//! redb adapter.
//!
//! Each relation gets a redb table named after it, keyed by `u64` row IDs.
//! A reserved table named `__meta`, keyed by relation name, carries per-relation
//! metadata (arity and next synthetic row ID).
//!
//! [`RedbTx`] wraps a real `redb::WriteTransaction`. Inserts go through the
//! transaction; [`Transaction::commit`] commits it; dropping the tx without
//! commit rolls back (redb's `WriteTransaction` drops the pending writes).

use std::collections::HashMap;

use redb::{Database, ReadableTable, TableDefinition, WriteTransaction};

use crate::codec::{decode_meta, decode_row, encode_meta, encode_row};
use crate::id::RowId;
use crate::value::Value;
use crate::{backend, CommittedTx, RowStream, Storage, StorageError, Transaction};

const META_TABLE: &str = "__meta";

fn meta_def() -> TableDefinition<'static, &'static str, &'static [u8]> {
    TableDefinition::new(META_TABLE)
}

fn rows_def(name: &str) -> TableDefinition<'_, u64, &'static [u8]> {
    TableDefinition::new(name)
}

/// redb-backed [`Storage`] implementation.
pub struct RedbStorage {
    db: Database,
}

impl RedbStorage {
    /// Open or create a redb database at `path`.
    ///
    /// # Errors
    /// Returns [`StorageError::Backend`] if redb fails to open the file.
    pub fn open(path: impl AsRef<std::path::Path>) -> Result<Self, StorageError> {
        let db = Database::create(path).map_err(backend)?;
        Ok(Self { db })
    }
}

impl Storage for RedbStorage {
    fn create_relation(&mut self, name: &str, arity: usize) -> Result<(), StorageError> {
        if name == META_TABLE {
            return Err(StorageError::Validation(format!(
                "relation name '{name}' is reserved"
            )));
        }
        let Ok(arity_u32) = u32::try_from(arity) else {
            unreachable!("arity exceeds u32::MAX")
        };
        let txn = self.db.begin_write().map_err(backend)?;
        {
            let mut meta = txn.open_table(meta_def()).map_err(backend)?;
            if meta.get(name).map_err(backend)?.is_some() {
                return Err(StorageError::RelationExists(name.to_string()));
            }
            let encoded = encode_meta(arity_u32, 0);
            meta.insert(name, &encoded[..]).map_err(backend)?;
            let _ = txn.open_table(rows_def(name)).map_err(backend)?;
        }
        txn.commit().map_err(backend)?;
        Ok(())
    }

    fn arity(&self, name: &str) -> Result<usize, StorageError> {
        let txn = self.db.begin_read().map_err(backend)?;
        let meta = txn.open_table(meta_def()).map_err(backend)?;
        let entry = meta
            .get(name)
            .map_err(backend)?
            .ok_or_else(|| StorageError::RelationNotFound(name.to_string()))?;
        let (arity, _) = decode_meta(entry.value())?;
        Ok(arity as usize)
    }

    fn scan_iter<'a>(&'a self, name: &str) -> Result<RowStream<'a>, StorageError> {
        let txn = self.db.begin_read().map_err(backend)?;
        let meta = txn.open_table(meta_def()).map_err(backend)?;
        if meta.get(name).map_err(backend)?.is_none() {
            return Err(StorageError::RelationNotFound(name.to_string()));
        }
        let table = txn.open_table(rows_def(name)).map_err(backend)?;
        let mut rows = Vec::new();
        for entry in table.iter().map_err(backend)? {
            let (key, value) = entry.map_err(backend)?;
            let id = RowId::from(key.value());
            rows.push((id, decode_row(value.value())?));
        }
        Ok(Box::new(rows.into_iter().map(Ok)))
    }

    fn transaction<'a>(&'a mut self) -> Result<Box<dyn Transaction + 'a>, StorageError> {
        let wtxn = self.db.begin_write().map_err(backend)?;
        Ok(Box::new(RedbTx {
            wtxn: Some(wtxn),
            next_ids: HashMap::new(),
        }))
    }
}

pub(crate) struct RedbTx {
    wtxn: Option<WriteTransaction>,
    next_ids: HashMap<String, (u32, u64)>,
}

impl RedbTx {
    /// Borrow the live `WriteTransaction`. Panics if commit already
    /// consumed it: unreachable via the public API since
    /// [`Transaction::commit`] consumes the boxed tx.
    fn live(&self) -> &WriteTransaction {
        match self.wtxn.as_ref() {
            Some(t) => t,
            None => unreachable!("transaction was already committed"),
        }
    }

    fn meta_for(&mut self, name: &str) -> Result<(u32, u64), StorageError> {
        if let Some(&entry) = self.next_ids.get(name) {
            return Ok(entry);
        }
        let decoded = {
            let meta = self.live().open_table(meta_def()).map_err(backend)?;
            let entry = meta
                .get(name)
                .map_err(backend)?
                .ok_or_else(|| StorageError::RelationNotFound(name.to_string()))?;
            decode_meta(entry.value())?
        };
        self.next_ids.insert(name.to_string(), decoded);
        Ok(decoded)
    }
}

fn row_id_as_u64(id: &RowId) -> Result<u64, StorageError> {
    let bytes = id.as_bytes();
    if bytes.len() != 8 {
        return Err(StorageError::Backend(
            format!("redb row id must be 8 bytes, got {}", bytes.len()).into(),
        ));
    }
    let mut buf = [0u8; 8];
    buf.copy_from_slice(bytes);
    Ok(u64::from_be_bytes(buf))
}

impl Transaction for RedbTx {
    fn insert(&mut self, name: &str, row: Vec<Value>) -> Result<RowId, StorageError> {
        let (arity, next_id) = self.meta_for(name)?;
        if row.len() != arity as usize {
            return Err(StorageError::ArityMismatch {
                expected: arity as usize,
                got: row.len(),
            });
        }
        {
            let mut rows = self.live().open_table(rows_def(name)).map_err(backend)?;
            let encoded = encode_row(&row);
            rows.insert(next_id, &encoded[..]).map_err(backend)?;
        }
        self.next_ids.insert(name.to_string(), (arity, next_id + 1));
        Ok(RowId::from(next_id))
    }

    fn delete(&mut self, name: &str, id: &RowId) -> Result<(), StorageError> {
        let key = row_id_as_u64(id)?;
        let wtxn = self.live();
        // Verify the relation exists by checking meta.
        let meta = wtxn.open_table(meta_def()).map_err(backend)?;
        if meta.get(name).map_err(backend)?.is_none() {
            return Err(StorageError::RelationNotFound(name.to_string()));
        }
        drop(meta);
        let mut rows = wtxn.open_table(rows_def(name)).map_err(backend)?;
        let _ = rows.remove(key).map_err(backend)?;
        Ok(())
    }

    fn commit(self: Box<Self>) -> Result<CommittedTx, StorageError> {
        let mut this = self;
        let Some(wtxn) = this.wtxn.take() else {
            unreachable!("transaction was already committed")
        };
        {
            let mut meta = wtxn.open_table(meta_def()).map_err(backend)?;
            for (name, (arity, next_id)) in this.next_ids.drain() {
                let encoded = encode_meta(arity, next_id);
                meta.insert(name.as_str(), &encoded[..]).map_err(backend)?;
            }
        }
        wtxn.commit().map_err(backend)?;
        Ok(CommittedTx::empty())
    }
}

#[cfg(test)]
mod tests {
    use super::{backend, RedbStorage};
    use crate::value::Value;
    use crate::{Storage, StorageError};

    fn i(x: i64) -> Value {
        Value::Int(x)
    }

    fn s(x: &str) -> Value {
        Value::Str(x.to_string())
    }

    fn open_temp() -> Result<RedbStorage, StorageError> {
        let dir = tempfile::tempdir().map_err(backend)?;
        let path = dir.path().join("test.redb");
        let storage = RedbStorage::open(&path)?;
        std::mem::forget(dir);
        Ok(storage)
    }

    #[test]
    fn create_insert_scan_roundtrip() -> Result<(), StorageError> {
        let mut storage = open_temp()?;
        storage.create_relation("edge", 2)?;
        let id0 = storage.insert("edge", vec![i(1), i(2)])?;
        let id1 = storage.insert("edge", vec![s("hello"), i(7)])?;
        let rows = storage.scan("edge")?;
        assert_eq!(
            rows,
            vec![(id0, vec![i(1), i(2)]), (id1, vec![s("hello"), i(7)])],
        );
        assert_eq!(storage.arity("edge")?, 2);
        Ok(())
    }

    #[test]
    fn batched_inserts_share_one_commit() -> Result<(), StorageError> {
        let mut storage = open_temp()?;
        storage.create_relation("edge", 2)?;
        let (a, b) = {
            let mut tx = storage.transaction()?;
            let a = tx.insert("edge", vec![i(1), i(2)])?;
            let b = tx.insert("edge", vec![i(3), i(4)])?;
            tx.commit()?;
            (a, b)
        };
        let rows = storage.scan("edge")?;
        assert_eq!(rows, vec![(a, vec![i(1), i(2)]), (b, vec![i(3), i(4)])]);
        Ok(())
    }

    #[test]
    fn dropped_transaction_is_rolled_back() -> Result<(), StorageError> {
        let mut storage = open_temp()?;
        storage.create_relation("edge", 2)?;
        {
            let mut tx = storage.transaction()?;
            tx.insert("edge", vec![i(1), i(2)])?;
        }
        assert!(storage.scan("edge")?.is_empty());
        Ok(())
    }

    #[test]
    fn delete_removes_row() -> Result<(), StorageError> {
        let mut storage = open_temp()?;
        storage.create_relation("edge", 1)?;
        let a = storage.insert("edge", vec![i(1)])?;
        let b = storage.insert("edge", vec![i(2)])?;
        storage.delete("edge", &a)?;
        let rows = storage.scan("edge")?;
        assert_eq!(rows, vec![(b, vec![i(2)])]);
        storage.delete("edge", &a)?;
        Ok(())
    }

    #[test]
    fn duplicate_create_returns_err() -> Result<(), StorageError> {
        let mut storage = open_temp()?;
        storage.create_relation("edge", 2)?;
        assert!(matches!(
            storage.create_relation("edge", 2),
            Err(StorageError::RelationExists(_))
        ));
        Ok(())
    }

    #[test]
    fn insert_wrong_arity_returns_err() -> Result<(), StorageError> {
        let mut storage = open_temp()?;
        storage.create_relation("edge", 2)?;
        assert!(matches!(
            storage.insert("edge", vec![i(1)]),
            Err(StorageError::ArityMismatch {
                expected: 2,
                got: 1,
            })
        ));
        Ok(())
    }
}