//! Operation log (OpLog) for tracking mutations.
//!
//! The OpLog records every write operation so that changes can be replayed,
//! replicated, or used for change-stream style notifications.
//! Each entry stores both the new and previous document state, enabling
//! point-in-time replay and revert.

use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

use bson::Document;
use dashmap::DashMap;
use serde::{Deserialize, Serialize};

/// The type of operation recorded in the oplog.
#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
pub enum OpType {
    Insert,
    Update,
    Delete,
}

/// A single oplog entry.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct OpLogEntry {
    /// Monotonically increasing sequence number.
    pub seq: u64,
    /// Timestamp in milliseconds since UNIX epoch.
    pub timestamp_ms: i64,
    /// Operation type.
    pub op: OpType,
    /// Database name.
    pub db: String,
    /// Collection name.
    pub collection: String,
    /// Document id (hex string).
    pub document_id: String,
    /// The new document snapshot (for insert/update; None for delete).
    pub document: Option<Document>,
    /// The previous document snapshot (for update/delete; None for insert).
    pub previous_document: Option<Document>,
}

/// Aggregate statistics about the oplog.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct OpLogStats {
    pub current_seq: u64,
    pub total_entries: usize,
    pub oldest_seq: u64,
    pub inserts: usize,
    pub updates: usize,
    pub deletes: usize,
}

/// In-memory operation log.
pub struct OpLog {
    /// All entries keyed by sequence number.
    entries: DashMap<u64, OpLogEntry>,
    /// Next sequence number.
    next_seq: AtomicU64,
}

impl OpLog {
    pub fn new() -> Self {
        Self {
            entries: DashMap::new(),
            next_seq: AtomicU64::new(1),
        }
    }

    /// Append an operation to the log and return its sequence number.
    pub fn append(
        &self,
        op: OpType,
        db: &str,
        collection: &str,
        document_id: &str,
        document: Option<Document>,
        previous_document: Option<Document>,
    ) -> u64 {
        let seq = self.next_seq.fetch_add(1, Ordering::SeqCst);
        let entry = OpLogEntry {
            seq,
            timestamp_ms: SystemTime::now()
                .duration_since(UNIX_EPOCH)
                .unwrap()
                .as_millis() as i64,
            op,
            db: db.to_string(),
            collection: collection.to_string(),
            document_id: document_id.to_string(),
            document,
            previous_document,
        };
        self.entries.insert(seq, entry);
        seq
    }

    /// Get a single entry by sequence number.
    pub fn get_entry(&self, seq: u64) -> Option<OpLogEntry> {
        self.entries.get(&seq).map(|e| e.value().clone())
    }

    /// Get all entries with sequence number >= `since`.
    pub fn entries_since(&self, since: u64) -> Vec<OpLogEntry> {
        let mut result: Vec<_> = self
            .entries
            .iter()
            .filter(|e| *e.key() >= since)
            .map(|e| e.value().clone())
            .collect();
        result.sort_by_key(|e| e.seq);
        result
    }

    /// Get entries in range [from_seq, to_seq] inclusive, sorted by seq.
    pub fn entries_range(&self, from_seq: u64, to_seq: u64) -> Vec<OpLogEntry> {
        let mut result: Vec<_> = self
            .entries
            .iter()
            .filter(|e| {
                let k = *e.key();
                k >= from_seq && k <= to_seq
            })
            .map(|e| e.value().clone())
            .collect();
        result.sort_by_key(|e| e.seq);
        result
    }

    /// Remove all entries with seq > `after_seq` and reset the next_seq counter.
    pub fn truncate_after(&self, after_seq: u64) {
        let keys_to_remove: Vec<u64> = self
            .entries
            .iter()
            .filter(|e| *e.key() > after_seq)
            .map(|e| *e.key())
            .collect();
        for key in keys_to_remove {
            self.entries.remove(&key);
        }
        self.next_seq.store(after_seq + 1, Ordering::SeqCst);
    }

    /// Get the current (latest) sequence number. Returns 0 if empty.
    pub fn current_seq(&self) -> u64 {
        self.next_seq.load(Ordering::SeqCst).saturating_sub(1)
    }

    /// Get aggregate statistics.
    pub fn stats(&self) -> OpLogStats {
        let mut inserts = 0usize;
        let mut updates = 0usize;
        let mut deletes = 0usize;
        let mut oldest_seq = u64::MAX;

        for entry in self.entries.iter() {
            match entry.value().op {
                OpType::Insert => inserts += 1,
                OpType::Update => updates += 1,
                OpType::Delete => deletes += 1,
            }
            if entry.value().seq < oldest_seq {
                oldest_seq = entry.value().seq;
            }
        }

        if oldest_seq == u64::MAX {
            oldest_seq = 0;
        }

        OpLogStats {
            current_seq: self.current_seq(),
            total_entries: self.entries.len(),
            oldest_seq,
            inserts,
            updates,
            deletes,
        }
    }

    /// Clear all entries.
    pub fn clear(&self) {
        self.entries.clear();
        self.next_seq.store(1, Ordering::SeqCst);
    }

    /// Number of entries in the log.
    pub fn len(&self) -> usize {
        self.entries.len()
    }

    /// Whether the log is empty.
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }
}

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