// Copyright 2018 foundationdb-rs developers, https://github.com/bluejekyll/foundationdb-rs/graphs/contributors
// Copyright 2013-2018 Apple, Inc and the FoundationDB project authors.
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

//! Implementations of the FDBTransaction C API
//!
//! https://apple.github.io/foundationdb/api-c.html#transaction

use foundationdb_sys as fdb;
use futures::{Async, Future, Stream};
use std;
use std::sync::Arc;

use database::*;
use error::{self, *};
use future::*;
use keyselector::*;
use options;
use subspace::Subspace;
use tuple::Encode;

/// In FoundationDB, a transaction is a mutable snapshot of a database.
///
/// All read and write operations on a transaction see and modify an otherwise-unchanging version of the database and only change the underlying database if and when the transaction is committed. Read operations do see the effects of previous write operations on the same transaction. Committing a transaction usually succeeds in the absence of conflicts.
///
/// Applications must provide error handling and an appropriate retry loop around the application code for a transaction. See the documentation for [fdb_transaction_on_error()](https://apple.github.io/foundationdb/api-c.html#transaction).
///
/// Transactions group operations into a unit with the properties of atomicity, isolation, and durability. Transactions also provide the ability to maintain an application’s invariants or integrity constraints, supporting the property of consistency. Together these properties are known as ACID.
///
/// Transactions are also causally consistent: once a transaction has been successfully committed, all subsequently created transactions will see the modifications made by it.
#[derive(Clone)]
pub struct Transaction {
    // Order of fields should not be changed, because Rust drops field top-to-bottom, and
    // transaction should be dropped before cluster.
    inner: Arc<TransactionInner>,
    database: Database,
}

/// Converts Rust `bool` into `fdb::fdb_bool_t`
fn fdb_bool(v: bool) -> fdb::fdb_bool_t {
    if v {
        1
    } else {
        0
    }
}

/// Foundationdb API uses `c_int` type as a length of a value, while Rust uses `usize` for. Rust
/// inteface uses `usize` if it represents length or size of something. Those `usize` values should
/// be converted to `c_int` before passed to ffi, because naive casting with `v as i32` will
/// convert some `usize` values to unsigned one.
/// TODO: check if inverse function is needed, `cint_to_usize(v: c_int) -> usize`?
fn usize_trunc(v: usize) -> std::os::raw::c_int {
    if v > std::i32::MAX as usize {
        std::i32::MAX
    } else {
        v as i32
    }
}

/// `RangeOption` represents a query parameters for range scan query.
#[derive(Debug, Clone)]
pub struct RangeOption {
    begin: KeySelector,
    end: KeySelector,
    limit: Option<usize>,
    target_bytes: usize,
    mode: options::StreamingMode,
    //TODO: move snapshot out from `RangeOption`, as other methods like `Transaction::get` do?
    snapshot: bool,
    reverse: bool,
}

impl<'a> Default for RangeOption {
    fn default() -> Self {
        Self {
            begin: KeySelector::first_greater_or_equal(&[]).to_owned(),
            end: KeySelector::first_greater_or_equal(&[]).to_owned(),
            limit: None,
            target_bytes: 0,
            mode: options::StreamingMode::Iterator,
            snapshot: false,
            reverse: false,
        }
    }
}

/// A Builder with which options need to used for a range query.
pub struct RangeOptionBuilder(RangeOption);

impl<T: Encode> From<T> for RangeOptionBuilder {
    fn from(t: T) -> Self {
        let (begin, end) = Subspace::from(t).range();

        Self::new(
            KeySelector::first_greater_or_equal(&begin),
            KeySelector::first_greater_or_equal(&end),
        )
    }
}

impl RangeOptionBuilder {
    /// Creates new builder with given key selectors.
    pub fn new(begin: KeySelector, end: KeySelector) -> Self {
        let mut opt = RangeOption::default();
        opt.begin = begin.to_owned();
        opt.end = end.to_owned();
        RangeOptionBuilder(opt)
    }

    /// If non-zero, indicates the maximum number of key-value pairs to return.
    pub fn limit(mut self, limit: usize) -> Self {
        if limit > 0 {
            self.0.limit = Some(limit);
        }
        self
    }

    /// If non-zero, indicates a (soft) cap on the combined number of bytes of keys and values to
    /// return for each item.
    pub fn target_bytes(mut self, target_bytes: usize) -> Self {
        self.0.target_bytes = target_bytes;
        self
    }

    /// One of the options::StreamingMode values indicating how the caller would like the data in
    /// the range returned.
    pub fn mode(mut self, mode: options::StreamingMode) -> Self {
        self.0.mode = mode;
        self
    }

    /// Non-zero if this is a snapshot read.
    pub fn snapshot(mut self, snapshot: bool) -> Self {
        self.0.snapshot = snapshot;
        self
    }

    /// If non-zero, key-value pairs will be returned in reverse lexicographical order beginning at
    /// the end of the range.
    pub fn reverse(mut self, reverse: bool) -> Self {
        self.0.reverse = reverse;
        self
    }

    /// Finalizes the construction of the RangeOption
    pub fn build(self) -> RangeOption {
        self.0
    }
}

// TODO: many implementations left
impl Transaction {
    pub(crate) fn new(database: Database, trx: *mut fdb::FDBTransaction) -> Self {
        let inner = Arc::new(TransactionInner::new(trx));
        Self { database, inner }
    }

    /// Called to set an option on an FDBTransaction.
    pub fn set_option(&self, opt: options::TransactionOption) -> Result<()> {
        unsafe { opt.apply(self.inner.inner) }
    }

    /// Returns a clone of this transactions Database
    pub fn database(&self) -> Database {
        self.database.clone()
    }

    fn into_database(self) -> Database {
        self.database
    }

    /// Modify the database snapshot represented by transaction to change the given key to have the given value.
    ///
    /// If the given key was not previously present in the database it is inserted. The modification affects the actual database only if transaction is later committed with `Transaction::commit`.
    ///
    /// # Arguments
    ///
    /// * `key_name` - the name of the key to be inserted into the database.
    /// * `value` - the value to be inserted into the database
    pub fn set(&self, key: &[u8], value: &[u8]) {
        let trx = self.inner.inner;
        unsafe {
            fdb::fdb_transaction_set(
                trx,
                key.as_ptr(),
                key.len() as i32,
                value.as_ptr(),
                value.len() as i32,
            )
        }
    }

    /// Modify the database snapshot represented by transaction to remove the given key from the database.
    ///
    /// If the key was not previously present in the database, there is no effect. The modification affects the actual database only if transaction is later committed with `Transaction::commit`.
    ///
    /// # Arguments
    ///
    /// * `key_name` - the name of the key to be removed from the database.
    pub fn clear(&self, key: &[u8]) {
        let trx = self.inner.inner;
        unsafe { fdb::fdb_transaction_clear(trx, key.as_ptr(), key.len() as i32) }
    }

    /// Reads a value from the database snapshot represented by transaction.
    ///
    /// Returns an FDBFuture which will be set to the value of key_name in the database. You must first wait for the FDBFuture to be ready, check for errors, call fdb_future_get_value() to extract the value, and then destroy the FDBFuture with fdb_future_destroy().
    ///
    /// See `FdbFutureResult::value` to see exactly how results are unpacked. If key_name is not present in the database, the result is not an error, but a zero for *out_present returned from that function.
    ///
    /// # Arguments
    ///
    /// * `key_name` - the name of the key to be looked up in the database
    ///
    /// TODO: implement: snapshot Non-zero if this is a snapshot read.
    pub fn get(&self, key: &[u8], snapshot: bool) -> TrxGet {
        let trx = self.inner.inner;

        let f = unsafe {
            fdb::fdb_transaction_get(
                trx,
                key.as_ptr() as *const _,
                key.len() as i32,
                fdb_bool(snapshot),
            )
        };
        TrxGet {
            inner: self.new_fut_trx(f),
        }
    }

    /// Modify the database snapshot represented by transaction to perform the operation indicated
    /// by operationType with operand param to the value stored by the given key.
    ///
    /// An atomic operation is a single database command that carries out several logical steps:
    /// reading the value of a key, performing a transformation on that value, and writing the
    /// result. Different atomic operations perform different transformations. Like other database
    /// operations, an atomic operation is used within a transaction; however, its use within a
    /// transaction will not cause the transaction to conflict.
    ///
    /// Atomic operations do not expose the current value of the key to the client but simply send
    /// the database the transformation to apply. In regard to conflict checking, an atomic
    /// operation is equivalent to a write without a read. It can only cause other transactions
    /// performing reads of the key to conflict.
    ///
    /// By combining these logical steps into a single, read-free operation, FoundationDB can
    /// guarantee that the transaction will not conflict due to the operation. This makes atomic
    /// operations ideal for operating on keys that are frequently modified. A common example is
    /// the use of a key-value pair as a counter.
    pub fn atomic_op(&self, key: &[u8], param: &[u8], op_type: options::MutationType) {
        let trx = self.inner.inner;
        unsafe {
            fdb::fdb_transaction_atomic_op(
                trx,
                key.as_ptr() as *const _,
                key.len() as i32,
                param.as_ptr() as *const _,
                param.len() as i32,
                op_type.code(),
            )
        }
    }

    /// Resolves a key selector against the keys in the database snapshot represented by
    /// transaction.
    ///
    /// Returns an FDBFuture which will be set to the key in the database matching the key
    /// selector. You must first wait for the FDBFuture to be ready, check for errors, call
    /// fdb_future_get_key() to extract the key, and then destroy the FDBFuture with
    /// fdb_future_destroy().
    pub fn get_key(&self, selector: KeySelector, snapshot: bool) -> TrxGetKey {
        let trx = self.inner.inner;

        let key = selector.key();

        let f = unsafe {
            fdb::fdb_transaction_get_key(
                trx,
                key.as_ptr() as *const _,
                key.len() as i32,
                fdb_bool(selector.or_equal()),
                selector.offset() as i32,
                fdb_bool(snapshot),
            )
        };
        TrxGetKey {
            inner: self.new_fut_trx(f),
        }
    }

    ///
    pub fn get_ranges(&self, opt: RangeOption) -> RangeStream {
        let iteration = 1;
        let inner = self.get_range(opt, iteration);

        RangeStream {
            iteration,

            trx: self.clone(),
            inner: Some(inner),
        }
    }

    /// Reads all key-value pairs in the database snapshot represented by transaction (potentially
    /// limited by limit, target_bytes, or mode) which have a key lexicographically greater than or
    /// equal to the key resolved by the begin key selector and lexicographically less than the key
    /// resolved by the end key selector.
    pub fn get_range(&self, opt: RangeOption, iteration: usize) -> TrxGetRange {
        let trx = self.inner.inner;

        let f = unsafe {
            let begin = &opt.begin;
            let end = &opt.end;
            let key_begin = begin.key();
            let key_end = end.key();

            fdb::fdb_transaction_get_range(
                trx,
                key_begin.as_ptr() as *const _,
                key_begin.len() as i32,
                fdb_bool(begin.or_equal()),
                begin.offset() as i32,
                key_end.as_ptr() as *const _,
                key_end.len() as i32,
                fdb_bool(end.or_equal()),
                end.offset() as i32,
                usize_trunc(opt.limit.unwrap_or(0)),
                usize_trunc(opt.target_bytes),
                opt.mode.code(),
                iteration as i32,
                fdb_bool(opt.snapshot),
                fdb_bool(opt.reverse),
            )
        };

        TrxGetRange {
            inner: self.new_fut_trx(f),
            opt: Some(opt),
        }
    }

    /// Modify the database snapshot represented by transaction to remove all keys (if any) which
    /// are lexicographically greater than or equal to the given begin key and lexicographically
    /// less than the given end_key.
    ///
    /// The modification affects the actual database only if transaction is later committed with
    /// `Tranasction::commit`.
    pub fn clear_range(&self, begin: &[u8], end: &[u8]) {
        let trx = self.inner.inner;
        unsafe {
            fdb::fdb_transaction_clear_range(
                trx,
                begin.as_ptr() as *const _,
                begin.len() as i32,
                end.as_ptr() as *const _,
                end.len() as i32,
            )
        }
    }

    /// Clears all keys based on the range of the Subspace
    pub fn clear_subspace_range<S: Into<Subspace>>(&self, subspace: S) {
        let subspace = subspace.into();
        let range = subspace.range();
        self.clear_range(&range.0, &range.1)
    }

    /// Attempts to commit the sets and clears previously applied to the database snapshot represented by transaction to the actual database.
    ///
    /// The commit may or may not succeed – in particular, if a conflicting transaction previously committed, then the commit must fail in order to preserve transactional isolation. If the commit does succeed, the transaction is durably committed to the database and all subsequently started transactions will observe its effects.
    ///
    /// It is not necessary to commit a read-only transaction – you can simply call fdb_transaction_destroy().
    ///
    /// Returns an `TrxCommit` representing an empty value.
    ///
    /// Callers will usually want to retry a transaction if the commit or a prior fdb_transaction_get_*() returns a retryable error (see fdb_transaction_on_error()).
    ///
    /// As with other client/server databases, in some failure scenarios a client may be unable to determine whether a transaction succeeded. In these cases, `Transaction::commit` will return a commit_unknown_result error. The fdb_transaction_on_error() function treats this error as retryable, so retry loops that don’t check for commit_unknown_result could execute the transaction twice. In these cases, you must consider the idempotence of the transaction. For more information, see Transactions with unknown results.
    ///
    /// Normally, commit will wait for outstanding reads to return. However, if those reads were snapshot reads or the transaction option for disabling “read-your-writes” has been invoked, any outstanding reads will immediately return errors.
    pub fn commit(self) -> TrxCommit {
        let trx = self.inner.inner;

        let f = unsafe { fdb::fdb_transaction_commit(trx) };
        let f = self.new_fut_trx(f);
        TrxCommit { inner: f }
    }

    /// Cancels the transaction. All pending or future uses of the transaction will return a
    /// transaction_cancelled error. The transaction can be used again after it is reset.
    ///
    /// # Warning
    ///
    /// * Be careful if you are using fdb_transaction_reset() and fdb_transaction_cancel()
    /// concurrently with the same transaction. Since they negate each other’s effects, a race
    /// condition between these calls will leave the transaction in an unknown state.
    ///
    /// * If your program attempts to cancel a transaction after fdb_transaction_commit() has been
    /// called but before it returns, unpredictable behavior will result. While it is guaranteed
    /// that the transaction will eventually end up in a cancelled state, the commit may or may not
    /// occur. Moreover, even if the call to fdb_transaction_commit() appears to return a
    /// transaction_cancelled error, the commit may have occurred or may occur in the future. This
    /// can make it more difficult to reason about the order in which transactions occur.
    pub fn cancel(self) {
        let trx = self.inner.inner;
        unsafe { fdb::fdb_transaction_cancel(trx) }
    }

    /// Retrieves the database version number at which a given transaction was committed.
    /// fdb_transaction_commit() must have been called on transaction and the resulting future must
    /// be ready and not an error before this function is called, or the behavior is undefined.
    /// Read-only transactions do not modify the database when committed and will have a committed
    /// version of -1. Keep in mind that a transaction which reads keys and then sets them to their
    /// current values may be optimized to a read-only transaction.
    ///
    /// Note that database versions are not necessarily unique to a given transaction and so cannot
    /// be used to determine in what order two transactions completed. The only use for this
    /// function is to manually enforce causal consistency when calling
    /// fdb_transaction_set_read_version() on another subsequent transaction.
    ///
    /// Most applications will not call this function.
    pub fn committed_version(&self) -> Result<i64> {
        let trx = self.inner.inner;

        let mut version: i64 = 0;
        let e = unsafe { fdb::fdb_transaction_get_committed_version(trx, &mut version as *mut _) };
        error::eval(e)?;
        Ok(version)
    }

    /// Returns a list of public network addresses as strings, one for each of the storage servers
    /// responsible for storing key_name and its associated value.
    ///
    /// Returns an FDBFuture which will be set to an array of strings. You must first wait for the
    /// FDBFuture to be ready, check for errors, call fdb_future_get_string_array() to extract the
    /// string array, and then destroy the FDBFuture with fdb_future_destroy().
    pub fn get_addresses_for_key(&self, key: &[u8]) -> TrxGetAddressesForKey {
        let trx = self.inner.inner;

        let f = unsafe {
            fdb::fdb_transaction_get_addresses_for_key(
                trx,
                key.as_ptr() as *const _,
                key.len() as i32,
            )
        };
        TrxGetAddressesForKey {
            inner: self.new_fut_trx(f),
        }
    }

    /// A watch’s behavior is relative to the transaction that created it. A watch will report a
    /// change in relation to the key’s value as readable by that transaction. The initial value
    /// used for comparison is either that of the transaction’s read version or the value as
    /// modified by the transaction itself prior to the creation of the watch. If the value changes
    /// and then changes back to its initial value, the watch might not report the change.
    ///
    /// Until the transaction that created it has been committed, a watch will not report changes
    /// made by other transactions. In contrast, a watch will immediately report changes made by
    /// the transaction itself. Watches cannot be created if the transaction has set the
    /// READ_YOUR_WRITES_DISABLE transaction option, and an attempt to do so will return an
    /// watches_disabled error.
    ///
    /// If the transaction used to create a watch encounters an error during commit, then the watch
    /// will be set with that error. A transaction whose commit result is unknown will set all of
    /// its watches with the commit_unknown_result error. If an uncommitted transaction is reset or
    /// destroyed, then any watches it created will be set with the transaction_cancelled error.
    ///
    /// Returns an FDBFuture representing an empty value that will be set once the watch has
    /// detected a change to the value at the specified key. You must first wait for the FDBFuture
    /// to be ready, check for errors, and then destroy the FDBFuture with fdb_future_destroy().
    ///
    /// By default, each database connection can have no more than 10,000 watches that have not yet
    /// reported a change. When this number is exceeded, an attempt to create a watch will return a
    /// too_many_watches error. This limit can be changed using the MAX_WATCHES database option.
    /// Because a watch outlives the transaction that creates it, any watch that is no longer
    /// needed should be cancelled by calling fdb_future_cancel() on its returned future.
    pub fn watch(&self, key: &[u8]) -> TrxWatch {
        let trx = self.inner.inner;

        let f =
            unsafe { fdb::fdb_transaction_watch(trx, key.as_ptr() as *const _, key.len() as i32) };
        TrxWatch {
            inner: self.new_fut_non_trx(f),
        }
    }

    /// Returns an FDBFuture which will be set to the versionstamp which was used by any
    /// versionstamp operations in this transaction. You must first wait for the FDBFuture to be
    /// ready, check for errors, call fdb_future_get_key() to extract the key, and then destroy the
    /// FDBFuture with fdb_future_destroy().
    ///
    /// The future will be ready only after the successful completion of a call to
    /// fdb_transaction_commit() on this Transaction. Read-only transactions do not modify the
    /// database when committed and will result in the future completing with an error.  Keep in
    /// mind that a transaction which reads keys and then sets them to their current values may be
    /// optimized to a read-only transaction.
    ///
    /// Most applications will not call this function.
    pub fn get_versionstamp(&self) -> TrxVersionstamp {
        let trx = self.inner.inner;

        let f = unsafe { fdb::fdb_transaction_get_versionstamp(trx) };
        TrxVersionstamp {
            inner: self.new_fut_non_trx(f),
        }
    }

    /// The transaction obtains a snapshot read version automatically at the time of the first call
    /// to fdb_transaction_get_*() (including this one) and (unless causal consistency has been
    /// deliberately compromised by transaction options) is guaranteed to represent all
    /// transactions which were reported committed before that call.
    pub fn get_read_version(&self) -> TrxReadVersion {
        let trx = self.inner.inner;

        let f = unsafe { fdb::fdb_transaction_get_read_version(trx) };
        TrxReadVersion {
            inner: self.new_fut_trx(f),
        }
    }

    /// Sets the snapshot read version used by a transaction. This is not needed in simple cases.
    /// If the given version is too old, subsequent reads will fail with error_code_past_version;
    /// if it is too new, subsequent reads may be delayed indefinitely and/or fail with
    /// error_code_future_version. If any of fdb_transaction_get_*() have been called on this
    /// transaction already, the result is undefined.
    pub fn set_read_version(&self, version: i64) {
        let trx = self.inner.inner;

        unsafe { fdb::fdb_transaction_set_read_version(trx, version) }
    }

    /// Reset transaction to its initial state. This is similar to calling
    /// fdb_transaction_destroy() followed by fdb_database_create_transaction(). It is not
    /// necessary to call fdb_transaction_reset() when handling an error with
    /// fdb_transaction_on_error() since the transaction has already been reset.
    ///
    /// # Warning
    ///
    /// The API is exposed mainly for `bindingtester`, and it is not recommended to call the API
    /// directly from application.
    #[doc(hidden)]
    pub fn reset(&self) {
        let trx = self.inner.inner;
        unsafe { fdb::fdb_transaction_reset(trx) }
    }

    /// Implements the recommended retry and backoff behavior for a transaction. This function
    /// knows which of the error codes generated by other fdb_transaction_*() functions represent
    /// temporary error conditions and which represent application errors that should be handled by
    /// the application. It also implements an exponential backoff strategy to avoid swamping the
    /// database cluster with excessive retries when there is a high level of conflict between
    /// transactions.
    ///
    /// # Warning
    ///
    /// The API is exposed mainly for `bindingtester`, and it is not recommended to call the API
    /// directly from application. Use `Database::transact` instead.
    #[doc(hidden)]
    pub fn on_error(&self, error: Error) -> TrxErrFuture {
        TrxErrFuture::new(self.clone(), error)
    }

    /// Adds a conflict range to a transaction without performing the associated read or write.
    ///
    /// # Note
    ///
    /// Most applications will use the serializable isolation that transactions provide by default
    /// and will not need to manipulate conflict ranges.
    pub fn add_conflict_range(
        &self,
        begin: &[u8],
        end: &[u8],
        ty: options::ConflictRangeType,
    ) -> Result<()> {
        let trx = self.inner.inner;
        unsafe {
            eval(fdb::fdb_transaction_add_conflict_range(
                trx,
                begin.as_ptr() as *const _,
                begin.len() as i32,
                end.as_ptr() as *const _,
                end.len() as i32,
                ty.code(),
            ))
        }
    }

    fn new_fut_trx(&self, f: *mut fdb::FDBFuture) -> TrxFuture {
        TrxFuture::new(self.clone(), f)
    }

    fn new_fut_non_trx(&self, f: *mut fdb::FDBFuture) -> NonTrxFuture {
        NonTrxFuture::new(self.database(), f)
    }
}

struct TransactionInner {
    inner: *mut fdb::FDBTransaction,
}
impl TransactionInner {
    fn new(inner: *mut fdb::FDBTransaction) -> Self {
        Self { inner }
    }
}
impl Drop for TransactionInner {
    fn drop(&mut self) {
        unsafe {
            fdb::fdb_transaction_destroy(self.inner);
        }
    }
}
unsafe impl Send for TransactionInner {}
unsafe impl Sync for TransactionInner {}

/// Represents the data of a `Transaction::get`
pub struct GetResult {
    trx: Transaction,
    inner: FdbFutureResult,
}
impl GetResult {
    /// Returns a clone of the Transaction this get is a part of
    pub fn transaction(&self) -> Transaction {
        self.trx.clone()
    }

    /// Returns the values associated with this get
    pub fn value(&self) -> Option<&[u8]> {
        self.inner
            .get_value()
            .expect("inner should resolve into value")
    }
}

/// A future results of a get operation
pub struct TrxGet {
    inner: TrxFuture,
}
impl Future for TrxGet {
    type Item = GetResult;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let (trx, inner) = try_ready!(self.inner.poll());
        // check if a future resolves to value type
        inner.get_value()?;

        Ok(Async::Ready(GetResult { trx, inner }))
    }
}

/// Represents the data of a `Transaction::get_key`
pub struct GetKeyResult {
    trx: Transaction,
    inner: FdbFutureResult,
}
impl GetKeyResult {
    /// Returns a clone of the Transaction this get is a part of
    pub fn transaction(&self) -> Transaction {
        self.trx.clone()
    }

    /// Returns the values associated with this get
    pub fn value(&self) -> &[u8] {
        self.inner.get_key().expect("inner should resolve into key")
    }
}

/// A future results of a `get_key` operation
pub struct TrxGetKey {
    inner: TrxFuture,
}
impl Future for TrxGetKey {
    type Item = GetKeyResult;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let (trx, inner) = try_ready!(self.inner.poll());
        inner.get_key()?;
        Ok(Async::Ready(GetKeyResult { trx, inner }))
    }
}

/// Represents the data of a `Transaction::get_range`. The result might not contains all results
/// specified by `Transaction::get_range`. A caller can test if the result is complete by either
/// checking `GetRangeResult::key_values().more()` is `true`, or checking `GetRangeResult::next` is
/// not `None`.
/// If a caller wants to fetch all matching results, they should call `Transcation::get_range` with
/// following `RangeOption` returned by `GetRangeResult::next`. The caller might want to use
/// `Transaction::get_ranges` which will fetch all results until it finishes.
pub struct GetRangeResult {
    trx: Transaction,
    opt: RangeOption,

    // This future should always resolves to keyvalue array.
    inner: FdbFutureResult,
}

impl GetRangeResult {
    /// Returns a clone of the Transaction this get is a part of
    pub fn transaction(&self) -> Transaction {
        self.trx.clone()
    }

    /// Returns the values associated with this get
    pub fn key_values(&self) -> KeyValues {
        self.inner
            .get_keyvalue_array()
            .expect("inner should resolve into keyvalue array")
    }

    /// Returns `None` if all results are returned, and returns `Some(_)` if there are more results
    /// to fetch. In this case, user can fetch remaining results by calling
    /// `Transaction::get_range` with returned `RangeOption`.
    pub fn next(&self) -> Option<RangeOption> {
        let kva = self.key_values();
        if !kva.more() {
            return None;
        }

        let slice = kva.as_ref();
        if slice.is_empty() {
            return None;
        }

        let last = slice.last().unwrap();
        let last_key = last.key();

        let mut opt = self.opt.clone();
        if let Some(limit) = opt.limit.as_mut() {
            *limit -= slice.len();
            if *limit == 0 {
                return None;
            }
        }

        if opt.reverse {
            opt.end = KeySelector::first_greater_or_equal(last_key).to_owned();
        } else {
            opt.begin = KeySelector::first_greater_than(last_key).to_owned();
        }
        Some(opt)
    }
}

/// A future results of a `get_range` operation
pub struct TrxGetRange {
    inner: TrxFuture,
    opt: Option<RangeOption>,
}

impl Future for TrxGetRange {
    type Item = GetRangeResult;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let (trx, inner) = try_ready!(self.inner.poll());
        // tests if the future resolves to keyvalue array.
        inner.get_keyvalue_array()?;

        let opt = self.opt.take().expect("should not poll after ready");
        Ok(Async::Ready(GetRangeResult { trx, inner, opt }))
    }
}

//TODO: proper naming
/// `RangeStream` represents a stream of `GetRangeResult`
pub struct RangeStream {
    iteration: usize,

    trx: Transaction,
    inner: Option<TrxGetRange>,
}

impl RangeStream {
    fn update_inner(&mut self, opt: RangeOption) {
        self.iteration += 1;
        self.inner = Some(self.trx.get_range(opt, self.iteration));
    }

    fn advance(&mut self, res: &GetRangeResult) {
        if let Some(opt) = res.next() {
            self.update_inner(opt)
        }
    }
}

impl<'a> Stream for RangeStream {
    type Item = GetRangeResult;
    type Error = (RangeOption, Error);

    fn poll(&mut self) -> std::result::Result<Async<Option<Self::Item>>, Self::Error> {
        if self.inner.is_none() {
            return Ok(Async::Ready(None));
        }

        let mut inner = self.inner.take().unwrap();
        match inner.poll() {
            Ok(Async::NotReady) => {
                self.inner = Some(inner);
                Ok(Async::NotReady)
            }
            Ok(Async::Ready(res)) => {
                self.advance(&res);
                Ok(Async::Ready(Some(res)))
            }
            Err(e) => {
                // `inner.opt == None` after it resolves, so `inner.opt.unwrap()` should not fail.
                Err((inner.opt.unwrap(), e))
            }
        }
    }
}

/// A future result of a `Transaction::commit`
pub struct TrxCommit {
    inner: TrxFuture,
}

impl Future for TrxCommit {
    type Item = Transaction;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let (trx, _res) = try_ready!(self.inner.poll());
        Ok(Async::Ready(trx))
    }
}

/// Represents the data of a `Transaction::get_addresses_for_key`
pub struct GetAddressResult {
    trx: Transaction,
    inner: FdbFutureResult,
}
impl GetAddressResult {
    /// Returns a clone of the Transaction this get is a part of
    pub fn transaction(&self) -> Transaction {
        self.trx.clone()
    }

    /// Returns the addresses for the key
    pub fn address(&self) -> Vec<&[u8]> {
        self.inner
            .get_string_array()
            .expect("inner should resolve into string array")
    }
}

/// A future result of a `Transaction::get_addresses_for_key`
pub struct TrxGetAddressesForKey {
    inner: TrxFuture,
}
impl Future for TrxGetAddressesForKey {
    type Item = GetAddressResult;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let (trx, inner) = try_ready!(self.inner.poll());
        inner.get_string_array()?;
        Ok(Async::Ready(GetAddressResult { trx, inner }))
    }
}

/// A future result of a `Transaction::watch`
pub struct TrxWatch {
    // `TrxWatch` can live longer then a parent transaction that registhers the watch, so it should
    // not maintain a reference to the transaction, which will prevent the transcation to be freed.
    inner: NonTrxFuture,
}
impl Future for TrxWatch {
    type Item = ();
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        try_ready!(self.inner.poll());
        Ok(Async::Ready(()))
    }
}

/// A versionstamp is a 10 byte, unique, monotonically (but not sequentially) increasing value for
/// each committed transaction. The first 8 bytes are the committed version of the database. The
/// last 2 bytes are monotonic in the serialization order for transactions.
#[derive(Clone, Copy)]
pub struct Versionstamp([u8; 10]);
impl Versionstamp {
    /// get versionstamp
    pub fn versionstamp(self) -> [u8; 10] {
        self.0
    }
}

/// A future result of a `Transaction::watch`
pub struct TrxVersionstamp {
    // `TrxVersionstamp` resolves after `Transaction::commit`, so like `TrxWatch` it does not
    // not maintain a reference to the transaction.
    inner: NonTrxFuture,
}
impl Future for TrxVersionstamp {
    type Item = Versionstamp;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let r = try_ready!(self.inner.poll());
        // returning future should resolve to key
        let key = r.get_key()?;
        let mut buf: [u8; 10] = Default::default();
        buf.copy_from_slice(key);
        Ok(Async::Ready(Versionstamp(buf)))
    }
}

/// A future result of a `Transaction::watch`
pub struct TrxReadVersion {
    inner: TrxFuture,
}

impl Future for TrxReadVersion {
    type Item = i64;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        let (_trx, r) = try_ready!(self.inner.poll());
        let version = r.get_version()?;
        Ok(Async::Ready(version))
    }
}

/// Futures that could be outlive transaction.
struct NonTrxFuture {
    // Order of fields should not be changed, because Rust drops field top-to-bottom, and future
    // should be dropped before database.
    inner: FdbFuture,
    // We should maintain refcount for database, to make FdbFuture not outlive database.
    #[allow(unused)]
    db: Database,
}

impl NonTrxFuture {
    fn new(db: Database, f: *mut fdb::FDBFuture) -> Self {
        let inner = unsafe { FdbFuture::new(f) };
        Self { inner, db }
    }
}

impl Future for NonTrxFuture {
    type Item = FdbFutureResult;
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        self.inner.poll()
    }
}

/// Abstraction over `fdb_transaction_on_err`.
pub struct TrxErrFuture {
    // A future from `fdb_transaction_on_err`. It resolves to `Ok(_)` after backoff interval if
    // undering transaction should be retried, and resolved to `Err(e)` if the error should be
    // reported to the user without retry.
    inner: NonTrxFuture,
    err: Option<Error>,
}
impl TrxErrFuture {
    fn new(trx: Transaction, err: Error) -> Self {
        let inner = unsafe { fdb::fdb_transaction_on_error(trx.inner.inner, err.code()) };

        Self {
            inner: NonTrxFuture::new(trx.into_database(), inner),
            err: Some(err),
        }
    }
}

impl Future for TrxErrFuture {
    type Item = Error;
    type Error = Error;
    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        try_ready!(self.inner.poll());
        let mut e = self.err.take().expect("should not poll after ready");
        e.set_should_retry(true);
        Ok(Async::Ready(e))
    }
}

/// Futures for transaction, which supports retry/backoff with `Database::transact`.
struct TrxFuture {
    // Order of fields should not be changed, because Rust drops field top-to-bottom, and future
    // should be dropped before transaction.
    inner: FdbFuture,
    trx: Option<Transaction>,
    f_err: Option<TrxErrFuture>,
}

impl TrxFuture {
    fn new(trx: Transaction, f: *mut fdb::FDBFuture) -> Self {
        let inner = unsafe { FdbFuture::new(f) };
        Self {
            inner,
            trx: Some(trx),
            f_err: None,
        }
    }
}

impl Future for TrxFuture {
    type Item = (Transaction, FdbFutureResult);
    type Error = Error;

    fn poll(&mut self) -> std::result::Result<Async<Self::Item>, Self::Error> {
        if self.f_err.is_none() {
            match self.inner.poll() {
                Ok(Async::Ready(res)) => {
                    return Ok(Async::Ready((
                        self.trx.take().expect("should not poll after ready"),
                        res,
                    )))
                }
                Ok(Async::NotReady) => return Ok(Async::NotReady),
                Err(e) => {
                    // The transaction will be dropped on `TrxErrFuture::new`. The `trx` is a last
                    // reference for the transaction, undering transaction will be destroyed at
                    // this point.
                    let trx = self.trx.take().expect("should not poll after error");
                    self.f_err = Some(TrxErrFuture::new(trx, e));
                    return self.poll();
                }
            }
        }

        match self.f_err.as_mut().unwrap().poll() {
            Ok(Async::Ready(e)) => Err(e),
            Ok(Async::NotReady) => Ok(Async::NotReady),
            Err(e) => Err(e),
        }
    }
}