//! Merkle tree manager with persistence.
//!
//! Manages the Merkle tree for assertion hashes with periodic checkpointing
//! to the KV store for crash recovery.
//!
//! # Persistence
//!
//! The tree is serialized and stored at key `\x00MERKLE_CHECKPOINT`.
//! On startup, the manager attempts to load from this checkpoint.
//! If not found or corrupt, it rebuilds from the assertion store.
//!
//! # Thread Safety
//!
//! All operations are protected by an RwLock, allowing concurrent reads
//! but exclusive writes.

use crate::error::{Result, SyncError};
use std::sync::Arc;
use stemedb_merkle::serialize::{deserialize_tree, serialize_tree};
use stemedb_merkle::{Hash, MerkleTree};
use stemedb_storage::KVStore;
use tokio::sync::RwLock;
use tracing::{debug, info, instrument, warn};

/// Key for storing the Merkle tree checkpoint.
const MERKLE_CHECKPOINT_KEY: &[u8] = b"\x00MERKLE_CHECKPOINT";

/// Manages a Merkle tree with persistence.
pub struct MerkleTreeManager<S> {
    tree: RwLock<MerkleTree>,
    store: Arc<S>,
}

impl<S: KVStore> MerkleTreeManager<S> {
    /// Load the Merkle tree from checkpoint, or create a new empty tree.
    ///
    /// # Arguments
    ///
    /// * `store` - KV store for persistence
    ///
    /// # Returns
    ///
    /// A manager with the tree loaded from checkpoint if available.
    #[instrument(skip(store))]
    pub async fn load_or_create(store: Arc<S>) -> Result<Self> {
        let tree = match store.get(MERKLE_CHECKPOINT_KEY).await? {
            Some(data) => match deserialize_tree(&data) {
                Ok(tree) => {
                    info!(leaf_count = tree.len(), "Loaded Merkle tree from checkpoint");
                    tree
                }
                Err(e) => {
                    warn!(error = %e, "Failed to deserialize Merkle checkpoint, starting fresh");
                    MerkleTree::new()
                }
            },
            None => {
                debug!("No Merkle checkpoint found, starting with empty tree");
                MerkleTree::new()
            }
        };

        Ok(Self { tree: RwLock::new(tree), store })
    }

    /// Insert a hash into the Merkle tree.
    ///
    /// This operation does NOT automatically checkpoint. Call `checkpoint()`
    /// periodically to persist the tree.
    #[instrument(skip(self, hash), fields(hash = %hex::encode(&hash[..8])))]
    pub async fn insert(&self, hash: Hash) -> Result<()> {
        let mut tree = self.tree.write().await;
        tree.insert(hash)?;
        debug!(leaf_count = tree.len(), "Inserted hash into Merkle tree");
        Ok(())
    }

    /// Get the current Merkle root.
    ///
    /// Returns `None` if the tree is empty.
    pub async fn root(&self) -> Result<Option<Hash>> {
        let tree = self.tree.read().await;
        match tree.root() {
            Ok(root) => Ok(Some(root)),
            Err(stemedb_merkle::TreeError::EmptyTree) => Ok(None),
            Err(e) => Err(SyncError::Merkle(e.to_string())),
        }
    }

    /// Get the number of leaves in the tree.
    pub async fn len(&self) -> usize {
        self.tree.read().await.len()
    }

    /// Check if the tree is empty.
    pub async fn is_empty(&self) -> bool {
        self.tree.read().await.is_empty()
    }

    /// Get all leaf hashes.
    ///
    /// Used for diff operations during anti-entropy sync.
    pub async fn leaves(&self) -> Vec<Hash> {
        self.tree.read().await.leaves().to_vec()
    }

    /// Checkpoint the tree to persistent storage.
    ///
    /// Should be called periodically (e.g., every 5 minutes) to ensure
    /// fast recovery after crash.
    #[instrument(skip(self))]
    pub async fn checkpoint(&self) -> Result<()> {
        let tree = self.tree.read().await;
        let data = serialize_tree(&tree)?;

        self.store.put(MERKLE_CHECKPOINT_KEY, &data).await?;
        info!(leaf_count = tree.len(), bytes = data.len(), "Checkpointed Merkle tree");

        Ok(())
    }

    /// Rebuild the tree from a list of hashes.
    ///
    /// Used during recovery if the checkpoint is corrupt or missing.
    #[instrument(skip(self, hashes), fields(hash_count = hashes.len()))]
    pub async fn rebuild_from_hashes(&self, hashes: Vec<Hash>) -> Result<()> {
        let mut tree = self.tree.write().await;
        *tree = MerkleTree::new();

        for hash in hashes {
            tree.insert(hash)?;
        }

        info!(leaf_count = tree.len(), "Rebuilt Merkle tree from hashes");
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use stemedb_storage::HybridStore;
    use tempfile::tempdir;

    async fn create_test_store() -> Arc<HybridStore> {
        let dir = tempdir().expect("create temp dir");
        Arc::new(HybridStore::open(dir.path()).expect("open store"))
    }

    #[tokio::test]
    async fn test_empty_tree() {
        let store = create_test_store().await;
        let manager = MerkleTreeManager::load_or_create(store).await.expect("create");

        assert!(manager.is_empty().await);
        assert_eq!(manager.len().await, 0);
        assert!(manager.root().await.expect("root").is_none());
    }

    #[tokio::test]
    async fn test_insert_and_root() {
        let store = create_test_store().await;
        let manager = MerkleTreeManager::load_or_create(store).await.expect("create");

        manager.insert([1u8; 32]).await.expect("insert");
        manager.insert([2u8; 32]).await.expect("insert");

        assert_eq!(manager.len().await, 2);
        assert!(!manager.is_empty().await);
        assert!(manager.root().await.expect("root").is_some());
    }

    #[tokio::test]
    async fn test_checkpoint_and_restore() {
        let dir = tempdir().expect("create temp dir");
        let path = dir.path().to_path_buf();

        // Create and populate
        {
            let store = Arc::new(HybridStore::open(&path).expect("open store"));
            let manager = MerkleTreeManager::load_or_create(store).await.expect("create");

            manager.insert([1u8; 32]).await.expect("insert");
            manager.insert([2u8; 32]).await.expect("insert");
            manager.insert([3u8; 32]).await.expect("insert");

            manager.checkpoint().await.expect("checkpoint");
        }

        // Reopen and verify
        {
            let store = Arc::new(HybridStore::open(&path).expect("open store"));
            let manager = MerkleTreeManager::load_or_create(store).await.expect("create");

            assert_eq!(manager.len().await, 3);
            let leaves = manager.leaves().await;
            assert_eq!(leaves.len(), 3);
            assert_eq!(leaves[0], [1u8; 32]);
            assert_eq!(leaves[1], [2u8; 32]);
            assert_eq!(leaves[2], [3u8; 32]);
        }
    }

    #[tokio::test]
    async fn test_rebuild_from_hashes() {
        let store = create_test_store().await;
        let manager = MerkleTreeManager::load_or_create(store).await.expect("create");

        let hashes = vec![[1u8; 32], [2u8; 32], [3u8; 32]];
        manager.rebuild_from_hashes(hashes).await.expect("rebuild");

        assert_eq!(manager.len().await, 3);
    }
}