//! Implementation of `trait Env` for `heed`.

//---------------------------------------------------------------------------------------------------- Import
use std::{
    cell::RefCell,
    num::NonZeroUsize,
    sync::{RwLock, RwLockReadGuard},
};

use heed::{DatabaseFlags, EnvFlags, EnvOpenOptions};

use crate::{
    backend::heed::{
        database::{HeedTableRo, HeedTableRw},
        storable::StorableHeed,
        types::HeedDb,
    },
    config::{Config, SyncMode},
    database::{DatabaseIter, DatabaseRo, DatabaseRw},
    env::{Env, EnvInner},
    error::{InitError, RuntimeError},
    key::{Key, KeyCompare},
    resize::ResizeAlgorithm,
    table::Table,
};

//---------------------------------------------------------------------------------------------------- ConcreteEnv
/// A strongly typed, concrete database environment, backed by `heed`.
pub struct ConcreteEnv {
    /// The actual database environment.
    ///
    /// # Why `RwLock`?
    /// We need mutual exclusive access to the environment for resizing.
    ///
    /// Using 2 atomics for mutual exclusion was considered:
    /// - `currently_resizing: AtomicBool`
    /// - `reader_count: AtomicUsize`
    ///
    /// This is how `monerod` does it:
    /// <https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L354-L355>
    ///
    /// `currently_resizing` would be set to `true` on resizes and
    /// `reader_count` would be spinned on until 0, at which point
    /// we are safe to resize.
    ///
    /// Although, 3 atomic operations (check atomic bool, `reader_count++`, `reader_count--`)
    /// turns out to be roughly as expensive as acquiring a non-contended `RwLock`,
    /// the CPU sleeping instead of spinning is much better too.
    ///
    /// # `unwrap()`
    /// This will be [`unwrap()`]ed everywhere.
    ///
    /// If lock is poisoned, we want all of Cuprate to panic.
    env: RwLock<heed::Env>,

    /// The configuration we were opened with
    /// (and in current use).
    pub(super) config: Config,
}

impl Drop for ConcreteEnv {
    fn drop(&mut self) {
        // INVARIANT: drop(ConcreteEnv) must sync.
        //
        // SOMEDAY:
        // "if the environment has the MDB_NOSYNC flag set the flushes will be omitted,
        // and with MDB_MAPASYNC they will be asynchronous."
        // <http://www.lmdb.tech/doc/group__mdb.html#ga85e61f05aa68b520cc6c3b981dba5037>
        //
        // We need to do `mdb_env_set_flags(&env, MDB_NOSYNC|MDB_ASYNCMAP, 0)`
        // to clear the no sync and async flags such that the below `self.sync()`
        // _actually_ synchronously syncs.
        if let Err(_e) = Env::sync(self) {
            // TODO: log error?
        }

        // TODO: log that we are dropping the database.

        // TODO: use tracing.
        // <https://github.com/LMDB/lmdb/blob/b8e54b4c31378932b69f1298972de54a565185b1/libraries/liblmdb/lmdb.h#L49-L61>
        let result = self.env.read().unwrap().clear_stale_readers();
        match result {
            Ok(n) => println!("LMDB stale readers cleared: {n}"),
            Err(e) => println!("LMDB stale reader clear error: {e:?}"),
        }
    }
}

//---------------------------------------------------------------------------------------------------- Env Impl
impl Env for ConcreteEnv {
    const MANUAL_RESIZE: bool = true;
    const SYNCS_PER_TX: bool = false;
    type EnvInner<'env> = RwLockReadGuard<'env, heed::Env>;
    type TxRo<'tx> = heed::RoTxn<'tx>;

    /// HACK:
    /// `heed::RwTxn` is wrapped in `RefCell` to allow:
    /// - opening a database with only a `&` to it
    /// - allowing 1 write tx to open multiple tables
    ///
    /// Our mutable accesses are safe and will not panic as:
    /// - Write transactions are `!Sync`
    /// - A table operation does not hold a reference to the inner cell
    ///   once the call is over
    /// - The function to manipulate the table takes the same type
    ///   of reference that the `RefCell` gets for that function
    ///
    /// Also see:
    /// - <https://github.com/Cuprate/cuprate/pull/102#discussion_r1548695610>
    /// - <https://github.com/Cuprate/cuprate/pull/104>
    type TxRw<'tx> = RefCell<heed::RwTxn<'tx>>;

    #[cold]
    #[inline(never)] // called once.
    fn open(config: Config) -> Result<Self, InitError> {
        // <https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1324>

        let mut env_open_options = EnvOpenOptions::new();

        // Map our `Config` sync mode to the LMDB environment flags.
        //
        // <https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1324>
        let flags = match config.sync_mode {
            SyncMode::Safe => EnvFlags::empty(),
            SyncMode::Async => EnvFlags::MAP_ASYNC,
            SyncMode::Fast => EnvFlags::NO_SYNC | EnvFlags::WRITE_MAP | EnvFlags::MAP_ASYNC,
            // SOMEDAY: dynamic syncs are not implemented.
            SyncMode::FastThenSafe | SyncMode::Threshold(_) => unimplemented!(),
        };

        // SAFETY: the flags we're setting are 'unsafe'
        // from a data durability perspective, although,
        // the user config wanted this.
        //
        // MAYBE: We may need to open/create tables with certain flags
        // <https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1324>
        // MAYBE: Set comparison functions for certain tables
        // <https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1324>
        unsafe {
            env_open_options.flags(flags);
        }

        // Set the memory map size to
        // (current disk size) + (a bit of leeway)
        // to account for empty databases where we
        // need to write same tables.
        #[expect(clippy::cast_possible_truncation, reason = "only 64-bit targets")]
        let disk_size_bytes = match std::fs::File::open(&config.db_file) {
            Ok(file) => file.metadata()?.len() as usize,
            // The database file doesn't exist, 0 bytes.
            Err(io_err) if io_err.kind() == std::io::ErrorKind::NotFound => 0,
            Err(io_err) => return Err(io_err.into()),
        };
        // Add leeway space.
        let memory_map_size = crate::resize::fixed_bytes(disk_size_bytes, 1_000_000 /* 1MB */);
        env_open_options.map_size(memory_map_size.get());

        // Set the max amount of database tables.
        // We know at compile time how many tables there are.
        // SOMEDAY: ...how many?
        env_open_options.max_dbs(32);

        // LMDB documentation:
        // ```
        // Number of slots in the reader table.
        // This value was chosen somewhat arbitrarily. 126 readers plus a
        // couple mutexes fit exactly into 8KB on my development machine.
        // ```
        // <https://github.com/LMDB/lmdb/blob/b8e54b4c31378932b69f1298972de54a565185b1/libraries/liblmdb/mdb.c#L794-L799>
        //
        // So, we're going to be following these rules:
        // - Use at least 126 reader threads
        // - Add 16 extra reader threads if <126
        //
        // FIXME: This behavior is from `monerod`:
        // <https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1324>
        // I believe this could be adjusted percentage-wise so very high
        // thread PCs can benefit from something like (cuprated + anything that uses the DB in the future).
        // For now:
        // - No other program using our DB exists
        // - Almost no-one has a 126+ thread CPU
        let reader_threads = u32::try_from(config.reader_threads.get()).unwrap_or(u32::MAX);
        env_open_options.max_readers(if reader_threads < 110 {
            126
        } else {
            reader_threads.saturating_add(16)
        });

        // Create the database directory if it doesn't exist.
        std::fs::create_dir_all(config.db_directory())?;
        // Open the environment in the user's PATH.
        // SAFETY: LMDB uses a memory-map backed file.
        // <https://docs.rs/heed/0.20.0/heed/struct.EnvOpenOptions.html#method.open>
        let env = unsafe { env_open_options.open(config.db_directory())? };

        Ok(Self {
            env: RwLock::new(env),
            config,
        })
    }

    fn config(&self) -> &Config {
        &self.config
    }

    fn sync(&self) -> Result<(), RuntimeError> {
        Ok(self.env.read().unwrap().force_sync()?)
    }

    fn resize_map(&self, resize_algorithm: Option<ResizeAlgorithm>) -> NonZeroUsize {
        let resize_algorithm = resize_algorithm.unwrap_or_else(|| self.config().resize_algorithm);

        let current_size_bytes = self.current_map_size();
        let new_size_bytes = resize_algorithm.resize(current_size_bytes);

        // SAFETY:
        // Resizing requires that we have
        // exclusive access to the database environment.
        // Our `heed::Env` is wrapped within a `RwLock`,
        // and we have a WriteGuard to it, so we're safe.
        //
        // <http://www.lmdb.tech/doc/group__mdb.html#gaa2506ec8dab3d969b0e609cd82e619e5>
        unsafe {
            // INVARIANT: `resize()` returns a valid `usize` to resize to.
            self.env
                .write()
                .unwrap()
                .resize(new_size_bytes.get())
                .unwrap();
        }

        new_size_bytes
    }

    #[inline]
    fn current_map_size(&self) -> usize {
        self.env.read().unwrap().info().map_size
    }

    #[inline]
    fn env_inner(&self) -> Self::EnvInner<'_> {
        self.env.read().unwrap()
    }
}

//---------------------------------------------------------------------------------------------------- EnvInner Impl
impl<'env> EnvInner<'env> for RwLockReadGuard<'env, heed::Env>
where
    Self: 'env,
{
    type Ro<'a> = heed::RoTxn<'a>;

    type Rw<'a> = RefCell<heed::RwTxn<'a>>;

    #[inline]
    fn tx_ro(&self) -> Result<Self::Ro<'_>, RuntimeError> {
        Ok(self.read_txn()?)
    }

    #[inline]
    fn tx_rw(&self) -> Result<Self::Rw<'_>, RuntimeError> {
        Ok(RefCell::new(self.write_txn()?))
    }

    #[inline]
    fn open_db_ro<T: Table>(
        &self,
        tx_ro: &Self::Ro<'_>,
    ) -> Result<impl DatabaseRo<T> + DatabaseIter<T>, RuntimeError> {
        // Open up a read-only database using our table's const metadata.
        //
        // INVARIANT: LMDB caches the ordering / comparison function from [`EnvInner::create_db`],
        // and we're relying on that since we aren't setting that here.
        // <https://github.com/Cuprate/cuprate/pull/198#discussion_r1659422277>
        Ok(HeedTableRo {
            db: self
                .open_database(tx_ro, Some(T::NAME))?
                .ok_or(RuntimeError::TableNotFound)?,
            tx_ro,
        })
    }

    #[inline]
    fn open_db_rw<T: Table>(
        &self,
        tx_rw: &Self::Rw<'_>,
    ) -> Result<impl DatabaseRw<T>, RuntimeError> {
        // Open up a read/write database using our table's const metadata.
        //
        // INVARIANT: LMDB caches the ordering / comparison function from [`EnvInner::create_db`],
        // and we're relying on that since we aren't setting that here.
        // <https://github.com/Cuprate/cuprate/pull/198#discussion_r1659422277>
        Ok(HeedTableRw {
            db: self.create_database(&mut tx_rw.borrow_mut(), Some(T::NAME))?,
            tx_rw,
        })
    }

    fn create_db<T: Table>(&self, tx_rw: &Self::Rw<'_>) -> Result<(), RuntimeError> {
        // Create a database using our:
        // - [`Table`]'s const metadata.
        // - (potentially) our [`Key`] comparison function
        let mut tx_rw = tx_rw.borrow_mut();
        let mut db = self.database_options();
        db.name(T::NAME);

        // Set the key comparison behavior.
        match <T::Key>::KEY_COMPARE {
            // Use LMDB's default comparison function.
            KeyCompare::Default => {
                db.create(&mut tx_rw)?;
            }

            // Instead of setting a custom [`heed::Comparator`],
            // use this LMDB flag; it is ~10% faster.
            KeyCompare::Number => {
                db.flags(DatabaseFlags::INTEGER_KEY).create(&mut tx_rw)?;
            }

            // Use a custom comparison function if specified.
            KeyCompare::Custom(_) => {
                db.key_comparator::<StorableHeed<T::Key>>()
                    .create(&mut tx_rw)?;
            }
        }

        Ok(())
    }

    #[inline]
    fn clear_db<T: Table>(&self, tx_rw: &mut Self::Rw<'_>) -> Result<(), RuntimeError> {
        let tx_rw = tx_rw.get_mut();

        // Open the table. We don't care about flags or key
        // comparison behavior since we're clearing it anyway.
        let db: HeedDb<T::Key, T::Value> = self
            .open_database(tx_rw, Some(T::NAME))?
            .ok_or(RuntimeError::TableNotFound)?;

        db.clear(tx_rw)?;

        Ok(())
    }
}

//---------------------------------------------------------------------------------------------------- Tests
#[cfg(test)]
mod tests {}