//! fjall adapter.
//!
//! Each relation gets a fjall [`PartitionHandle`](fjall::PartitionHandle) of
//! the same name. A reserved partition named `__meta` carries per-relation
//! metadata (arity and next synthetic row ID).
//!
//! fjall has no native cross-partition write transactions, so the adapter
//! buffers inserts inside [`FjallTx`] and applies them when
//! [`Transaction::commit`] is called; dropping the tx without commit discards
//! the buffer.

use std::collections::HashMap;

use fjall::{Keyspace, PartitionCreateOptions, PartitionHandle};

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

const META_PARTITION: &str = "__meta";

/// fjall-backed [`Storage`] implementation.
pub struct FjallStorage {
    keyspace: Keyspace,
    meta: PartitionHandle,
}

impl FjallStorage {
    /// Open or create a fjall keyspace at `path`.
    ///
    /// # Errors
    /// Returns [`StorageError::Backend`] if fjall fails to open the path.
    pub fn open(path: impl AsRef<std::path::Path>) -> Result<Self, StorageError> {
        let keyspace = fjall::Config::new(path).open().map_err(backend)?;
        let meta = keyspace
            .open_partition(META_PARTITION, PartitionCreateOptions::default())
            .map_err(backend)?;
        Ok(Self { keyspace, meta })
    }

    fn relation_partition(&self, name: &str) -> Result<PartitionHandle, StorageError> {
        self.keyspace
            .open_partition(name, PartitionCreateOptions::default())
            .map_err(backend)
    }
}

impl Storage for FjallStorage {
    fn create_relation(&mut self, name: &str, arity: usize) -> Result<(), StorageError> {
        if name == META_PARTITION {
            return Err(StorageError::Validation(format!(
                "relation name '{name}' is reserved"
            )));
        }
        if self.meta.contains_key(name.as_bytes()).map_err(backend)? {
            return Err(StorageError::RelationExists(name.to_string()));
        }
        let Ok(arity_u32) = u32::try_from(arity) else {
            unreachable!("arity exceeds u32::MAX")
        };
        self.meta
            .insert(name.as_bytes(), encode_meta(arity_u32, 0))
            .map_err(backend)?;
        let _ = self.relation_partition(name)?;
        Ok(())
    }

    fn arity(&self, name: &str) -> Result<usize, StorageError> {
        let raw = self
            .meta
            .get(name.as_bytes())
            .map_err(backend)?
            .ok_or_else(|| StorageError::RelationNotFound(name.to_string()))?;
        let (arity, _) = decode_meta(raw.as_ref())?;
        Ok(arity as usize)
    }

    fn scan_iter<'a>(&'a self, name: &str) -> Result<RowStream<'a>, StorageError> {
        let _ = self.arity(name)?;
        let partition = self.relation_partition(name)?;
        let iter = partition.iter().map(|res| {
            let (key, value) = res.map_err(backend)?;
            Ok((RowId::new(key.as_ref()), decode_row(value.as_ref())?))
        });
        Ok(Box::new(iter))
    }

    fn transaction<'a>(&'a mut self) -> Result<Box<dyn Transaction + 'a>, StorageError> {
        Ok(Box::new(FjallTx {
            keyspace: &self.keyspace,
            meta: &self.meta,
            pending: Vec::new(),
            deletes: Vec::new(),
            next_ids: HashMap::new(),
        }))
    }
}

pub(crate) struct FjallTx<'a> {
    keyspace: &'a Keyspace,
    meta: &'a PartitionHandle,
    pending: Vec<(String, RowId, Vec<Value>)>,
    deletes: Vec<(String, RowId)>,
    next_ids: HashMap<String, (u32, u64)>,
}

impl FjallTx<'_> {
    fn meta_for(&mut self, name: &str) -> Result<(u32, u64), StorageError> {
        if let Some(&entry) = self.next_ids.get(name) {
            return Ok(entry);
        }
        let raw = self
            .meta
            .get(name.as_bytes())
            .map_err(backend)?
            .ok_or_else(|| StorageError::RelationNotFound(name.to_string()))?;
        let entry = decode_meta(raw.as_ref())?;
        self.next_ids.insert(name.to_string(), entry);
        Ok(entry)
    }
}

impl Transaction for FjallTx<'_> {
    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 id = RowId::from(next_id);
        self.next_ids.insert(name.to_string(), (arity, next_id + 1));
        self.pending.push((name.to_string(), id.clone(), row));
        Ok(id)
    }

    fn delete(&mut self, name: &str, id: &RowId) -> Result<(), StorageError> {
        if !self.meta.contains_key(name.as_bytes()).map_err(backend)? {
            return Err(StorageError::RelationNotFound(name.to_string()));
        }
        self.deletes.push((name.to_string(), id.clone()));
        Ok(())
    }

    fn commit(self: Box<Self>) -> Result<CommittedTx, StorageError> {
        let FjallTx {
            keyspace,
            meta,
            pending,
            deletes,
            next_ids,
        } = *self;
        for (name, id, row) in pending {
            let partition = keyspace
                .open_partition(&name, PartitionCreateOptions::default())
                .map_err(backend)?;
            partition
                .insert(id.as_bytes(), encode_row(&row))
                .map_err(backend)?;
        }
        for (name, id) in deletes {
            let partition = keyspace
                .open_partition(&name, PartitionCreateOptions::default())
                .map_err(backend)?;
            partition.remove(id.as_bytes()).map_err(backend)?;
        }
        for (name, (arity, next_id)) in next_ids {
            meta.insert(name.as_bytes(), encode_meta(arity, next_id))
                .map_err(backend)?;
        }
        Ok(CommittedTx::empty())
    }
}

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

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

    fn open_temp() -> Result<FjallStorage, StorageError> {
        let dir = tempfile::tempdir().map_err(backend)?;
        let storage = FjallStorage::open(dir.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![i(2), i(3)])?;
        let rows = storage.scan("edge")?;
        assert_eq!(rows, vec![(id0, vec![i(1), i(2)]), (id1, vec![i(2), i(3)])]);
        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(())
    }
}