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use crate::{client::ClientBorrowMut, error::Error, prepare::Prepare, query::Query};

use super::Transaction;

/// The isolation level of a database transaction.
#[derive(Debug, Copy, Clone)]
#[non_exhaustive]
pub enum IsolationLevel {
    /// Equivalent to `ReadCommitted`.
    ReadUncommitted,
    /// An individual statement in the transaction will see rows committed before it began.
    ReadCommitted,
    /// All statements in the transaction will see the same view of rows committed before the first query in the
    /// transaction.
    RepeatableRead,
    /// The reads and writes in this transaction must be able to be committed as an atomic "unit" with respect to reads
    /// and writes of all other concurrent serializable transactions without interleaving.
    Serializable,
}

impl IsolationLevel {
    const PREFIX: &str = " ISOLATION LEVEL ";
    const READ_UNCOMMITTED: &str = "READ UNCOMMITTED,";
    const READ_COMMITTED: &str = "READ COMMITTED,";
    const REPEATABLE_READ: &str = "REPEATABLE READ,";
    const SERIALIZABLE: &str = "SERIALIZABLE,";

    fn write(self, str: &mut String) {
        str.reserve(const { Self::PREFIX.len() + Self::READ_UNCOMMITTED.len() });
        str.push_str(Self::PREFIX);
        str.push_str(match self {
            IsolationLevel::ReadUncommitted => Self::READ_UNCOMMITTED,
            IsolationLevel::ReadCommitted => Self::READ_COMMITTED,
            IsolationLevel::RepeatableRead => Self::REPEATABLE_READ,
            IsolationLevel::Serializable => Self::SERIALIZABLE,
        });
    }
}

/// A builder for database transactions.
pub struct TransactionBuilder {
    isolation_level: Option<IsolationLevel>,
    read_only: Option<bool>,
    deferrable: Option<bool>,
}

impl TransactionBuilder {
    pub(crate) fn new() -> Self {
        Self {
            isolation_level: None,
            read_only: None,
            deferrable: None,
        }
    }

    /// Sets the isolation level of the transaction.
    pub fn isolation_level(mut self, isolation_level: IsolationLevel) -> Self {
        self.isolation_level = Some(isolation_level);
        self
    }

    /// Sets the access mode of the transaction.
    pub fn read_only(mut self, read_only: bool) -> Self {
        self.read_only = Some(read_only);
        self
    }

    /// Sets the deferrability of the transaction.
    ///
    /// If the transaction is also serializable and read only, creation of the transaction may block, but when it
    /// completes the transaction is able to run with less overhead and a guarantee that it will not be aborted due to
    /// serialization failure.
    pub fn deferrable(mut self, deferrable: bool) -> Self {
        self.deferrable = Some(deferrable);
        self
    }

    /// Begins the transaction.
    ///
    /// The transaction will roll back by default - use the `commit` method to commit it.
    pub async fn begin<C>(self, cli: &mut C) -> Result<Transaction<C>, Error>
    where
        C: Prepare + Query + ClientBorrowMut,
    {
        // marker check to ensure exclusive borrowing Client. see ClientBorrowMut for detail
        let _c = cli._borrow_mut();

        let mut query = String::from("START TRANSACTION");

        let Self {
            isolation_level,
            read_only,
            deferrable,
        } = self;

        if let Some(isolation_level) = isolation_level {
            isolation_level.write(&mut query);
        }

        if let Some(read_only) = read_only {
            let s = if read_only { " READ ONLY," } else { " READ WRITE," };
            query.push_str(s);
        }

        if let Some(deferrable) = deferrable {
            let s = if deferrable { " DEFERRABLE" } else { " NOT DEFERRABLE" };
            query.push_str(s);
        }

        if query.ends_with(',') {
            query.pop();
        }

        cli._execute(&query, &[]).await.map(|_| Transaction::new(cli))
    }
}