use diesel::connection::TransactionManagerStatus;
use diesel::connection::{
    InTransactionStatus, TransactionDepthChange, ValidTransactionManagerStatus,
};
use diesel::result::Error;
use diesel::QueryResult;
use scoped_futures::ScopedBoxFuture;
use std::borrow::Cow;
use std::num::NonZeroU32;

use crate::AsyncConnection;
// TODO: refactor this to share more code with diesel

/// Manages the internal transaction state for a connection.
///
/// You will not need to interact with this trait, unless you are writing an
/// implementation of [`AsyncConnection`].
#[async_trait::async_trait]
pub trait TransactionManager<Conn: AsyncConnection>: Send {
    /// Data stored as part of the connection implementation
    /// to track the current transaction state of a connection
    type TransactionStateData;

    /// Begin a new transaction or savepoint
    ///
    /// If the transaction depth is greater than 0,
    /// this should create a savepoint instead.
    /// This function is expected to increment the transaction depth by 1.
    async fn begin_transaction(conn: &mut Conn) -> QueryResult<()>;

    /// Rollback the inner-most transaction or savepoint
    ///
    /// If the transaction depth is greater than 1,
    /// this should rollback to the most recent savepoint.
    /// This function is expected to decrement the transaction depth by 1.
    async fn rollback_transaction(conn: &mut Conn) -> QueryResult<()>;

    /// Commit the inner-most transaction or savepoint
    ///
    /// If the transaction depth is greater than 1,
    /// this should release the most recent savepoint.
    /// This function is expected to decrement the transaction depth by 1.
    async fn commit_transaction(conn: &mut Conn) -> QueryResult<()>;

    /// Fetch the current transaction status as mutable
    ///
    /// Used to ensure that `begin_test_transaction` is not called when already
    /// inside of a transaction, and that operations are not run in a `InError`
    /// transaction manager.
    #[doc(hidden)]
    fn transaction_manager_status_mut(conn: &mut Conn) -> &mut TransactionManagerStatus;

    /// Executes the given function inside of a database transaction
    ///
    /// Each implementation of this function needs to fulfill the documented
    /// behaviour of [`AsyncConnection::transaction`]
    async fn transaction<'a, F, R, E>(conn: &mut Conn, callback: F) -> Result<R, E>
    where
        F: for<'r> FnOnce(&'r mut Conn) -> ScopedBoxFuture<'a, 'r, Result<R, E>> + Send + 'a,
        E: From<Error> + Send,
        R: Send,
    {
        Self::begin_transaction(conn).await?;
        match callback(&mut *conn).await {
            Ok(value) => {
                Self::commit_transaction(conn).await?;
                Ok(value)
            }
            Err(user_error) => match Self::rollback_transaction(conn).await {
                Ok(()) => Err(user_error),
                Err(Error::BrokenTransactionManager) => {
                    // In this case we are probably more interested by the
                    // original error, which likely caused this
                    Err(user_error)
                }
                Err(rollback_error) => Err(rollback_error.into()),
            },
        }
    }

    /// This methods checks if the connection manager is considered to be broken
    /// by connection pool implementations
    ///
    /// A connection manager is considered to be broken by default if it either
    /// contains an open transaction (because you don't want to have connections
    /// with open transactions in your pool) or when the transaction manager is
    /// in an error state.
    #[doc(hidden)]
    fn is_broken_transaction_manager(conn: &mut Conn) -> bool {
        match Self::transaction_manager_status_mut(conn).transaction_state() {
            // all transactions are closed
            // so we don't consider this connection broken
            Ok(ValidTransactionManagerStatus {
                in_transaction: None,
                ..
            }) => false,
            // The transaction manager is in an error state
            // Therefore we consider this connection broken
            Err(_) => true,
            // The transaction manager contains a open transaction
            // we do consider this connection broken
            // if that transaction was not opened by `begin_test_transaction`
            Ok(ValidTransactionManagerStatus {
                in_transaction: Some(s),
                ..
            }) => !s.test_transaction,
        }
    }
}

/// An implementation of `TransactionManager` which can be used for backends
/// which use ANSI standard syntax for savepoints such as SQLite and PostgreSQL.
#[derive(Default, Debug)]
pub struct AnsiTransactionManager {
    pub(crate) status: TransactionManagerStatus,
}

// /// Status of the transaction manager
// #[derive(Debug)]
// pub enum TransactionManagerStatus {
//     /// Valid status, the manager can run operations
//     Valid(ValidTransactionManagerStatus),
//     /// Error status, probably following a broken connection. The manager will no longer run operations
//     InError,
// }

// impl Default for TransactionManagerStatus {
//     fn default() -> Self {
//         TransactionManagerStatus::Valid(ValidTransactionManagerStatus::default())
//     }
// }

// impl TransactionManagerStatus {
//     /// Returns the transaction depth if the transaction manager's status is valid, or returns
//     /// [`Error::BrokenTransactionManager`] if the transaction manager is in error.
//     pub fn transaction_depth(&self) -> QueryResult<Option<NonZeroU32>> {
//         match self {
//             TransactionManagerStatus::Valid(valid_status) => Ok(valid_status.transaction_depth()),
//             TransactionManagerStatus::InError => Err(Error::BrokenTransactionManager),
//         }
//     }

//     /// If in transaction and transaction manager is not broken, registers that the
//     /// connection can not be used anymore until top-level transaction is rolled back
//     pub(crate) fn set_top_level_transaction_requires_rollback(&mut self) {
//         if let TransactionManagerStatus::Valid(ValidTransactionManagerStatus {
//             in_transaction:
//                 Some(InTransactionStatus {
//                     top_level_transaction_requires_rollback,
//                     ..
//                 }),
//         }) = self
//         {
//             *top_level_transaction_requires_rollback = true;
//         }
//     }

//     /// Sets the transaction manager status to InError
//     ///
//     /// Subsequent attempts to use transaction-related features will result in a
//     /// [`Error::BrokenTransactionManager`] error
//     pub fn set_in_error(&mut self) {
//         *self = TransactionManagerStatus::InError
//     }

//     fn transaction_state(&mut self) -> QueryResult<&mut ValidTransactionManagerStatus> {
//         match self {
//             TransactionManagerStatus::Valid(valid_status) => Ok(valid_status),
//             TransactionManagerStatus::InError => Err(Error::BrokenTransactionManager),
//         }
//     }

//     pub(crate) fn set_test_transaction_flag(&mut self) {
//         if let TransactionManagerStatus::Valid(ValidTransactionManagerStatus {
//             in_transaction: Some(s),
//         }) = self
//         {
//             s.test_transaction = true;
//         }
//     }
// }

// /// Valid transaction status for the manager. Can return the current transaction depth
// #[allow(missing_copy_implementations)]
// #[derive(Debug, Default)]
// pub struct ValidTransactionManagerStatus {
//     in_transaction: Option<InTransactionStatus>,
// }

// #[allow(missing_copy_implementations)]
// #[derive(Debug)]
// struct InTransactionStatus {
//     transaction_depth: NonZeroU32,
//     top_level_transaction_requires_rollback: bool,
//     test_transaction: bool,
// }

// impl ValidTransactionManagerStatus {
//     /// Return the current transaction depth
//     ///
//     /// This value is `None` if no current transaction is running
//     /// otherwise the number of nested transactions is returned.
//     pub fn transaction_depth(&self) -> Option<NonZeroU32> {
//         self.in_transaction.as_ref().map(|it| it.transaction_depth)
//     }

//     /// Update the transaction depth by adding the value of the `transaction_depth_change` parameter if the `query` is
//     /// `Ok(())`
//     pub fn change_transaction_depth(
//         &mut self,
//         transaction_depth_change: TransactionDepthChange,
//     ) -> QueryResult<()> {
//         match (&mut self.in_transaction, transaction_depth_change) {
//             (Some(in_transaction), TransactionDepthChange::IncreaseDepth) => {
//                 // Can be replaced with saturating_add directly on NonZeroU32 once
//                 // <https://github.com/rust-lang/rust/issues/84186> is stable
//                 in_transaction.transaction_depth =
//                     NonZeroU32::new(in_transaction.transaction_depth.get().saturating_add(1))
//                         .expect("nz + nz is always non-zero");
//                 Ok(())
//             }
//             (Some(in_transaction), TransactionDepthChange::DecreaseDepth) => {
//                 // This sets `transaction_depth` to `None` as soon as we reach zero
//                 match NonZeroU32::new(in_transaction.transaction_depth.get() - 1) {
//                     Some(depth) => in_transaction.transaction_depth = depth,
//                     None => self.in_transaction = None,
//                 }
//                 Ok(())
//             }
//             (None, TransactionDepthChange::IncreaseDepth) => {
//                 self.in_transaction = Some(InTransactionStatus {
//                     transaction_depth: NonZeroU32::new(1).expect("1 is non-zero"),
//                     top_level_transaction_requires_rollback: false,
//                     test_transaction: false,
//                 });
//                 Ok(())
//             }
//             (None, TransactionDepthChange::DecreaseDepth) => {
//                 // We screwed up something somewhere
//                 // we cannot decrease the transaction count if
//                 // we are not inside a transaction
//                 Err(Error::NotInTransaction)
//             }
//         }
//     }
// }

// /// Represents a change to apply to the depth of a transaction
// #[derive(Debug, Clone, Copy)]
// pub enum TransactionDepthChange {
//     /// Increase the depth of the transaction (corresponds to `BEGIN` or `SAVEPOINT`)
//     IncreaseDepth,
//     /// Decreases the depth of the transaction (corresponds to `COMMIT`/`RELEASE SAVEPOINT` or `ROLLBACK`)
//     DecreaseDepth,
// }

impl AnsiTransactionManager {
    fn get_transaction_state<Conn>(
        conn: &mut Conn,
    ) -> QueryResult<&mut ValidTransactionManagerStatus>
    where
        Conn: AsyncConnection<TransactionManager = Self>,
    {
        conn.transaction_state().status.transaction_state()
    }

    /// Begin a transaction with custom SQL
    ///
    /// This is used by connections to implement more complex transaction APIs
    /// to set things such as isolation levels.
    /// Returns an error if already inside of a transaction.
    pub async fn begin_transaction_sql<Conn>(conn: &mut Conn, sql: &str) -> QueryResult<()>
    where
        Conn: AsyncConnection<TransactionManager = Self>,
    {
        let state = Self::get_transaction_state(conn)?;
        match state.transaction_depth() {
            None => {
                conn.batch_execute(sql).await?;
                Self::get_transaction_state(conn)?
                    .change_transaction_depth(TransactionDepthChange::IncreaseDepth)?;
                Ok(())
            }
            Some(_depth) => Err(Error::AlreadyInTransaction),
        }
    }
}

#[async_trait::async_trait]
impl<Conn> TransactionManager<Conn> for AnsiTransactionManager
where
    Conn: AsyncConnection<TransactionManager = Self>,
{
    type TransactionStateData = Self;

    async fn begin_transaction(conn: &mut Conn) -> QueryResult<()> {
        let transaction_state = Self::get_transaction_state(conn)?;
        let start_transaction_sql = match transaction_state.transaction_depth() {
            None => Cow::from("BEGIN"),
            Some(transaction_depth) => {
                Cow::from(format!("SAVEPOINT diesel_savepoint_{transaction_depth}"))
            }
        };
        conn.batch_execute(&start_transaction_sql).await?;
        Self::get_transaction_state(conn)?
            .change_transaction_depth(TransactionDepthChange::IncreaseDepth)?;

        Ok(())
    }

    async fn rollback_transaction(conn: &mut Conn) -> QueryResult<()> {
        let transaction_state = Self::get_transaction_state(conn)?;

        let (
            (rollback_sql, rolling_back_top_level),
            requires_rollback_maybe_up_to_top_level_before_execute,
        ) = match transaction_state.in_transaction {
            Some(ref in_transaction) => (
                match in_transaction.transaction_depth.get() {
                    1 => (Cow::Borrowed("ROLLBACK"), true),
                    depth_gt1 => (
                        Cow::Owned(format!(
                            "ROLLBACK TO SAVEPOINT diesel_savepoint_{}",
                            depth_gt1 - 1
                        )),
                        false,
                    ),
                },
                in_transaction.requires_rollback_maybe_up_to_top_level,
            ),
            None => return Err(Error::NotInTransaction),
        };

        match conn.batch_execute(&rollback_sql).await {
            Ok(()) => {
                match Self::get_transaction_state(conn)?
                    .change_transaction_depth(TransactionDepthChange::DecreaseDepth)
                {
                    Ok(()) => {}
                    Err(Error::NotInTransaction) if rolling_back_top_level => {
                        // Transaction exit may have already been detected by connection
                        // implementation. It's fine.
                    }
                    Err(e) => return Err(e),
                }
                Ok(())
            }
            Err(rollback_error) => {
                let tm_status = Self::transaction_manager_status_mut(conn);
                match tm_status {
                    TransactionManagerStatus::Valid(ValidTransactionManagerStatus {
                        in_transaction:
                            Some(InTransactionStatus {
                                transaction_depth,
                                requires_rollback_maybe_up_to_top_level,
                                ..
                            }),
                        ..
                    }) if transaction_depth.get() > 1 => {
                        // A savepoint failed to rollback - we may still attempt to repair
                        // the connection by rolling back higher levels.

                        // To make it easier on the user (that they don't have to really
                        // look at actual transaction depth and can just rely on the number
                        // of times they have called begin/commit/rollback) we still
                        // decrement here:
                        *transaction_depth = NonZeroU32::new(transaction_depth.get() - 1)
                            .expect("Depth was checked to be > 1");
                        *requires_rollback_maybe_up_to_top_level = true;
                        if requires_rollback_maybe_up_to_top_level_before_execute {
                            // In that case, we tolerate that savepoint releases fail
                            // -> we should ignore errors
                            return Ok(());
                        }
                    }
                    TransactionManagerStatus::Valid(ValidTransactionManagerStatus {
                        in_transaction: None,
                        ..
                    }) => {
                        // we would have returned `NotInTransaction` if that was already the state
                        // before we made our call
                        // => Transaction manager status has been fixed by the underlying connection
                        // so we don't need to set_in_error
                    }
                    _ => tm_status.set_in_error(),
                }
                Err(rollback_error)
            }
        }
    }

    /// If the transaction fails to commit due to a `SerializationFailure` or a
    /// `ReadOnlyTransaction` a rollback will be attempted. If the rollback succeeds,
    /// the original error will be returned, otherwise the error generated by the rollback
    /// will be returned. In the second case the connection will be considered broken
    /// as it contains a uncommitted unabortable open transaction.
    async fn commit_transaction(conn: &mut Conn) -> QueryResult<()> {
        let transaction_state = Self::get_transaction_state(conn)?;
        let transaction_depth = transaction_state.transaction_depth();
        let (commit_sql, committing_top_level) = match transaction_depth {
            None => return Err(Error::NotInTransaction),
            Some(transaction_depth) if transaction_depth.get() == 1 => {
                (Cow::Borrowed("COMMIT"), true)
            }
            Some(transaction_depth) => (
                Cow::Owned(format!(
                    "RELEASE SAVEPOINT diesel_savepoint_{}",
                    transaction_depth.get() - 1
                )),
                false,
            ),
        };
        match conn.batch_execute(&commit_sql).await {
            Ok(()) => {
                match Self::get_transaction_state(conn)?
                    .change_transaction_depth(TransactionDepthChange::DecreaseDepth)
                {
                    Ok(()) => {}
                    Err(Error::NotInTransaction) if committing_top_level => {
                        // Transaction exit may have already been detected by connection.
                        // It's fine
                    }
                    Err(e) => return Err(e),
                }
                Ok(())
            }
            Err(commit_error) => {
                if let TransactionManagerStatus::Valid(ValidTransactionManagerStatus {
                    in_transaction:
                        Some(InTransactionStatus {
                            requires_rollback_maybe_up_to_top_level: true,
                            ..
                        }),
                    ..
                }) = conn.transaction_state().status
                {
                    match Self::rollback_transaction(conn).await {
                        Ok(()) => {}
                        Err(rollback_error) => {
                            conn.transaction_state().status.set_in_error();
                            return Err(Error::RollbackErrorOnCommit {
                                rollback_error: Box::new(rollback_error),
                                commit_error: Box::new(commit_error),
                            });
                        }
                    }
                }
                Err(commit_error)
            }
        }
    }

    fn transaction_manager_status_mut(conn: &mut Conn) -> &mut TransactionManagerStatus {
        &mut conn.transaction_state().status
    }
}