//! Geomerge adapter.
//!
//! Geomerge schemas are immutable after store construction: there is no
//! public API to register a new table on a live `Store`. The adapter expects
//! all relations to be declared up front via a `FlatTheory` passed to
//! [`GeomergeStorage::from_theory`] (or a pre-built `Store` via
//! [`GeomergeStorage::from_store`]). [`Storage::create_relation`] is then a
//! verifier that the relation exists and that its arity matches.
//!
//! ## Deletion
//!
//! Geomerge upstream's `txn::ops::Op` enum currently has only `Op::Add`;
//! there is no retract or remove op. The adapter therefore uses the default
//! [`Transaction::delete`] implementation, which returns
//! [`StorageError::Backend`].
//!
//! ## Row Identifier Encoding
//!
//! Geomerge's [`RowId`](geomerge::table::RowId) is `{ commit: CommitHash, counter: u32 }`.
//! The adapter uses two encodings under [`crate::id::RowId`]:
//!
//! - **Existing (36 bytes):** 32-byte commit hash followed by a 4-byte BE
//!   counter. Returned by [`Storage::scan`] and stable across calls.
//! - **Pending (4 bytes):** just a 4-byte BE counter, returned by
//!   [`Transaction::insert`] to reference an in-flight row from later
//!   inserts in the same transaction. Pending ids become invalid after
//!   commit; post-commit identifiers must be looked up via
//!   [`Storage::scan`].
//!
//! Foreign-key references inside a transaction work because geomerge's
//! `TxnCellValue::Id(RowRef::Pending(TempRowId(counter)))` accepts the
//! counter the adapter tracked locally. The local counter is assumed to
//! match geomerge's internal `TempRowId` counter (both start at 0 and
//! increment per `Transaction::add`).

use std::collections::{HashMap, HashSet};

use geomerge::commit::hash::CommitHash;
use geomerge::ir::{self, Path};
use geomerge::store::Store;
use geomerge::table::{CellValue, RowId as GmRowId};
use geomerge::txn::ops::{RowRef, TempRowId, TxnCellValue};

use crate::id::RowId;
use crate::value::Value;
use crate::{CommittedTx, RowStream, Storage, StorageError, Transaction, backend};

const GM_ROW_ID_LEN: usize = 32 + 4;
const PENDING_ROW_ID_LEN: usize = 4;

fn validation(msg: impl Into<String>) -> StorageError {
    StorageError::Validation(msg.into())
}

fn encode_gm_row_id(id: &GmRowId) -> RowId {
    let mut bytes = [0u8; GM_ROW_ID_LEN];
    bytes[..32].copy_from_slice(&id.commit.0);
    bytes[32..].copy_from_slice(&id.counter.to_be_bytes());
    RowId::new(bytes)
}

fn decode_gm_row_id(bytes: &[u8]) -> Result<GmRowId, StorageError> {
    if bytes.len() != GM_ROW_ID_LEN {
        return Err(validation(format!(
            "expected {GM_ROW_ID_LEN}-byte geomerge RowId, got {} bytes",
            bytes.len()
        )));
    }
    let mut hash = [0u8; 32];
    hash.copy_from_slice(&bytes[..32]);
    let mut counter_buf = [0u8; 4];
    counter_buf.copy_from_slice(&bytes[32..]);
    Ok(GmRowId {
        commit: CommitHash(hash),
        counter: u32::from_be_bytes(counter_buf),
    })
}

fn encode_pending_row_id(counter: u32) -> RowId {
    RowId::new(counter.to_be_bytes())
}

fn decode_pending_row_id(bytes: &[u8]) -> Result<TempRowId, StorageError> {
    if bytes.len() != PENDING_ROW_ID_LEN {
        return Err(validation(format!(
            "expected {PENDING_ROW_ID_LEN}-byte pending RowId, got {} bytes",
            bytes.len()
        )));
    }
    let mut counter_buf = [0u8; 4];
    counter_buf.copy_from_slice(bytes);
    Ok(TempRowId::from(u32::from_be_bytes(counter_buf)))
}

/// Geomerge-backed [`Storage`] implementation.
pub struct GeomergeStorage {
    store: Store,
    declared: HashSet<String>,
}

impl Default for GeomergeStorage {
    fn default() -> Self {
        Self::new()
    }
}

impl GeomergeStorage {
    /// Build an empty store. No relations are available until the store is
    /// rebuilt via a theory.
    #[must_use]
    pub fn new() -> Self {
        Self {
            store: Store::new(),
            declared: HashSet::new(),
        }
    }

    /// Build a store from a pre-defined `FlatTheory`. All `create_relation`
    /// calls must reference relations declared in the theory.
    ///
    /// # Errors
    /// Returns [`StorageError::Backend`] if geomerge rejects the theory.
    pub fn from_theory(theory: ir::FlatTheory) -> Result<Self, StorageError> {
        let store = Store::try_from_theory(theory).map_err(|e| backend(*e))?;
        Ok(Self {
            store,
            declared: HashSet::new(),
        })
    }

    /// Wrap an existing `Store`, e.g. after decoding via
    /// `geomerge::commit::pst::decode_store`.
    #[must_use]
    pub fn from_store(store: Store) -> Self {
        Self {
            store,
            declared: HashSet::new(),
        }
    }

    /// Borrow the underlying geomerge store (for backend-specific operations
    /// like persistence, dump, or law inspection that aren't on the trait).
    #[must_use]
    pub fn store(&self) -> &Store {
        &self.store
    }
}

impl Storage for GeomergeStorage {
    fn create_relation(&mut self, name: &str, arity: usize) -> Result<(), StorageError> {
        if self.declared.contains(name) {
            return Err(StorageError::RelationExists(name.to_string()));
        }
        let path: Path = name.into();
        let table = self.store.table_at(&path).ok_or_else(|| {
            validation(format!(
                "relation '{name}' is not declared in the loaded geomerge theory; \
                 geomerge does not support runtime relation creation"
            ))
        })?;
        let declared_arity = table.schema().columns.len();
        if declared_arity != arity {
            return Err(StorageError::ArityMismatch {
                expected: declared_arity,
                got: arity,
            });
        }
        self.declared.insert(name.to_string());
        Ok(())
    }

    fn arity(&self, name: &str) -> Result<usize, StorageError> {
        let path: Path = name.into();
        let table = self
            .store
            .table_at(&path)
            .ok_or_else(|| StorageError::RelationNotFound(name.to_string()))?;
        Ok(table.schema().columns.len())
    }

    fn scan_iter<'a>(&'a self, name: &str) -> Result<RowStream<'a>, StorageError> {
        let path: Path = name.into();
        let table = self
            .store
            .table_at(&path)
            .ok_or_else(|| StorageError::RelationNotFound(name.to_string()))?;
        let arity = table.schema().columns.len();
        let mut rows = Vec::with_capacity(table.row_count());
        for r in 0..table.row_count() {
            let gm_id = table
                .row_id_at(r)
                .ok_or_else(|| validation(format!("missing row id at {r} in '{name}'")))?;
            let id = encode_gm_row_id(&gm_id);
            let mut row = Vec::with_capacity(arity);
            for c in 0..arity {
                let cell = table
                    .cell_at(r, c)
                    .ok_or_else(|| validation(format!("missing cell at ({r}, {c}) in '{name}'")))?;
                row.push(cell_to_value(cell));
            }
            rows.push((id, row));
        }
        Ok(Box::new(rows.into_iter().map(Ok)))
    }

    fn transaction<'a>(&'a mut self) -> Result<Box<dyn Transaction + 'a>, StorageError> {
        let txn = self.store.transaction();
        Ok(Box::new(GeomergeTx {
            txn: Some(txn),
            counter: 0,
        }))
    }
}

pub(crate) struct GeomergeTx<'a> {
    txn: Option<geomerge::txn::Transaction<'a>>,
    /// Mirrors geomerge's internal `TempRowId` counter for this transaction.
    /// Both start at 0 and increment by 1 per `Transaction::add`.
    counter: u32,
}

impl Transaction for GeomergeTx<'_> {
    fn insert(&mut self, name: &str, row: Vec<Value>) -> Result<RowId, StorageError> {
        let path: Path = name.into();
        let values: Vec<TxnCellValue> = row
            .into_iter()
            .map(value_to_txn_cell)
            .collect::<Result<Vec<_>, _>>()?;
        let Some(txn) = self.txn.as_mut() else {
            unreachable!("transaction was already committed")
        };
        txn.add(&path, values)
            .map_err(|e| validation(e.to_string()))?;
        let id = encode_pending_row_id(self.counter);
        self.counter += 1;
        Ok(id)
    }

    fn commit(self: Box<Self>) -> Result<CommittedTx, StorageError> {
        let mut this = self;
        let Some(txn) = this.txn.take() else {
            unreachable!("transaction was already committed")
        };
        // Law violations (totality, foreign-key, etc.) surface here.
        let commit_hash = txn.commit().map_err(|e| validation(e.to_string()))?;
        // Every counter we returned during this tx (0..self.counter) now
        // corresponds to a real RowId { commit: commit_hash, counter }.
        let mut resolutions = HashMap::with_capacity(this.counter as usize);
        for counter in 0..this.counter {
            let pending = encode_pending_row_id(counter);
            let real = encode_gm_row_id(&GmRowId {
                commit: commit_hash,
                counter,
            });
            resolutions.insert(pending, real);
        }
        Ok(CommittedTx::from_mappings(resolutions))
    }
}

fn cell_to_value(cell: &CellValue) -> Value {
    match cell {
        CellValue::Int(i) => Value::Int(*i),
        CellValue::Str(s) => Value::Str(s.clone()),
        CellValue::Id(id) => Value::Id(encode_gm_row_id(id)),
    }
}

fn value_to_txn_cell(value: Value) -> Result<TxnCellValue, StorageError> {
    match value {
        Value::Int(i) => Ok(TxnCellValue::Int(i)),
        Value::Str(s) => Ok(TxnCellValue::Str(s)),
        Value::Id(id) => {
            let bytes = id.as_bytes();
            match bytes.len() {
                PENDING_ROW_ID_LEN => Ok(TxnCellValue::Id(RowRef::Pending(decode_pending_row_id(
                    bytes,
                )?))),
                GM_ROW_ID_LEN => Ok(TxnCellValue::Id(RowRef::Existing(decode_gm_row_id(bytes)?))),
                len => Err(validation(format!(
                    "geomerge RowId must be {PENDING_ROW_ID_LEN} (pending) or \
                     {GM_ROW_ID_LEN} (existing) bytes, got {len}"
                ))),
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use geomerge::ir::{ColType, FlatTheory, PrimType, Schema, TableEntry};

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

    fn int_schema(arity: usize) -> Schema {
        Schema {
            columns: (0..arity)
                .map(|_| ColType::PrimType {
                    prim: PrimType::PrimInt,
                })
                .collect(),
            primary_key: None,
        }
    }

    fn theory_with_one_int_table(name: &str, arity: usize) -> FlatTheory {
        FlatTheory {
            tables: vec![TableEntry {
                path: name.into(),
                table: int_schema(arity),
            }],
            laws: Vec::new(),
        }
    }

    #[test]
    fn empty_store_has_no_relations() {
        let storage = GeomergeStorage::new();
        assert!(matches!(
            storage.arity("edge"),
            Err(StorageError::RelationNotFound(_))
        ));
    }

    #[test]
    fn create_relation_on_undeclared_returns_validation_error() {
        let mut storage = GeomergeStorage::new();
        assert!(matches!(
            storage.create_relation("edge", 2),
            Err(StorageError::Validation(_))
        ));
    }

    #[test]
    fn theory_loaded_insert_scan_roundtrip() -> Result<(), StorageError> {
        let theory = theory_with_one_int_table("edge", 2);
        let mut storage = GeomergeStorage::from_theory(theory)?;
        storage.create_relation("edge", 2)?;
        storage.insert("edge", vec![i(1), i(2)])?;
        storage.insert("edge", vec![i(3), i(4)])?;
        let rows = storage.scan("edge")?;
        assert_eq!(rows.len(), 2);
        assert_eq!(rows[0].1, vec![i(1), i(2)]);
        assert_eq!(rows[1].1, vec![i(3), i(4)]);
        // Scanned IDs use the 36-byte existing form.
        assert_eq!(rows[0].0.as_bytes().len(), GM_ROW_ID_LEN);
        assert_eq!(rows[1].0.as_bytes().len(), GM_ROW_ID_LEN);
        Ok(())
    }

    #[test]
    fn single_storage_insert_returns_post_commit_row_id() -> Result<(), StorageError> {
        let theory = theory_with_one_int_table("edge", 1);
        let mut storage = GeomergeStorage::from_theory(theory)?;
        storage.create_relation("edge", 1)?;
        // `Storage::insert` opens its own tx, commits, and resolves the
        // pending RowId to its post-commit form. The returned id should be
        // the 36-byte (existing) shape, not the 4-byte (pending) shape.
        let id = storage.insert("edge", vec![i(1)])?;
        assert_eq!(id.as_bytes().len(), GM_ROW_ID_LEN);
        // And it should equal what scan returns.
        let rows = storage.scan("edge")?;
        assert_eq!(rows.len(), 1);
        assert_eq!(rows[0].0, id);
        Ok(())
    }

    #[test]
    fn committed_tx_resolves_pending_ids() -> Result<(), StorageError> {
        let theory = theory_with_one_int_table("edge", 1);
        let mut storage = GeomergeStorage::from_theory(theory)?;
        storage.create_relation("edge", 1)?;
        let (committed, pending_a, pending_b) = {
            let mut tx = storage.transaction()?;
            let a = tx.insert("edge", vec![i(1)])?;
            let b = tx.insert("edge", vec![i(2)])?;
            (tx.commit()?, a, b)
        };
        let real_a = committed.resolve(&pending_a);
        let real_b = committed.resolve(&pending_b);
        assert_eq!(real_a.as_bytes().len(), GM_ROW_ID_LEN);
        assert_eq!(real_b.as_bytes().len(), GM_ROW_ID_LEN);
        assert_ne!(real_a, real_b);
        // The resolved ids should match what scan reports.
        let rows = storage.scan("edge")?;
        assert!(rows.iter().any(|(id, _)| id == &real_a));
        assert!(rows.iter().any(|(id, _)| id == &real_b));
        Ok(())
    }

    #[test]
    fn batched_inserts_in_one_transaction() -> Result<(), StorageError> {
        let theory = theory_with_one_int_table("edge", 2);
        let mut storage = GeomergeStorage::from_theory(theory)?;
        storage.create_relation("edge", 2)?;
        {
            let mut tx = storage.transaction()?;
            let p0 = tx.insert("edge", vec![i(1), i(2)])?;
            let p1 = tx.insert("edge", vec![i(3), i(4)])?;
            // Pending ids are 4-byte counters within the tx.
            assert_eq!(p0.as_bytes().len(), PENDING_ROW_ID_LEN);
            assert_eq!(p1.as_bytes().len(), PENDING_ROW_ID_LEN);
            tx.commit()?;
        }
        let rows = storage.scan("edge")?;
        assert_eq!(rows.len(), 2);
        Ok(())
    }

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

    #[test]
    fn delete_is_not_supported() -> Result<(), StorageError> {
        let theory = theory_with_one_int_table("edge", 1);
        let mut storage = GeomergeStorage::from_theory(theory)?;
        storage.create_relation("edge", 1)?;
        let id = storage.insert("edge", vec![i(1)])?;
        let result = storage.delete("edge", &id);
        assert!(matches!(result, Err(StorageError::Unsupported(_))));
        Ok(())
    }

    #[test]
    fn insert_wrong_type_returns_validation_error() -> Result<(), StorageError> {
        let theory = theory_with_one_int_table("edge", 2);
        let mut storage = GeomergeStorage::from_theory(theory)?;
        storage.create_relation("edge", 2)?;
        let result = storage.insert("edge", vec![Value::Str("not an int".to_string()), i(2)]);
        assert!(matches!(result, Err(StorageError::Validation(_))));
        Ok(())
    }
}