export class EventSourcing {

  eventNames;
  eventLog;
  snapshots;
  snapshotInterval;
  lastSnapshotIndex;

  constructor(options = {}) {
    this.eventNames = new Map();
    this.eventLog = [];
    this.snapshots = [];
    this.snapshotInterval = options.snapshotInterval || 100;
    this.lastSnapshotIndex = 0;
    this.persistence = options.persistence || null;
    this.maxLogSize = options.maxLogSize || Infinity;
    this.sequenceNumber = 0;

    // Load persisted events if persistence is enabled
    if (this.persistence) {
      this.loadEvents();
    }
  }

  on(event, subscriber) {
    if (!this.eventNames.has(event)) {
      this.eventNames.set(event, new Set());
    }
    this.eventNames.get(event).add(subscriber);
    return () => this.off(event, subscriber);
  }

  off(event, subscriber) {
    this.eventNames.get(event)?.delete(subscriber);
  }

  emit(event, data, options = {}) {
    const timestamp = new Date().toISOString();
    const eventEntry = {
      id: this.generateEventId(),
      sequence: ++this.sequenceNumber,
      event,
      data,
      timestamp,
      metadata: options.metadata || {},
      version: options.version || 1
    };

    // Add to event log
    this.eventLog.push(eventEntry);

    // Persist event if persistence is enabled
    if (this.persistence && !options.skipPersistence) {
      this.persistEvent(eventEntry);
    }

    // Emit to current subscribers (live projection)
    if (!options.skipEmit) {
      const subscribers = this.eventNames.get(event) ?? new Set();
      for (const subscriber of subscribers) {
        try {
          subscriber(data, eventEntry);
        } catch (error) {
          console.error(`Error in event subscriber for ${event}:`, error);
        }
      }
    }

    // Create snapshot if needed
    if (this.shouldCreateSnapshot()) {
      this.createSnapshot();
    }

    // Trim log if it exceeds max size
    if (this.eventLog.length > this.maxLogSize) {
      this.trimEventLog();
    }

    return eventEntry.id;
  }

  // Event Sourcing Core Methods

  replay(fromSequence = 0, toSequence = Infinity) {
    const events = this.getEventsBySequenceRange(fromSequence, toSequence);

    for (const eventEntry of events) {
      const subscribers = this.eventNames.get(eventEntry.event) ?? new Set();
      for (const subscriber of subscribers) {
        try {
          subscriber(eventEntry.data, eventEntry);
        } catch (error) {
          console.error(`Error during replay for ${eventEntry.event}:`, error);
        }
      }
    }

    return events.length;
  }

  replayFromSnapshot(snapshotIndex = -1) {
    const snapshot = snapshotIndex === -1
      ? this.getLatestSnapshot()
      : this.snapshots[snapshotIndex];

    if (!snapshot) {
      return this.replay();
    }

    // Restore state from snapshot if it has state data
    if (snapshot.state) {
      this.restoreFromSnapshot(snapshot);
    }

    // Replay events after the snapshot
    return this.replay(snapshot.lastSequence + 1);
  }

  // Query Methods

  getEvents(filter = {}) {
    let events = [...this.eventLog];

    if (filter.event) {
      events = events.filter(e => e.event === filter.event);
    }

    if (filter.fromTimestamp) {
      events = events.filter(e => new Date(e.timestamp) >= new Date(filter.fromTimestamp));
    }

    if (filter.toTimestamp) {
      events = events.filter(e => new Date(e.timestamp) <= new Date(filter.toTimestamp));
    }

    if (filter.fromSequence !== undefined) {
      events = events.filter(e => e.sequence >= filter.fromSequence);
    }

    if (filter.toSequence !== undefined) {
      events = events.filter(e => e.sequence <= filter.toSequence);
    }

    if (filter.metadata) {
      events = events.filter(e => {
        return Object.entries(filter.metadata).every(([key, value]) =>
          e.metadata[key] === value
        );
      });
    }

    if (filter.limit) {
      events = events.slice(0, filter.limit);
    }

    return events;
  }

  getEventsBySequenceRange(from, to) {
    return this.eventLog.filter(e => e.sequence >= from && e.sequence <= to);
  }

  getEventsByTimeRange(fromTimestamp, toTimestamp) {
    const from = new Date(fromTimestamp);
    const to = new Date(toTimestamp);
    return this.eventLog.filter(e => {
      const eventTime = new Date(e.timestamp);
      return eventTime >= from && eventTime <= to;
    });
  }

  getLastEvent(eventType) {
    for (let i = this.eventLog.length - 1; i >= 0; i--) {
      if (!eventType || this.eventLog[i].event === eventType) {
        return this.eventLog[i];
      }
    }
    return null;
  }

  // Projection Methods

  project(projectionFn, filter = {}) {
    const events = this.getEvents(filter);
    let state = {};

    for (const event of events) {
      state = projectionFn(state, event);
    }

    return state;
  }

  projectFromSnapshot(projectionFn, snapshotIndex = -1) {
    const snapshot = snapshotIndex === -1
      ? this.getLatestSnapshot()
      : this.snapshots[snapshotIndex];

    let state = snapshot?.projectionState || {};
    const fromSequence = snapshot ? snapshot.lastSequence + 1 : 0;

    const events = this.getEventsBySequenceRange(fromSequence, Infinity);

    for (const event of events) {
      state = projectionFn(state, event);
    }

    return state;
  }

  // Snapshot Methods

  createSnapshot(customState = null, options = {}) {
    const snapshot = {
      id: this.generateSnapshotId(),
      timestamp: new Date().toISOString(),
      lastSequence: this.sequenceNumber,
      eventCount: this.eventLog.length,
      state: customState,
      projectionState: null
    };

    this.snapshots.push(snapshot);
    this.lastSnapshotIndex = this.eventLog.length;

    if (this.persistence) {
      this.persistSnapshot(snapshot);
    }

    // Clear event log if requested
    if (options.clearLog) {
      this.clearEventLog(options.keepEvents || 0);
    }

    return snapshot;
  }

  clearEventLog(keepRecentCount = 0) {
    if (keepRecentCount > 0) {
      this.eventLog = this.eventLog.slice(-keepRecentCount);
    } else {
      this.eventLog = [];
    }

    // Update last snapshot index
    this.lastSnapshotIndex = this.eventLog.length;

    if (this.persistence) {
      this.persistence.clearEvents?.(keepRecentCount);
    }

    return this.eventLog.length;
  }

  createSnapshotAndClear(customState = null, keepRecentEvents = 0) {
    const snapshot = this.createSnapshot(customState, {
      clearLog: true,
      keepEvents: keepRecentEvents
    });

    return {
      snapshot,
      eventsCleared: snapshot.eventCount - this.eventLog.length,
      eventsKept: this.eventLog.length
    };
  }

  shouldCreateSnapshot() {
    return this.eventLog.length - this.lastSnapshotIndex >= this.snapshotInterval;
  }

  getLatestSnapshot() {
    return this.snapshots[this.snapshots.length - 1] || null;
  }

  restoreFromSnapshot(snapshot) {
    // Override this method in subclasses to restore specific state
    if (snapshot.state && typeof snapshot.state === 'object') {
      Object.assign(this, snapshot.state);
    }
  }

  // Persistence Methods

  async loadEvents() {
    if (!this.persistence) return;

    try {
      const data = await this.persistence.load();
      if (data.events) {
        this.eventLog = data.events;
        this.sequenceNumber = Math.max(...this.eventLog.map(e => e.sequence), 0);
      }
      if (data.snapshots) {
        this.snapshots = data.snapshots;
      }
    } catch (error) {
      console.error('Failed to load events:', error);
    }
  }

  async persistEvent(event) {
    if (!this.persistence) return;

    try {
      await this.persistence.saveEvent(event);
    } catch (error) {
      console.error('Failed to persist event:', error);
    }
  }

  async persistSnapshot(snapshot) {
    if (!this.persistence) return;

    try {
      await this.persistence.saveSnapshot(snapshot);
    } catch (error) {
      console.error('Failed to persist snapshot:', error);
    }
  }

  async saveAll() {
    if (!this.persistence) return;

    try {
      await this.persistence.saveAll({
        events: this.eventLog,
        snapshots: this.snapshots
      });
    } catch (error) {
      console.error('Failed to save all data:', error);
    }
  }

  // Utility Methods

  generateEventId() {
    return `evt_${Date.now()}_${Math.random().toString(36).substr(2, 9)}`;
  }

  generateSnapshotId() {
    return `snap_${Date.now()}_${Math.random().toString(36).substr(2, 9)}`;
  }

  trimEventLog() {
    const latestSnapshot = this.getLatestSnapshot();
    if (latestSnapshot) {
      // Keep events after the latest snapshot
      const keepFromIndex = this.eventLog.findIndex(e => e.sequence > latestSnapshot.lastSequence);
      if (keepFromIndex > 0) {
        this.eventLog = this.eventLog.slice(keepFromIndex);
      }
    } else {
      // No snapshots, keep only the most recent events
      const keepCount = Math.floor(this.maxLogSize * 0.8);
      this.eventLog = this.eventLog.slice(-keepCount);
    }
  }

  // Statistics and Monitoring

  getStats() {
    const eventTypes = new Map();
    this.eventLog.forEach(e => {
      eventTypes.set(e.event, (eventTypes.get(e.event) || 0) + 1);
    });

    return {
      totalEvents: this.eventLog.length,
      currentSequence: this.sequenceNumber,
      eventTypes: Object.fromEntries(eventTypes),
      subscriberCount: Array.from(this.eventNames.values()).reduce((sum, set) => sum + set.size, 0),
      snapshotCount: this.snapshots.length,
      lastEventTimestamp: this.eventLog.length > 0 ? this.eventLog[this.eventLog.length - 1].timestamp : null,
      memoryUsage: this.estimateMemoryUsage()
    };
  }

  estimateMemoryUsage() {
    const eventSize = JSON.stringify(this.eventLog).length;
    const snapshotSize = JSON.stringify(this.snapshots).length;
    return {
      events: `${Math.round(eventSize / 1024)}KB`,
      snapshots: `${Math.round(snapshotSize / 1024)}KB`,
      total: `${Math.round((eventSize + snapshotSize) / 1024)}KB`
    };
  }

  // Advanced Features

  createProjection(name, projectionFn, options = {}) {
    const projection = {
      name,
      fn: projectionFn,
      state: options.initialState || {},
      lastProcessedSequence: 0,
      ...options
    };

    // Process existing events
    const events = this.eventLog.filter(e => e.sequence > projection.lastProcessedSequence);
    for (const event of events) {
      projection.state = projectionFn(projection.state, event);
      projection.lastProcessedSequence = event.sequence;
    }

    // Subscribe to new events
    this.on('*', (data, event) => {
      if (event.sequence > projection.lastProcessedSequence) {
        projection.state = projectionFn(projection.state, event);
        projection.lastProcessedSequence = event.sequence;
      }
    });

    return projection;
  }

  // Batch operations

  emitBatch(events, options = {}) {
    const results = [];
    const batchId = this.generateEventId();

    for (const { event, data, metadata = {} } of events) {
      const eventId = this.emit(event, data, {
        ...options,
        metadata: { ...metadata, batchId }
      });
      results.push(eventId);
    }

    return results;
  }

  terminate() {
    // Save all data before terminating
    if (this.persistence) {
      this.saveAll();
    }

    this.eventNames.forEach((subscribers) => subscribers.clear());
    this.eventNames.clear();
    this.eventLog = [];
    this.snapshots = [];
  }
}




class EventSourcingStream extends EventSourcing {
  name = "StreamSource";
  source; // source emitter
  constructor(...argv) {
    super(...argv);
    this.unsubscribe = new Set();
  }
  emitValue(value) {
    this.emit("value", value);
  }
  subscribe(subscriber) {
    this.on("value", (v) => subscriber(v, this));
    return () => this.off("value", subscriber);
  }
  get path() {
    const sources = [];
    const getNextItem = (node) => node.source;
    let next = this;
    while (next) {
      sources.unshift(next);
      next = getNextItem(next);
    }
    return sources;
  }
  terminate() {
    super.terminate();
    this.unsubscribe.forEach((subscriber) => subscriber());
    this.unsubscribe.clear();
    this.path.forEach(fragment.terminate());
  }
}





// Simple in-memory persistence adapter example
export class EventSourcingMemoryPersistence {
 constructor() {
    this.data = { events: [], snapshots: [] };
  }

  async load() {
    return { ...this.data };
  }

  async saveEvent(event) {
    this.data.events.push(event);
  }

  async saveSnapshot(snapshot) {
    this.data.snapshots.push(snapshot);
  }

  async saveAll(data) {
    this.data = { ...data };
  }

  async clearEvents(keepRecentCount) {
    this.data.events = keepRecentCount > 0
      ? this.data.events.slice(-keepRecentCount)
      : [];
  }
}

// LocalStorage persistence adapter example
export class EventSourcingLocalStoragePersistence {
  constructor(key = 'event_store') {
    this.key = key;
  }

  async load() {
    const data = localStorage.getItem(this.key);
    return data ? JSON.parse(data) : { events: [], snapshots: [] };
  }

  async saveEvent(event) {
    const data = await this.load();
    data.events.push(event);
    localStorage.setItem(this.key, JSON.stringify(data));
  }

  async saveSnapshot(snapshot) {
    const data = await this.load();
    data.snapshots.push(snapshot);
    localStorage.setItem(this.key, JSON.stringify(data));
  }

  async saveAll(data) {
    localStorage.setItem(this.key, JSON.stringify(data));
  }

  async clearEvents(keepRecentCount) {
    const data = await this.load();
    data.events = keepRecentCount > 0
      ? data.events.slice(-keepRecentCount)
      : [];
    localStorage.setItem(this.key, JSON.stringify(data));
  }
}



export class ReactiveEventSourcingStream extends EventSourcingStream {

  name = "EventSourcingEngine";

  fromEvent(...argv) {
    return fromEvent(this, ...argv);
  }

  iterate(...argv) {
    return iterate(this, ...argv);
  }

  map(...argv) {
    return map(this, ...argv);
  }

  filter(...argv) {
    return filter(this, ...argv);
  }

  debounce(...argv) {
    return debounce(this, ...argv);
  }

  distinctUntilChanged(...argv) {
    return distinctUntilChanged(this, ...argv);
  }

  scan(...argv) {
    return scan(this, ...argv);
  }

  delay(...argv) {
    return delay(this, ...argv);
  }

  throttle(...argv) {
    return throttle(this, ...argv);
  }

  withLatestFrom(...argv) {
    return withLatestFrom(this, ...argv);
  }

  merge(...argv) {
    return merge(this, ...argv);
  }

  log(...argv) {
    return log(this, ...argv);
  }

}

// Usage Examples:

/*
// Basic usage with event sourcing
const emitter = new EventSourcing({
  snapshotInterval: 50,
  maxLogSize: 1000,
  persistence: new EventSourcingMemoryPersistence()
});

// Listen to events
emitter.on('user.created', (data, event) => {
  console.log('User created:', data.name, 'at', event.timestamp);
});

// Emit events with metadata
emitter.emit('user.created',
  { id: 1, name: 'John' },
  { metadata: { source: 'api', userId: 'admin' } }
);

// Query events
const userEvents = emitter.getEvents({
  event: 'user.created',
  fromTimestamp: '2023-01-01'
});

// Create projections
const userProjection = emitter.project((state, event) => {
  if (event.event === 'user.created') {
    state.users = state.users || [];
    state.users.push(event.data);
  }
  return state;
});

// Create snapshots
emitter.createSnapshot({ customData: 'state' });

// Replay events
emitter.replay(0, 100); // Replay events 0-100

// Get statistics
console.log(emitter.getStats());
*/