_size as usize))
        }
    }

    /// Deletes an item from a database.
    ///
    /// This function removes key/data pairs from the database. If the database
    /// does not support sorted duplicate data items (`DatabaseFlags::DUP_SORT`)
    /// the data parameter is ignored.  If the database supports sorted
    /// duplicates and the data parameter is `None`, all of the duplicate data
    /// items for the key will be deleted. Otherwise, if the data parameter is
    /// `Some` only the matching data item will be deleted. This function will
    /// return `Error::NotFound` if the specified key/data pair is not in the
    /// database.
    pub fn del<K>(&mut self, database: Database, key: &K, data: Option<&[u8]>) -> Result<()>
    where
        K: AsRef<[u8]>,
    {
        let key = key.as_ref();
        let mut key_val: ffi::MDB_val = ffi::MDB_val {
            mv_size: key.len() as size_t,
            mv_data: key.as_ptr() as *mut c_void,
        };
        let data_val: Option<ffi::MDB_val> = data.map(|data| ffi::MDB_val {
            mv_size: data.len() as size_t,
            mv_data: data.as_ptr() as *mut c_void,
        });

        if let Some(mut d) = data_val {
            unsafe { lmdb_result(ffi::mdb_del(self.txn(), database.dbi(), &mut key_val, &mut d)) }
        } else {
            unsafe { lmdb_result(ffi::mdb_del(self.txn(), database.dbi(), &mut key_val, ptr::null_mut())) }
        }
    }

    /// Empties the given database. All items will be removed.
    pub fn clear_db(&mut self, db: Database) -> Result<()> {
        unsafe { lmdb_result(ffi::mdb_drop(self.txn(), db.dbi(), 0)) }
    }

    /// Drops the database from the environment.
    ///
    /// ## Safety
    ///
    /// This method is unsafe in the same ways as `Environment::close_db`, and
    /// should be used accordingly.
    pub unsafe fn drop_db(&mut self, db: Database) -> Result<()> {
        lmdb_result(ffi::mdb_drop(self.txn, db.dbi(), 1))
    }

    /// Begins a new nested transaction inside of this transaction.
    pub fn begin_nested_txn<'txn>(&'txn mut self) -> Result<RwTransaction<'txn>> {
        let mut nested: *mut ffi::MDB_txn = ptr::null_mut();
        unsafe {
            let env: *mut ffi::MDB_env = ffi::mdb_txn_env(self.txn());
            ffi::mdb_txn_begin(env, self.txn(), 0, &mut nested);
        }
        Ok(RwTransaction {
            txn: nested,
            _marker: PhantomData,
        })
    }
}

impl<'env> Transaction for RwTransaction<'env> {
    fn txn(&self) -> *mut ffi::MDB_txn {
        self.txn
    }
}

#[cfg(test)]
mod test {

    use std::io::Write;
    use std::sync::{
        Arc,
        Barrier,
    };
    use std::thread::{
        self,
        JoinHandle,
    };

    use tempdir::TempDir;

    use super::*;
    use cursor::Cursor;
    use error::*;
    use flags::*;

    #[test]
    fn test_put_get_del() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.open_db(None).unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key1", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val3", WriteFlags::empty()).unwrap();
        txn.commit().unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        assert_eq!(b"val1", txn.get(db, b"key1").unwrap());
        assert_eq!(b"val2", txn.get(db, b"key2").unwrap());
        assert_eq!(b"val3", txn.get(db, b"key3").unwrap());
        assert_eq!(txn.get(db, b"key"), Err(Error::NotFound));

        txn.del(db, b"key1", None).unwrap();
        assert_eq!(txn.get(db, b"key1"), Err(Error::NotFound));
    }

    #[test]
    fn test_put_get_del_multi() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.create_db(None, DatabaseFlags::DUP_SORT).unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key1", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key1", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key1", b"val3", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val3", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val3", WriteFlags::empty()).unwrap();
        txn.commit().unwrap();

        let txn = env.begin_rw_txn().unwrap();
        {
            let mut cur = txn.open_ro_cursor(db).unwrap();
            let iter = cur.iter_dup_of(b"key1");
            let vals = iter.map(|x| x.unwrap()).map(|(_, x)| x).collect::<Vec<_>>();
            assert_eq!(vals, vec![b"val1", b"val2", b"val3"]);
        }
        txn.commit().unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        txn.del(db, b"key1", Some(b"val2")).unwrap();
        txn.del(db, b"key2", None).unwrap();
        txn.commit().unwrap();

        let txn = env.begin_rw_txn().unwrap();
        {
            let mut cur = txn.open_ro_cursor(db).unwrap();
            let iter = cur.iter_dup_of(b"key1");
            let vals = iter.map(|x| x.unwrap()).map(|(_, x)| x).collect::<Vec<_>>();
            assert_eq!(vals, vec![b"val1", b"val3"]);

            let iter = cur.iter_dup_of(b"key2");
            assert_eq!(0, iter.count());
        }
        txn.commit().unwrap();
    }

    #[test]
    fn test_reserve() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.open_db(None).unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        {
            let mut writer = txn.reserve(db, b"key1", 4, WriteFlags::empty()).unwrap();
            writer.write_all(b"val1").unwrap();
        }
        txn.commit().unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        assert_eq!(b"val1", txn.get(db, b"key1").unwrap());
        assert_eq!(txn.get(db, b"key"), Err(Error::NotFound));

        txn.del(db, b"key1", None).unwrap();
        assert_eq!(txn.get(db, b"key1"), Err(Error::NotFound));
    }

    #[test]
    fn test_inactive_txn() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.open_db(None).unwrap();

        {
            let mut txn = env.begin_rw_txn().unwrap();
            txn.put(db, b"key", b"val", WriteFlags::empty()).unwrap();
            txn.commit().unwrap();
        }

        let txn = env.begin_ro_txn().unwrap();
        let inactive = txn.reset();
        let active = inactive.renew().unwrap();
        assert!(active.get(db, b"key").is_ok());
    }

    #[test]
    fn test_nested_txn() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.open_db(None).unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key1", b"val1", WriteFlags::empty()).unwrap();

        {
            let mut nested = txn.begin_nested_txn().unwrap();
            nested.put(db, b"key2", b"val2", WriteFlags::empty()).unwrap();
            assert_eq!(nested.get(db, b"key1").unwrap(), b"val1");
            assert_eq!(nested.get(db, b"key2").unwrap(), b"val2");
        }

        assert_eq!(txn.get(db, b"key1").unwrap(), b"val1");
        assert_eq!(txn.get(db, b"key2"), Err(Error::NotFound));
    }

    #[test]
    fn test_clear_db() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.open_db(None).unwrap();

        {
            let mut txn = env.begin_rw_txn().unwrap();
            txn.put(db, b"key", b"val", WriteFlags::empty()).unwrap();
            txn.commit().unwrap();
        }

        {
            let mut txn = env.begin_rw_txn().unwrap();
            txn.clear_db(db).unwrap();
            txn.commit().unwrap();
        }

        let txn = env.begin_ro_txn().unwrap();
        assert_eq!(txn.get(db, b"key"), Err(Error::NotFound));
    }

    #[test]
    fn test_drop_db() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().set_max_dbs(2).open(dir.path()).unwrap();
        let db = env.create_db(Some("test"), DatabaseFlags::empty()).unwrap();

        {
            let mut txn = env.begin_rw_txn().unwrap();
            txn.put(db, b"key", b"val", WriteFlags::empty()).unwrap();
            txn.commit().unwrap();
        }
        {
            let mut txn = env.begin_rw_txn().unwrap();
            unsafe {
                txn.drop_db(db).unwrap();
            }
            txn.commit().unwrap();
        }

        assert_eq!(env.open_db(Some("test")), Err(Error::NotFound));
    }

    #[test]
    fn test_concurrent_readers_single_writer() {
        let dir = TempDir::new("test").unwrap();
        let env: Arc<Environment> = Arc::new(Environment::new().open(dir.path()).unwrap());

        let n = 10usize; // Number of concurrent readers
        let barrier = Arc::new(Barrier::new(n + 1));
        let mut threads: Vec<JoinHandle<bool>> = Vec::with_capacity(n);

        let key = b"key";
        let val = b"val";

        for _ in 0..n {
            let reader_env = env.clone();
            let reader_barrier = barrier.clone();

            threads.push(thread::spawn(move || {
                let db = reader_env.open_db(None).unwrap();
                {
                    let txn = reader_env.begin_ro_txn().unwrap();
                    assert_eq!(txn.get(db, key), Err(Error::NotFound));
                    txn.abort();
                }
                reader_barrier.wait();
                reader_barrier.wait();
                {
                    let txn = reader_env.begin_ro_txn().unwrap();
                    txn.get(db, key).unwrap() == val
                }
            }));
        }

        let db = env.open_db(None).unwrap();
        let mut txn = env.begin_rw_txn().unwrap();
        barrier.wait();
        txn.put(db, key, val, WriteFlags::empty()).unwrap();
        txn.commit().unwrap();
        barrier.wait();

        assert!(threads.into_iter().all(|b| b.join().unwrap()))
    }

    #[test]
    fn test_concurrent_writers() {
        let dir = TempDir::new("test").unwrap();
        let env = Arc::new(Environment::new().open(dir.path()).unwrap());

        let n = 10usize; // Number of concurrent writers
        let mut threads: Vec<JoinHandle<bool>> = Vec::with_capacity(n);

        let key = "key";
        let val = "val";

        for i in 0..n {
            let writer_env = env.clone();

            threads.push(thread::spawn(move || {
                let db = writer_env.open_db(None).unwrap();
                let mut txn = writer_env.begin_rw_txn().unwrap();
                txn.put(db, &format!("{}{}", key, i), &format!("{}{}", val, i), WriteFlags::empty()).unwrap();
                txn.commit().is_ok()
            }));
        }
        assert!(threads.into_iter().all(|b| b.join().unwrap()));

        let db = env.open_db(None).unwrap();
        let txn = env.begin_ro_txn().unwrap();

        for i in 0..n {
            assert_eq!(format!("{}{}", val, i).as_bytes(), txn.get(db, &format!("{}{}", key, i)).unwrap());
        }
    }

    #[test]
    fn test_stat() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.create_db(None, DatabaseFlags::empty()).unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key1", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val3", WriteFlags::empty()).unwrap();
        txn.commit().unwrap();

        {
            let txn = env.begin_ro_txn().unwrap();
            let stat = txn.stat(db).unwrap();
            assert_eq!(stat.entries(), 3);
        }

        let mut txn = env.begin_rw_txn().unwrap();
        txn.del(db, b"key1", None).unwrap();
        txn.del(db, b"key2", None).unwrap();
        txn.commit().unwrap();

        {
            let txn = env.begin_ro_txn().unwrap();
            let stat = txn.stat(db).unwrap();
            assert_eq!(stat.entries(), 1);
        }

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key4", b"val4", WriteFlags::empty()).unwrap();
        txn.put(db, b"key5", b"val5", WriteFlags::empty()).unwrap();
        txn.put(db, b"key6", b"val6", WriteFlags::empty()).unwrap();
        txn.commit().unwrap();

        {
            let txn = env.begin_ro_txn().unwrap();
            let stat = txn.stat(db).unwrap();
            assert_eq!(stat.entries(), 4);
        }
    }

    #[test]
    fn test_stat_dupsort() {
        let dir = TempDir::new("test").unwrap();
        let env = Environment::new().open(dir.path()).unwrap();
        let db = env.create_db(None, DatabaseFlags::DUP_SORT).unwrap();

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key1", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key1", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key1", b"val3", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key2", b"val3", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key3", b"val3", WriteFlags::empty()).unwrap();
        txn.commit().unwrap();

        {
            let txn = env.begin_ro_txn().unwrap();
            let stat = txn.stat(db).unwrap();
            assert_eq!(stat.entries(), 9);
        }

        let mut txn = env.begin_rw_txn().unwrap();
        txn.del(db, b"key1", Some(b"val2")).unwrap();
        txn.del(db, b"key2", None).unwrap();
        txn.commit().unwrap();

        {
            let txn = env.begin_ro_txn().unwrap();
            let stat = txn.stat(db).unwrap();
            assert_eq!(stat.entries(), 5);
        }

        let mut txn = env.begin_rw_txn().unwrap();
        txn.put(db, b"key4", b"val1", WriteFlags::empty()).unwrap();
        txn.put(db, b"key4", b"val2", WriteFlags::empty()).unwrap();
        txn.put(db, b"key4", b"val3", WriteFlags::empty()).unwrap();
        txn.commit().unwrap();

        {
            let txn = env.begin_ro_txn().unwrap();
            let stat = txn.stat(db).unwrap();
            assert_eq!(stat.entries(), 8);
        }
    }
}