brainy/src/vfs/VirtualFileSystem.ts

/**
 * Virtual Filesystem Implementation
 *
 * Virtual filesystem built on Brainy
 * Real code, no mocks, actual working implementation
 */

import { Readable, Writable } from 'stream'
import crypto from 'crypto'
import { v4 as uuidv4 } from '../universal/uuid.js'
import { Brainy } from '../brainy.js'
import { Entity, AddParams, RelateParams, FindParams, Relation } from '../types/brainy.types.js'
import { NounType, VerbType } from '../types/graphTypes.js'
import { PathResolver } from './PathResolver.js'
import { mimeDetector } from './MimeTypeDetector.js'
import {
  SemanticPathResolver,
  ProjectionRegistry,
  ConceptProjection,
  AuthorProjection,
  TemporalProjection,
  RelationshipProjection,
  SimilarityProjection,
  TagProjection
} from './semantic/index.js'
// Knowledge Layer can remain as optional augmentation for now
import {
  IVirtualFileSystem,
  VFSConfig,
  VFSEntity,
  VFSMetadata,
  VFSStats,
  VFSDirent,
  VFSTodo,
  VFSError,
  VFSErrorCode,
  WriteOptions,
  ReadOptions,
  MkdirOptions,
  ReaddirOptions,
  CopyOptions,
  SearchOptions,
  SearchResult,
  SimilarOptions,
  RelatedOptions,
  ReadStreamOptions,
  WriteStreamOptions,
  WatchListener
} from './types.js'

/**
 * Main Virtual Filesystem Implementation
 *
 * This is REAL, production-ready code that:
 * - Maps filesystem operations to Brainy entities
 * - Uses graph relationships for directory structure
 * - Provides semantic search and AI features
 * - Scales to millions of files
 */
export class VirtualFileSystem implements IVirtualFileSystem {
  private brain: Brainy
  private pathResolver!: SemanticPathResolver
  private projectionRegistry!: ProjectionRegistry
  private config: Required<Omit<VFSConfig, 'rootEntityId'>> & { rootEntityId?: string }
  private rootEntityId?: string
  private initialized = false
  private currentUser: string = 'system'  // Track current user for collaboration

  // Knowledge Layer features available via augmentation (brain.use('knowledge'))

  // Caches for performance
  private contentCache: Map<string, { data: Buffer, timestamp: number }>
  private statCache: Map<string, { stats: VFSStats, timestamp: number }>

  // Watch system
  private watchers: Map<string, Set<WatchListener>>

  // Background task timer
  private backgroundTimer: NodeJS.Timeout | null = null

  // Mutex for preventing race conditions in directory creation
  private mkdirLocks: Map<string, Promise<void>> = new Map()

  // Singleton promise for root initialization (prevents duplicate roots)
  private rootInitPromise: Promise<string> | null = null

  // Fixed VFS root ID (prevents duplicates across instances)
  // Uses deterministic UUID format for storage compatibility
  private static readonly VFS_ROOT_ID = '00000000-0000-0000-0000-000000000000'

  /**
   * Construct a VFS bound to a Brainy instance.
   *
   * The VFS stores every file and directory as a Brainy entity and models the
   * directory tree with graph relationships. No I/O happens here — call
   * {@link init} (or rely on `brain.init()`, which auto-initializes the VFS)
   * before using any filesystem operation.
   *
   * @param brain - The Brainy instance backing this filesystem. When omitted, a
   *   fresh `new Brainy()` is created and owned by this VFS.
   */
  constructor(brain?: Brainy) {
    this.brain = brain || new Brainy()
    this.contentCache = new Map()
    this.statCache = new Map()
    this.watchers = new Map()

    // Default configuration (will be overridden in init)
    this.config = this.getDefaultConfig()
  }

  /**
   * Access to BlobStorage for unified file storage
   */
  private get blobStorage() {
    // Bracket access reaches Brainy's private `storage` field; `blobStorage`
    // is a public (optional) member of BaseStorage, set during brain.init().
    const storage = this.brain['storage']
    if (!storage?.blobStorage) {
      throw new Error(
        'BlobStorage not available. The storage adapter must run ' +
          'initializeBlobStorage() before the VFS is used (brain.init() does this).'
      )
    }
    return storage.blobStorage
  }

  /**
   * Initialize the VFS
   */
  async init(config?: VFSConfig): Promise<void> {
    if (this.initialized) return

    // Merge config with defaults
    this.config = { ...this.getDefaultConfig(), ...config }

    // VFS is now auto-initialized during brain.init()
    // Brain is guaranteed to be initialized when this is called
    // Removed brain.init() check to prevent infinite recursion

    // Create or find root entity
    this.rootEntityId = await this.initializeRoot()

    // Clean up old UUID-based roots (one-time migration)
    await this.cleanupOldRoots()

    // Initialize projection registry with auto-discovery of built-in projections
    this.projectionRegistry = new ProjectionRegistry()
    this.registerBuiltInProjections()

    // Initialize semantic path resolver (zero-config, uses brain.config)
    this.pathResolver = new SemanticPathResolver(
      this.brain,
      this, // Pass VFS instance for resolvePath
      this.rootEntityId,
      this.projectionRegistry
    )

    // Knowledge Layer is now a separate augmentation
    // Enable with: brain.use('knowledge')

    // Start background tasks
    this.startBackgroundTasks()

    this.initialized = true
  }

  /**
   * Create or find the root directory entity
   */
  /**
   * Auto-register built-in projection strategies
   * Zero-config: All semantic dimensions work out of the box
   */
  private registerBuiltInProjections(): void {
    const projections = [
      ConceptProjection,
      AuthorProjection,
      TemporalProjection,
      RelationshipProjection,
      SimilarityProjection,
      TagProjection
    ]

    for (const ProjectionClass of projections) {
      try {
        this.projectionRegistry.register(new ProjectionClass())
      } catch (err) {
        // Silently skip if already registered (e.g., in tests)
        if (!(err instanceof Error && err.message.includes('already registered'))) {
          throw err
        }
      }
    }
  }

  /**
   * CRITICAL FIX - Prevent duplicate root creation
   * Uses singleton promise pattern to ensure only ONE root initialization
   * happens even with concurrent init() calls
   */
  private async initializeRoot(): Promise<string> {
    // If initialization already in progress, wait for it (automatic mutex)
    if (this.rootInitPromise) {
      return await this.rootInitPromise
    }

    // Start initialization and cache the promise
    this.rootInitPromise = this.doInitializeRoot()

    try {
      const rootId = await this.rootInitPromise
      return rootId
    } catch (error) {
      // On error, clear promise so retry is possible
      this.rootInitPromise = null
      throw error
    }
    // NOTE: On success, we intentionally keep the promise cached
    // This prevents re-initialization and serves as a cache
  }

  /**
   * Atomic root initialization with fixed ID
   * Uses deterministic ID to prevent duplicates across all VFS instances
   *
   * ARCHITECTURAL FIX: Instead of query-then-create (race condition),
   * we use a fixed ID so storage-level uniqueness prevents duplicates.
   */
  private async doInitializeRoot(): Promise<string> {
    const rootId = VirtualFileSystem.VFS_ROOT_ID

    // Try to get existing root by fixed ID (O(1) lookup, not query)
    try {
      const existingRoot = await this.brain.get(rootId)

      if (existingRoot) {
        // Root exists - verify metadata is correct
        const metadata = existingRoot.metadata || existingRoot

        if (!metadata.vfsType || metadata.vfsType !== 'directory') {
          console.warn('⚠️  VFS: Root metadata incomplete, repairing...')
          await this.brain.update({
            id: rootId,
            // Re-assert system visibility on repair so a pre-8.0 root (created before the
            // tier existed) is moved out of the default-visible counts/find() too.
            visibility: 'system',
            metadata: this.getRootMetadata()
          })
        }

        return rootId
      }
    } catch (error) {
      // Root doesn't exist yet - proceed to creation
    }

    // Create root with fixed ID (idempotent - fails gracefully if exists)
    try {
      console.log('VFS: Creating root directory (fixed ID: 00000000-0000-0000-0000-000000000000)')

      await this.brain.add({
        id: rootId,  // Fixed ID - storage ensures uniqueness
        data: '/',
        type: NounType.Collection,
        subtype: 'vfs-root',  // Standard subtype for the VFS root collection (7.30+)
        // visibility 'system' (8.0): the VFS root is Brainy's own plumbing, not user data,
        // so it is hidden everywhere by default (getNounCount()/find()/stats()) and surfaces
        // only via find({ includeSystem: true }). 'system' is intentionally not part of the
        // public AddParams.visibility union ('public' | 'internal') — this is the single
        // sanctioned internal setter, hence the cast.
        visibility: 'system' as 'public' | 'internal',
        metadata: this.getRootMetadata()
      })

      return rootId
    } catch (error: any) {
      // If creation failed due to duplicate ID, another instance created it
      // This is normal in concurrent scenarios - just return the fixed ID
      const errorMsg = error?.message?.toLowerCase() || ''
      if (errorMsg.includes('already exists') ||
          errorMsg.includes('duplicate') ||
          errorMsg.includes('eexist')) {
        console.log('VFS: Root already created by another instance, using existing')
        return rootId
      }

      // Unexpected error
      throw error
    }
  }

  /**
   * Get standard root metadata
   * Centralized to ensure consistency
   */
  private getRootMetadata(): VFSMetadata {
    return {
      path: '/',
      name: '',
      vfsType: 'directory',
      isVFS: true,
      isVFSEntity: true,
      size: 0,
      permissions: 0o755,
      owner: 'root',
      group: 'root',
      accessed: Date.now(),
      modified: Date.now()
    }
  }

  /**
   * Cleanup old UUID-based VFS roots
   * Called during init to remove duplicate roots created before fixed-ID fix
   *
   * This is a one-time migration helper that can be removed in future versions.
   */
  private async cleanupOldRoots(): Promise<void> {
    try {
      // Find any old VFS roots with UUID-based IDs (not our fixed ID)
      const oldRoots = await this.brain.find({
        type: NounType.Collection,
        where: {
          path: '/',
          vfsType: 'directory'
        },
        limit: 100,
        excludeVFS: false
      })

      // Filter out our fixed-ID root
      const duplicates = oldRoots.filter(r => r.id !== VirtualFileSystem.VFS_ROOT_ID)

      if (duplicates.length > 0) {
        console.log(`VFS: Found ${duplicates.length} old UUID-based root(s), cleaning up...`)

        for (const duplicate of duplicates) {
          try {
            await this.brain.remove(duplicate.id)
            console.log(`VFS: Deleted old root ${duplicate.id.substring(0, 8)}`)
          } catch (error) {
            console.warn(`VFS: Failed to delete old root ${duplicate.id}:`, error)
          }
        }

        console.log('VFS: Cleanup complete - all old roots removed')
      }
    } catch (error) {
      // Non-critical error - log and continue
      console.warn('VFS: Cleanup of old roots failed (non-critical):', error)
    }
  }

  // ============= File Operations =============

  /**
   * Read a file's content
   */
  async readFile(path: string, options?: ReadOptions): Promise<Buffer> {
    await this.ensureInitialized()

    // Check cache first
    if (options?.cache !== false && this.contentCache.has(path)) {
      const cached = this.contentCache.get(path)!
      if (Date.now() - cached.timestamp < (this.config.cache?.ttl || 300000)) {
        return cached.data
      }
    }

    // Resolve path to entity
    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Verify it's a file
    if (entity.metadata.vfsType !== 'file') {
      throw new VFSError(VFSErrorCode.EISDIR, `Is a directory: ${path}`, path, 'readFile')
    }

    // Unified blob storage - ONE path only
    if (!entity.metadata.storage?.type || entity.metadata.storage.type !== 'blob') {
      throw new VFSError(
        VFSErrorCode.EIO,
        `File has no blob storage: ${path}. Requires blob storage format.`,
        path,
        'readFile'
      )
    }

    // CRITICAL FIX - Isolate blob errors from VFS tree corruption
    // Blob read errors MUST NOT cascade to VFS tree structure
    try {
      // Read from BlobStorage (handles decompression automatically)
      const content = await this.blobStorage.read(entity.metadata.storage.hash)

      // REMOVED updateAccessTime() for performance
      // Access time updates caused 50-100ms GCS write on EVERY file read
      // Modern file systems use 'noatime' for same reason (performance)
      // Field 'accessed' still exists in metadata for backward compat but won't update
      // await this.updateAccessTime(entityId)  // ← REMOVED

      // Cache the content
      if (options?.cache !== false) {
        this.contentCache.set(path, { data: content, timestamp: Date.now() })
      }

      // Apply encoding if requested
      if (options?.encoding) {
        return Buffer.from(content.toString(options.encoding))
      }

      return content
    } catch (blobError) {
      // Blob error isolated - VFS tree structure remains intact
      const errorMsg = blobError instanceof Error ? blobError.message : String(blobError)

      console.error(`VFS: Cannot read blob for ${path}:`, errorMsg)

      // Throw VFSError (not blob error) - prevents cascading corruption
      throw new VFSError(
        VFSErrorCode.EIO,
        `File read failed: ${errorMsg}`,
        path,
        'readFile'
      )
    }
  }

  /**
   * Write a file
   */
  async writeFile(path: string, data: Buffer | string, options?: WriteOptions): Promise<void> {
    await this.ensureInitialized()

    // Convert string to buffer
    const buffer = Buffer.isBuffer(data) ? data : Buffer.from(data, options?.encoding)

    // Check size limits
    if (this.config.limits?.maxFileSize && buffer.length > this.config.limits.maxFileSize) {
      throw new VFSError(VFSErrorCode.ENOSPC, `File too large: ${buffer.length} bytes`, path, 'writeFile')
    }

    // Parse path to get parent and name
    const parentPath = this.getParentPath(path)
    const name = this.getBasename(path)

    // Ensure parent directory exists
    const parentId = await this.ensureDirectory(parentPath)

    // Check if file already exists
    let existingId: string | null = null
    try {
      existingId = await this.pathResolver.resolve(path, { cache: false })
      // Verify the entity still exists in the brain
      const existing = await this.brain.get(existingId)
      if (!existing) {
        existingId = null  // Entity was deleted but cache wasn't cleared
      }
    } catch (err) {
      // File doesn't exist, which is fine
      existingId = null
    }

    // Detect MIME type BEFORE the blob write so the store's auto-compression
    // policy can skip zstd for already-compressed media (JPEG, MP4, ZIP, …).
    const mimeType = mimeDetector.detectMimeType(name, buffer)

    // Unified blob storage for ALL files (no size-based branching)
    // Store in BlobStorage (content-addressable, auto-deduplication)
    const blobHash = await this.blobStorage.write(buffer, { mimeType })

    // Get blob metadata (size, compression info)
    const blobMetadata = await this.blobStorage.getMetadata(blobHash)

    const storageStrategy: VFSMetadata['storage'] = {
      type: 'blob',
      hash: blobHash,
      size: buffer.length,
      compressed: blobMetadata ? blobMetadata.compression !== 'none' : undefined
    }

    // Create metadata
    const metadata: VFSMetadata = {
      path,
      name,
      parent: parentId,
      vfsType: 'file',
      isVFS: true,  // Mark as VFS entity (internal)
      isVFSEntity: true,  // Explicit flag for developer filtering
      size: buffer.length,
      mimeType,
      extension: this.getExtension(name),
      permissions: options?.mode || this.config.permissions?.defaultFile || 0o644,
      owner: 'user',  // In production, get from auth context
      group: 'users',
      accessed: Date.now(),
      modified: Date.now(),
      storage: storageStrategy
      // No rawData - content is in BlobStorage
      // Backward compatibility: readFile() checks for rawData for legacy files
    }

    // Extract additional metadata if enabled
    if (this.config.intelligence?.autoExtract && options?.extractMetadata !== false) {
      Object.assign(metadata, await this.extractMetadata(buffer, mimeType))
    }

    if (existingId) {
      // Update existing file
      // No entity.data - content is in BlobStorage
      await this.brain.update({
        id: existingId,
        metadata
      })

      // Ensure Contains relationship exists (fix for missing relationships)
      const existingRelations = await this.brain.related({
        from: parentId,
        to: existingId,
        type: VerbType.Contains
      })

      // Create relationship if it doesn't exist
      if (existingRelations.length === 0) {
        await this.brain.relate({
          from: parentId,
          to: existingId,
          type: VerbType.Contains,
          subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
          metadata: { isVFS: true }  // Mark as VFS relationship
        })
      }
    } else {
      // Create new file entity
      // For embedding: use text content, for storage: use raw data
      const embeddingData = mimeDetector.isTextFile(mimeType) ? buffer.toString('utf-8') : `File: ${name} (${mimeType}, ${buffer.length} bytes)`

      const entity = await this.brain.add({
        data: embeddingData,  // Always provide string for embeddings
        type: this.getFileNounType(mimeType),
        subtype: 'vfs-file',  // Standard subtype for VFS file entities (7.30+)
        metadata
      })

      // Create parent-child relationship (no need to check for duplicates on new entities)
      await this.brain.relate({
        from: parentId,
        to: entity,
        type: VerbType.Contains,
        subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
        metadata: { isVFS: true }  // Mark as VFS relationship
      })

      // Update path resolver cache
      await this.pathResolver.createPath(path, entity)
    }

    // Invalidate caches
    this.invalidateCaches(path)

    // Trigger watchers
    this.triggerWatchers(path, existingId ? 'change' : 'rename')

    // Knowledge Layer hooks will be added by augmentation if enabled

    // Knowledge Layer hooks will be added by augmentation if enabled
  }

  /**
   * Append to a file
   */
  async appendFile(path: string, data: Buffer | string, options?: WriteOptions): Promise<void> {
    await this.ensureInitialized()

    // Read existing content
    let existing: Buffer
    try {
      existing = await this.readFile(path)
    } catch (err) {
      // File doesn't exist, create it
      return this.writeFile(path, data, options)
    }

    // Append new data
    const newData = Buffer.isBuffer(data) ? data : Buffer.from(data, options?.encoding)
    const combined = Buffer.concat([existing, newData])

    // Write combined content
    await this.writeFile(path, combined, options)
  }

  /**
   * Delete a file
   */
  async unlink(path: string): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Verify it's a file
    if (entity.metadata.vfsType !== 'file') {
      throw new VFSError(VFSErrorCode.EISDIR, `Is a directory: ${path}`, path, 'unlink')
    }

    // Delete blob from BlobStorage (decrements ref count)
    if (entity.metadata.storage?.type === 'blob') {
      await this.blobStorage.delete(entity.metadata.storage.hash)
    }

    // Delete the entity
    await this.brain.remove(entityId)

    // Invalidate caches
    this.pathResolver.invalidatePath(path)
    this.invalidateCaches(path)

    // Trigger watchers
    this.triggerWatchers(path, 'rename')

    // Knowledge Layer hooks will be added by augmentation if enabled
  }

  // ============= Tree Operations (NEW) =============

  /**
   * Get only direct children of a directory - guaranteed no self-inclusion
   * This is the SAFE way to get children for building tree UIs
   */
  async getDirectChildren(path: string): Promise<VFSEntity[]> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Verify it's a directory
    if (entity.metadata.vfsType !== 'directory') {
      throw new VFSError(VFSErrorCode.ENOTDIR, `Not a directory: ${path}`, path, 'getDirectChildren')
    }

    // Use the safe getChildren from PathResolver
    const children = await this.pathResolver.getChildren(entityId)

    // Double-check no self-inclusion (paranoid safety)
    return children.filter(child => child.metadata.path !== path)
  }

  /**
   * Gather descendants using graph traversal + bulk fetch
   *
   * ARCHITECTURE:
   * 1. Traverse graph to collect entity IDs (in-memory, fast)
   * 2. Batch-fetch all entities in ONE storage call
   * 3. Return flat list of VFSEntity objects
   *
   * This is the ONLY correct approach:
   * - Uses GraphAdjacencyIndex (in-memory graph) to traverse relationships
   * - Makes ONE storage call to fetch all entities (not N calls)
   * - Respects maxDepth to limit scope (billion-scale safe)
   *
   * Performance (GCS):
   * - OLD: 111 directories × 50ms each = 5,550ms
   * - NEW: Graph traversal (1ms) + 1 batch fetch (100ms) = 101ms
   * - 55x faster on cloud storage
   *
   * @param rootId - Root directory entity ID
   * @param maxDepth - Maximum depth to traverse
   * @returns All descendant entities (flat list)
   */
  private async gatherDescendants(rootId: string, maxDepth: number): Promise<VFSEntity[]> {
    const entityIds = new Set<string>()
    const visited = new Set<string>([rootId])
    let currentLevel = [rootId]
    let depth = 0

    // Phase 1: Traverse graph in-memory to collect all entity IDs
    // GraphAdjacencyIndex is in-memory LSM-tree, so this is fast (<10ms for 10k relationships)
    while (currentLevel.length > 0 && depth < maxDepth) {
      const nextLevel: string[] = []

      // Get all Contains relationships for this level (in-memory query)
      for (const parentId of currentLevel) {
        const relations = await this.brain.related({
          from: parentId,
          type: VerbType.Contains
        })

        // Collect child IDs
        for (const rel of relations) {
          if (!visited.has(rel.to)) {
            visited.add(rel.to)
            entityIds.add(rel.to)
            nextLevel.push(rel.to)  // Queue for next level
          }
        }
      }

      currentLevel = nextLevel
      depth++
    }

    // Phase 2: Batch-fetch all entities in ONE storage call
    // This is the optimization: ONE GCS call instead of 111+ GCS calls
    const entityIdArray = Array.from(entityIds)
    if (entityIdArray.length === 0) {
      return []
    }

    const entitiesMap = await this.brain.batchGet(entityIdArray)

    // Convert to VFSEntity array
    const entities: VFSEntity[] = []
    for (const id of entityIdArray) {
      const entity = entitiesMap.get(id)
      if (entity && entity.metadata?.vfsType) {
        entities.push(entity as VFSEntity)
      }
    }

    return entities
  }

  /**
   * Get a properly structured tree for the given path
   *
   * Graph traversal + ONE batch fetch (55x faster on cloud storage)
   *
   * Architecture:
   * 1. Resolve path to entity ID
   * 2. Traverse graph in-memory to collect all descendant IDs
   * 3. Batch-fetch all entities in ONE storage call
   * 4. Build tree structure
   *
   * Performance:
   * - GCS: 5,300ms → ~100ms (53x faster)
   * - FileSystem: 200ms → ~50ms (4x faster)
   */
  async getTreeStructure(path: string, options?: {
    maxDepth?: number
    includeHidden?: boolean
    sort?: 'name' | 'modified' | 'size'
  }): Promise<any> {
    await this.ensureInitialized()
    const { VFSTreeUtils } = await import('./TreeUtils.js')

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    if (entity.metadata.vfsType !== 'directory') {
      throw new VFSError(VFSErrorCode.ENOTDIR, `Not a directory: ${path}`, path, 'getTreeStructure')
    }

    const maxDepth = options?.maxDepth ?? 10

    // Gather all descendants (graph traversal + ONE batch fetch)
    const allEntities = await this.gatherDescendants(entityId, maxDepth)

    // Build tree structure
    return VFSTreeUtils.buildTree(allEntities, path, options || {})
  }

  /**
   * Get all descendants of a directory (flat list)
   *
   * Same optimization as getTreeStructure
   */
  async getDescendants(path: string, options?: {
    includeAncestor?: boolean
    type?: 'file' | 'directory'
  }): Promise<VFSEntity[]> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    if (entity.metadata.vfsType !== 'directory') {
      throw new VFSError(VFSErrorCode.ENOTDIR, `Not a directory: ${path}`, path, 'getDescendants')
    }

    // Gather all descendants (no depth limit for this API)
    const descendants = await this.gatherDescendants(entityId, Infinity)

    // Filter by type if specified
    const filtered = options?.type
      ? descendants.filter(d => d.metadata.vfsType === options.type)
      : descendants

    // Include ancestor if requested
    if (options?.includeAncestor) {
      return [entity, ...filtered]
    }

    return filtered
  }

  /**
   * Inspect a path and return structured information
   * This is the recommended method for file explorers to use
   */
  async inspect(path: string): Promise<{
    node: VFSEntity
    children: VFSEntity[]
    parent: VFSEntity | null
    stats: VFSStats
  }> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)
    const stats = await this.stat(path)

    let children: VFSEntity[] = []
    if (entity.metadata.vfsType === 'directory') {
      children = await this.getDirectChildren(path)
    }

    let parent: VFSEntity | null = null
    if (path !== '/') {
      const parentPath = path.substring(0, path.lastIndexOf('/')) || '/'
      const parentId = await this.pathResolver.resolve(parentPath)
      parent = await this.getEntityById(parentId)
    }

    return {
      node: entity,
      children,
      parent,
      stats
    }
  }

  // ============= Directory Operations =============

  /**
   * Create a directory
   */
  async mkdir(path: string, options?: MkdirOptions): Promise<void> {
    await this.ensureInitialized()

    // Use mutex to prevent race conditions when creating the same directory concurrently
    // If another call is already creating this directory, wait for it to complete
    const existingLock = this.mkdirLocks.get(path)
    if (existingLock) {
      await existingLock
      // After waiting, check if directory now exists
      try {
        const existing = await this.pathResolver.resolve(path)
        const entity = await this.getEntityById(existing)
        if (entity.metadata.vfsType === 'directory') {
          return  // Directory was created by the other call
        }
      } catch (err) {
        // Still doesn't exist, proceed to create
      }
    }

    // Create a lock promise for this path
    let resolveLock: () => void
    const lockPromise = new Promise<void>(resolve => { resolveLock = resolve })
    this.mkdirLocks.set(path, lockPromise)

    try {
      // Check if already exists
      try {
        const existing = await this.pathResolver.resolve(path)
        const entity = await this.getEntityById(existing)
        if (entity.metadata.vfsType === 'directory') {
          if (!options?.recursive) {
            throw new VFSError(VFSErrorCode.EEXIST, `Directory exists: ${path}`, path, 'mkdir')
          }
          return  // Already exists and recursive is true
        } else {
          // Path exists but it's not a directory
          throw new VFSError(VFSErrorCode.EEXIST, `File exists: ${path}`, path, 'mkdir')
        }
      } catch (err) {
        // Only proceed if it's a ENOENT error (path doesn't exist)
        if (err instanceof VFSError && err.code !== VFSErrorCode.ENOENT) {
          throw err  // Re-throw non-ENOENT errors
        }
        // Doesn't exist, proceed to create
      }

      // Parse path
      const parentPath = this.getParentPath(path)
      const name = this.getBasename(path)

      // Ensure parent exists (recursive mkdir if needed)
      let parentId: string
      if (parentPath === '/' || parentPath === null) {
        parentId = this.rootEntityId!
      } else if (options?.recursive) {
        parentId = await this.ensureDirectory(parentPath)
      } else {
        try {
          parentId = await this.pathResolver.resolve(parentPath)
        } catch (err) {
          throw new VFSError(VFSErrorCode.ENOENT, `Parent directory not found: ${parentPath}`, path, 'mkdir')
        }
      }

      // Create directory entity
      const metadata: VFSMetadata = {
        path,
        name,
        parent: parentId,
        vfsType: 'directory',
        isVFS: true,  // Mark as VFS entity (internal)
        isVFSEntity: true,  // Explicit flag for developer filtering
        size: 0,
        permissions: options?.mode || this.config.permissions?.defaultDirectory || 0o755,
        owner: 'user',
        group: 'users',
        accessed: Date.now(),
        modified: Date.now(),
        ...options?.metadata
      }

      const entity = await this.brain.add({
        data: path,  // Directory path as string content
        type: NounType.Collection,
        subtype: 'vfs-directory',  // Standard subtype for VFS directory entities (7.30+)
        metadata
      })

      // Create parent-child relationship (no need to check for duplicates on new entities)
      if (parentId !== entity) {  // Don't relate to self (root)
        await this.brain.relate({
          from: parentId,
          to: entity,
          type: VerbType.Contains,
          subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
          metadata: {
            isVFS: true,  // Mark as VFS relationship
            relationshipType: 'vfs'  // Standardized relationship type metadata
          }
        })
      }

      // Update path resolver cache
      await this.pathResolver.createPath(path, entity)

      // Trigger watchers
      this.triggerWatchers(path, 'rename')
    } finally {
      // Release the lock
      resolveLock!()
      this.mkdirLocks.delete(path)
    }
  }

  /**
   * Remove a directory
   *
   * Optimized for cloud storage using batch operations
   * - Uses gatherDescendants() for efficient graph traversal + batch fetch
   * - Uses removeMany() for chunked transactional deletion
   * - Parallel blob cleanup with chunking
   *
   * Performance improvement: 4-8x faster on cloud storage (GCS, S3, R2, Azure)
   * - 15 files on GCS: 120s → 15-30s
   */
  async rmdir(path: string, options?: { recursive?: boolean }): Promise<void> {
    await this.ensureInitialized()

    if (path === '/') {
      throw new VFSError(VFSErrorCode.EACCES, 'Cannot remove root directory', path, 'rmdir')
    }

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Verify it's a directory
    if (entity.metadata.vfsType !== 'directory') {
      throw new VFSError(VFSErrorCode.ENOTDIR, `Not a directory: ${path}`, path, 'rmdir')
    }

    // Check if empty (unless recursive)
    const children = await this.pathResolver.getChildren(entityId)
    if (children.length > 0 && !options?.recursive) {
      throw new VFSError(VFSErrorCode.ENOTEMPTY, `Directory not empty: ${path}`, path, 'rmdir')
    }

    // OPTIMIZED batch deletion for recursive case
    if (options?.recursive && children.length > 0) {
      // Phase 1: Gather all descendants in ONE batch fetch
      const descendants = await this.gatherDescendants(entityId, Infinity)

      // Phase 2: Parallel blob cleanup (chunked to avoid overwhelming storage)
      // Blob deletion is reference-counted, so safe to call for all files
      const blobFiles = descendants.filter(d =>
        d.metadata.vfsType === 'file' && d.metadata.storage?.type === 'blob'
      )

      const BLOB_CHUNK_SIZE = 20  // Parallel delete 20 blobs at a time
      for (let i = 0; i < blobFiles.length; i += BLOB_CHUNK_SIZE) {
        const chunk = blobFiles.slice(i, i + BLOB_CHUNK_SIZE)
        await Promise.all(chunk.map(f =>
          this.blobStorage.delete(f.metadata.storage!.hash)
        ))
      }

      // Phase 3: Batch delete all entities (including root directory)
      const allIds = [...descendants.map(d => d.id), entityId]
      await this.brain.removeMany({ ids: allIds, continueOnError: false })
    } else {
      // No children or not recursive - just delete the directory entity
      await this.brain.remove(entityId)
    }

    // Invalidate caches (recursive invalidation handles all descendants)
    this.pathResolver.invalidatePath(path, true)
    this.invalidateCaches(path, true)

    // Trigger watchers
    this.triggerWatchers(path, 'rename')
  }

  /**
   * Read directory contents
   */
  async readdir(path: string, options?: ReaddirOptions): Promise<string[] | VFSDirent[]> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Verify it's a directory
    if (entity.metadata.vfsType !== 'directory') {
      throw new VFSError(VFSErrorCode.ENOTDIR, `Not a directory: ${path}`, path, 'readdir')
    }

    // Get children
    let children = await this.pathResolver.getChildren(entityId)

    // Apply filters
    if (options?.filter) {
      children = this.filterDirectoryEntries(children, options.filter)
    }

    // Sort if requested
    if (options?.sort) {
      children = this.sortDirectoryEntries(children, options.sort, options.order)
    }

    // Apply pagination
    if (options?.offset) {
      children = children.slice(options.offset)
    }
    if (options?.limit) {
      children = children.slice(0, options.limit)
    }

    // REMOVED updateAccessTime() for performance
    // Directory access time updates caused 50-100ms GCS write on EVERY readdir
    // await this.updateAccessTime(entityId)  // ← REMOVED

    // Return appropriate format
    if (options?.withFileTypes) {
      return children.map(child => ({
        name: child.metadata.name,
        path: child.metadata.path,
        type: child.metadata.vfsType,
        entityId: child.id
      } as VFSDirent))
    }

    return children.map(child => child.metadata.name)
  }

  // ============= Metadata Operations =============

  /**
   * Get file/directory statistics
   */
  async stat(path: string): Promise<VFSStats> {
    await this.ensureInitialized()

    // Check cache
    if (this.statCache.has(path)) {
      const cached = this.statCache.get(path)!
      if (Date.now() - cached.timestamp < 5000) {  // 5 second cache
        return cached.stats
      }
    }

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    const stats: VFSStats = {
      size: entity.metadata.size,
      mode: entity.metadata.permissions,
      uid: 1000,  // In production, map owner to UID
      gid: 1000,  // In production, map group to GID
      atime: new Date(entity.metadata.accessed),
      mtime: new Date(entity.metadata.modified),
      ctime: new Date(entity.updatedAt || entity.createdAt),
      birthtime: new Date(entity.createdAt),
      isFile: () => entity.metadata.vfsType === 'file',
      isDirectory: () => entity.metadata.vfsType === 'directory',
      isSymbolicLink: () => entity.metadata.vfsType === 'symlink',
      path,
      entityId: entity.id,
      vector: entity.vector,
      connections: await this.countRelationships(entityId)
    }

    // Cache stats
    this.statCache.set(path, { stats, timestamp: Date.now() })

    return stats
  }

  /**
   * lstat - same as stat for now (symlinks not fully implemented)
   */
  async lstat(path: string): Promise<VFSStats> {
    return this.stat(path)
  }

  /**
   * Check if path exists
   */
  async exists(path: string): Promise<boolean> {
    await this.ensureInitialized()

    try {
      await this.pathResolver.resolve(path)
      return true
    } catch (err) {
      return false
    }
  }

  // ============= Semantic Operations =============

  /**
   * Search files with natural language
   */
  async search(query: string, options?: SearchOptions): Promise<SearchResult[]> {
    await this.ensureInitialized()

    // Build find params
    const params: FindParams = {
      query,
      type: [NounType.File, NounType.Document, NounType.Media],
      limit: options?.limit || 10,
      offset: options?.offset,
      where: {
        vfsType: 'file'  // Search VFS files
      }
    }

    // Add path filter if specified
    if (options?.path) {
      params.where = {
        ...params.where,
        path: { $startsWith: options.path }
      }
    }

    // Add metadata filters
    if (options?.where) {
      Object.assign(params.where || {}, options.where)
    }

    // Execute search using Brainy's Triple Intelligence
    const results = await this.brain.find(params)

    // Convert to search results
    return results.map(r => {
      const entity = r.entity as VFSEntity
      return {
        path: entity.metadata.path,
        entityId: entity.id,
        score: r.score,
        type: entity.metadata.vfsType,
        size: entity.metadata.size,
        modified: new Date(entity.metadata.modified),
        explanation: r.explanation
      }
    })
  }

  /**
   * Find files similar to a given file
   */
  async findSimilar(path: string, options?: SimilarOptions): Promise<SearchResult[]> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)

    // Use Brainy's similarity search
    const results = await this.brain.similar({
      to: entityId,
      limit: options?.limit || 10,
      threshold: options?.threshold || 0.7,
      type: [NounType.File, NounType.Document, NounType.Media],
      where: {
        vfsType: 'file'  // Find similar VFS files
      }
    })

    return results.map(r => {
      const entity = r.entity as VFSEntity
      return {
        path: entity.metadata.path,
        entityId: entity.id,
        score: r.score,
        type: entity.metadata.vfsType,
        size: entity.metadata.size,
        modified: new Date(entity.metadata.modified)
      }
    })
  }

  // ============= Helper Methods =============

  private async ensureInitialized(): Promise<void> {
    if (!this.initialized) {
      throw new VFSError(
        VFSErrorCode.EINVAL,
        'VFS not initialized. Call await vfs.init() before using VFS operations.\n\n' +
        '✅ After brain.import():\n' +
        '  await brain.import(file, { vfsPath: "/imports/data" })\n' +
        '  const vfs = brain.vfs\n' +
        '  await vfs.init()  // ← Required! Safe to call multiple times\n' +
        '  const files = await vfs.readdir("/imports/data")\n\n' +
        '✅ Direct VFS usage:\n' +
        '  const vfs = brain.vfs\n' +
        '  await vfs.init()  // ← Always required before first use\n' +
        '  await vfs.writeFile("/docs/readme.md", "Hello")\n\n' +
        '📖 Docs: https://github.com/soulcraftlabs/brainy/blob/main/docs/vfs/QUICK_START.md',
        '<unknown>',
        'VFS'
      )
    }
  }

  private async ensureDirectory(path: string): Promise<string> {
    if (!path || path === '/') {
      return this.rootEntityId!
    }

    try {
      const entityId = await this.pathResolver.resolve(path)
      const entity = await this.getEntityById(entityId)
      if (entity.metadata.vfsType !== 'directory') {
        throw new VFSError(VFSErrorCode.ENOTDIR, `Not a directory: ${path}`, path)
      }
      return entityId
    } catch (err) {
      // Only create directory if it doesn't exist (ENOENT error)
      if (err instanceof VFSError && err.code === VFSErrorCode.ENOENT) {
        await this.mkdir(path, { recursive: true })
        return await this.pathResolver.resolve(path)
      }
      // Re-throw other errors (like ENOTDIR)
      throw err
    }
  }

  /**
   * Fetch a Brainy entity by id and normalize it into a {@link VFSEntity}.
   *
   * Backfills VFS metadata for legacy entities that stored fields at the top
   * level (pre-nested-metadata layout) and guarantees the root directory always
   * carries valid directory metadata.
   *
   * @param id - The Brainy entity id to fetch.
   * @returns The entity with a populated `metadata.vfsType` and related VFS fields.
   * @throws {VFSError} ENOENT when no entity exists for the given id.
   */
  async getEntityById(id: string): Promise<VFSEntity> {
    const entity = await this.brain.get(id)

    if (!entity) {
      throw new VFSError(VFSErrorCode.ENOENT, `Entity not found: ${id}`)
    }

    // Ensure entity has proper VFS metadata structure
    // Handle both nested and flat metadata structures for compatibility
    if (!entity.metadata || !entity.metadata.vfsType) {
      // Check if metadata is at top level (legacy structure). Legacy entities
      // carried VFS fields as extra top-level properties not modeled on Entity.
      const anyEntity = entity as Entity & Record<string, unknown>
      if (anyEntity.vfsType || anyEntity.path) {
        entity.metadata = {
          path: anyEntity.path || '/',
          name: anyEntity.name || '',
          vfsType: anyEntity.vfsType || (anyEntity.path === '/' ? 'directory' : 'file'),
          size: anyEntity.size || 0,
          permissions: anyEntity.permissions || (anyEntity.vfsType === 'directory' ? 0o755 : 0o644),
          owner: anyEntity.owner || 'user',
          group: anyEntity.group || 'users',
          accessed: anyEntity.accessed || Date.now(),
          modified: anyEntity.modified || Date.now(),
          ...entity.metadata  // Preserve any existing nested metadata
        }
      } else if (entity.id === this.rootEntityId) {
        // Special case: ensure root directory always has proper metadata
        entity.metadata = {
          path: '/',
          name: '',
          vfsType: 'directory',
          size: 0,
          permissions: 0o755,
          owner: 'root',
          group: 'root',
          accessed: Date.now(),
          modified: Date.now(),
          ...entity.metadata
        }
      }
    }

    return entity as VFSEntity
  }

  private getParentPath(path: string): string {
    const normalized = path.replace(/\/+/g, '/').replace(/\/$/, '')
    const lastSlash = normalized.lastIndexOf('/')
    if (lastSlash <= 0) return '/'
    return normalized.substring(0, lastSlash)
  }

  private getBasename(path: string): string {
    const normalized = path.replace(/\/+/g, '/').replace(/\/$/, '')
    const lastSlash = normalized.lastIndexOf('/')
    return normalized.substring(lastSlash + 1)
  }

  private getExtension(filename: string): string | undefined {
    const lastDot = filename.lastIndexOf('.')
    if (lastDot === -1 || lastDot === 0) return undefined
    return filename.substring(lastDot + 1).toLowerCase()
  }

  // MIME detection moved to MimeTypeDetector service
  // Removed detectMimeType() and isTextFile() - now using mimeDetector singleton

  private getFileNounType(mimeType: string): NounType {
    if (mimeType.startsWith('text/') || mimeType.includes('json')) {
      return NounType.Document
    }
    if (mimeType.startsWith('image/') || mimeType.startsWith('video/') || mimeType.startsWith('audio/')) {
      return NounType.Media
    }
    return NounType.File
  }

  // Removed compression methods (shouldCompress, compress, decompress)
  // BlobStorage handles all compression automatically with zstd

  private async generateEmbedding(buffer: Buffer, mimeType: string): Promise<number[] | undefined> {
    try {
      // Use text content for text files, description for binary
      let content: string
      if (mimeDetector.isTextFile(mimeType)) {
        // Use first 10KB for embedding
        content = buffer.toString('utf8', 0, Math.min(10240, buffer.length))
      } else {
        // For binary files, create a description
        content = `Binary file: ${mimeType}, size: ${buffer.length} bytes`
      }

      // Ensure content is actually a string
      if (typeof content !== 'string') {
        console.debug('Content is not a string:', typeof content, content)
        return undefined
      }

      // Ensure content is not empty or invalid
      if (!content || content.length === 0) {
        console.debug('Content is empty')
        return undefined
      }

      const vector = await this.brain.embed(content)
      return vector
    } catch (error) {
      console.debug('Failed to generate embedding:', error)
      return undefined
    }
  }

  private async extractMetadata(buffer: Buffer, mimeType: string): Promise<Partial<VFSMetadata>> {
    const metadata: Partial<VFSMetadata> = {}

    // Extract basic metadata based on content type
    if (mimeDetector.isTextFile(mimeType)) {
      const text = buffer.toString('utf8')
      metadata.lineCount = text.split('\n').length
      metadata.wordCount = text.split(/\s+/).filter(w => w).length
      metadata.charset = 'utf-8'

      // Extract concepts using brain.extractConcepts() (neural extraction)
      if (this.config.intelligence?.autoConcepts) {
        try {
          const concepts = await this.brain.extractConcepts(text, { limit: 20 })
          metadata.conceptNames = concepts // Flattened for O(log n) queries
        } catch (error) {
          // Concept extraction is optional - don't fail if it errors
          console.debug('Concept extraction failed:', error)
        }
      }
    }

    // Extract hash for integrity
    const crypto = await import('crypto')
    metadata.hash = crypto.createHash('sha256').update(buffer).digest('hex')

    return metadata
  }

  // REMOVED updateAccessTime() method entirely
  // Access time updates caused 50-100ms GCS write on EVERY file/dir read
  // Modern file systems use 'noatime' for same reason
  // Field 'accessed' still exists in metadata for backward compat but won't update

  private async countRelationships(entityId: string): Promise<number> {
    const relations = await this.brain.related({ from: entityId })
    const relationsTo = await this.brain.related({ to: entityId })
    return relations.length + relationsTo.length
  }

  private filterDirectoryEntries(entries: VFSEntity[], filter: any): VFSEntity[] {
    return entries.filter(entry => {
      if (filter.type && entry.metadata.vfsType !== filter.type) return false
      if (filter.pattern && !this.matchGlob(entry.metadata.name, filter.pattern)) return false
      if (filter.minSize && entry.metadata.size < filter.minSize) return false
      if (filter.maxSize && entry.metadata.size > filter.maxSize) return false
      if (filter.modifiedAfter && entry.metadata.modified < filter.modifiedAfter.getTime()) return false
      if (filter.modifiedBefore && entry.metadata.modified > filter.modifiedBefore.getTime()) return false
      return true
    })
  }

  private sortDirectoryEntries(entries: VFSEntity[], sort: string, order?: 'asc' | 'desc'): VFSEntity[] {
    const sorted = [...entries].sort((a, b) => {
      let comparison = 0
      switch (sort) {
        case 'name':
          comparison = a.metadata.name.localeCompare(b.metadata.name)
          break
        case 'size':
          comparison = a.metadata.size - b.metadata.size
          break
        case 'modified':
          comparison = a.metadata.modified - b.metadata.modified
          break
        case 'created':
          comparison = a.createdAt - b.createdAt
          break
      }
      return order === 'desc' ? -comparison : comparison
    })
    return sorted
  }

  private matchGlob(name: string, pattern: string): boolean {
    // Simple glob matching (in production, use proper glob library)
    const regex = pattern
      .replace(/\*/g, '.*')
      .replace(/\?/g, '.')
    return new RegExp(`^${regex}$`).test(name)
  }

  private invalidateCaches(path: string, recursive = false): void {
    this.contentCache.delete(path)
    this.statCache.delete(path)

    if (recursive) {
      const prefix = path.endsWith('/') ? path : path + '/'
      for (const cachedPath of this.contentCache.keys()) {
        if (cachedPath.startsWith(prefix)) {
          this.contentCache.delete(cachedPath)
        }
      }
      for (const cachedPath of this.statCache.keys()) {
        if (cachedPath.startsWith(prefix)) {
          this.statCache.delete(cachedPath)
        }
      }
    }
  }

  private triggerWatchers(path: string, event: 'rename' | 'change'): void {
    const watchers = this.watchers.get(path)
    if (watchers) {
      for (const listener of watchers) {
        listener(event, path)
      }
    }
  }

  private async updateChildrenPaths(parentId: string, oldParentPath: string, newParentPath: string): Promise<void> {
    // Get all children recursively
    const children = await this.pathResolver.getChildren(parentId)

    for (const child of children) {
      const oldChildPath = child.metadata.path as string
      const relativePath = oldChildPath.substring(oldParentPath.length)
      const newChildPath = newParentPath + relativePath

      // Update child entity — metadata-only (mirrors rename() above). Spreading
      // the whole child forwards its `vector` field into update(), which fails
      // dimension validation when the child was fetched without vectors (and
      // would needlessly touch the vector index when it wasn't).
      await this.brain.update({
        id: child.id,
        metadata: {
          ...child.metadata,
          path: newChildPath,
          modified: Date.now()
        }
      })

      // Update path cache
      this.pathResolver.invalidatePath(oldChildPath)
      await this.pathResolver.createPath(newChildPath, child.id)

      // Recursively update if it's a directory
      if (child.metadata.vfsType === 'directory') {
        await this.updateChildrenPaths(child.id, oldChildPath, newChildPath)
      }
    }
  }

  private startBackgroundTasks(): void {
    // Clean up caches periodically
    this.backgroundTimer = setInterval(() => {
      const now = Date.now()

      // Clean content cache
      for (const [path, entry] of this.contentCache) {
        if (now - entry.timestamp > (this.config.cache?.ttl || 300000)) {
          this.contentCache.delete(path)
        }
      }

      // Clean stat cache
      for (const [path, entry] of this.statCache) {
        if (now - entry.timestamp > 5000) {
          this.statCache.delete(path)
        }
      }
    }, 60000)  // Every minute
    // Cache maintenance must never keep the host process alive.
    if (this.backgroundTimer && typeof this.backgroundTimer.unref === 'function') {
      this.backgroundTimer.unref()
    }
  }

  private getDefaultConfig(): Required<Omit<VFSConfig, 'rootEntityId'>> & { rootEntityId?: string } {
    return {
      root: '/',
      rootEntityId: undefined,
      cache: {
        enabled: true,
        maxPaths: 100_000,
        maxContent: 100_000_000,  // 100MB
        ttl: 5 * 60 * 1000  // 5 minutes
      },
      storage: {
        inline: {
          maxSize: 100_000  // 100KB
        },
        chunking: {
          enabled: true,
          chunkSize: 5_000_000,  // 5MB
          parallel: 4
        },
        compression: {
          enabled: true,
          minSize: 10_000,  // 10KB
          algorithm: 'gzip'
        }
      },
      intelligence: {
        enabled: true,
        autoEmbed: true,
        autoExtract: true,
        autoTag: false,
        autoConcepts: false
      },
      permissions: {
        defaultFile: 0o644,
        defaultDirectory: 0o755,
        umask: 0o022
      },
      limits: {
        maxFileSize: 1_000_000_000,  // 1GB
        maxPathLength: 4096,
        maxDirectoryEntries: 100_000
      }
    }
  }

  // ============= Lifecycle, POSIX & Extended Operations =============

  /**
   * Release all resources held by the VFS.
   *
   * Stops background cache eviction, tears down the path resolver, and clears the
   * content cache and watcher registry. After close the VFS is marked
   * uninitialized; call {@link init} again before reusing it.
   *
   * @returns A promise that resolves once cleanup is complete.
   */
  async close(): Promise<void> {
    // Cleanup PathResolver resources
    if (this.pathResolver) {
      this.pathResolver.cleanup()
    }

    // Stop background tasks
    if (this.backgroundTimer) {
      clearInterval(this.backgroundTimer)
      this.backgroundTimer = null
    }

    // Clear caches
    this.contentCache.clear()

    // Clear watchers
    this.watchers.clear()

    this.initialized = false
  }

  /**
   * Change the permission bits of a file or directory (POSIX `chmod`).
   *
   * @param path - The VFS path whose permissions should change.
   * @param mode - The new permission bits (e.g. `0o644`), stored in entity metadata.
   * @returns A promise that resolves once the permissions are persisted.
   */
  async chmod(path: string, mode: number): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Update permissions in metadata
    await this.brain.update({
      ...entity,
      id: entityId,
      metadata: {
        ...entity.metadata,
        permissions: mode,
        modified: Date.now()
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * Change the owner and group of a file or directory (POSIX `chown`).
   *
   * @param path - The VFS path whose ownership should change.
   * @param uid - The new owner user id, stored in entity metadata.
   * @param gid - The new owning group id, stored in entity metadata.
   * @returns A promise that resolves once the ownership is persisted.
   */
  async chown(path: string, uid: number, gid: number): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Update ownership in metadata
    await this.brain.update({
      ...entity,
      id: entityId,
      metadata: {
        ...entity.metadata,
        uid,
        gid,
        modified: Date.now()
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * Set the access and modification timestamps of a file or directory (POSIX `utimes`).
   *
   * @param path - The VFS path to update.
   * @param atime - The new access time.
   * @param mtime - The new modification time.
   * @returns A promise that resolves once the timestamps are persisted.
   */
  async utimes(path: string, atime: Date, mtime: Date): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Update timestamps in metadata
    await this.brain.update({
      ...entity,
      id: entityId,
      metadata: {
        ...entity.metadata,
        accessed: atime.getTime(),
        modified: mtime.getTime()
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * Rename or move a file or directory in place.
   *
   * Updates the entity's path metadata without rewriting content (preserving the
   * underlying blob and entity id). When the parent directory changes, a new
   * `Contains` edge is added to the destination parent. Renaming a directory
   * cascades the path change to every descendant.
   *
   * @param oldPath - The existing VFS path.
   * @param newPath - The destination VFS path; must not already exist.
   * @returns A promise that resolves once the rename is complete.
   * @throws {VFSError} ENOENT when `oldPath` does not exist.
   * @throws {VFSError} EEXIST when `newPath` already exists.
   */
  async rename(oldPath: string, newPath: string): Promise<void> {
    await this.ensureInitialized()

    // Check if source exists
    const entityId = await this.pathResolver.resolve(oldPath)
    const entity = await this.brain.get(entityId)

    if (!entity) {
      throw new VFSError(VFSErrorCode.ENOENT, `No such file or directory: ${oldPath}`, oldPath, 'rename')
    }

    // Check if target already exists
    try {
      await this.pathResolver.resolve(newPath)
      throw new VFSError(VFSErrorCode.EEXIST, `File exists: ${newPath}`, newPath, 'rename')
    } catch (err: any) {
      if (err.code !== VFSErrorCode.ENOENT) throw err
    }

    // Update parent relationships if needed
    const oldParentPath = this.getParentPath(oldPath)
    const newParentPath = this.getParentPath(newPath)

    if (oldParentPath !== newParentPath) {
      // Remove from old parent
      if (oldParentPath) {
        const oldParentId = await this.pathResolver.resolve(oldParentPath)
        // unrelate takes the relation ID, not params - need to find and remove relation
        // For now, skip unrelate as it's not critical for rename
      }

      // Add to new parent
      if (newParentPath && newParentPath !== '/') {
        const newParentId = await this.pathResolver.resolve(newParentPath)
        await this.brain.relate({
          from: newParentId,
          to: entityId,
          type: VerbType.Contains,
          subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
          metadata: { isVFS: true }  // Mark as VFS relationship
        })
      }
    }

    // A rename is a path/metadata change, never a content change — issue a
    // metadata-only update. Spreading the whole entity here used to forward
    // its vector field into update(), which failed dimension validation when
    // the entity was fetched without vectors (and would needlessly touch the
    // vector index when it wasn't).
    await this.brain.update({
      id: entityId,
      metadata: {
        ...entity.metadata,
        path: newPath,
        name: this.getBasename(newPath),
        modified: Date.now()
      }
    })

    // Update path cache
    this.pathResolver.invalidatePath(oldPath, true)
    await this.pathResolver.createPath(newPath, entityId)

    // If it's a directory, update all children paths
    if (entity.metadata.vfsType === 'directory') {
      await this.updateChildrenPaths(entityId, oldPath, newPath)
    }

    // Trigger watchers
    this.triggerWatchers(oldPath, 'rename')
    this.triggerWatchers(newPath, 'rename')
  }

  /**
   * Copy a file or directory to a new path.
   *
   * Files are duplicated as new entities (sharing content via the content-addressed
   * blob store); directories are copied recursively. Unless `overwrite` is set, an
   * existing destination is rejected.
   *
   * @param src - The source VFS path to copy from.
   * @param dest - The destination VFS path to copy to.
   * @param options - Copy options such as `overwrite`, `deepCopy`, `preserveVector`,
   *   and `preserveRelationships`.
   * @returns A promise that resolves once the copy is complete.
   * @throws {VFSError} ENOENT when `src` does not exist.
   * @throws {VFSError} EEXIST when `dest` exists and `overwrite` is not set.
   */
  async copy(src: string, dest: string, options?: CopyOptions): Promise<void> {
    await this.ensureInitialized()

    // Get source entity
    const srcEntityId = await this.pathResolver.resolve(src)
    const srcEntity = await this.brain.get(srcEntityId)

    if (!srcEntity) {
      throw new VFSError(VFSErrorCode.ENOENT, `No such file or directory: ${src}`, src, 'copy')
    }

    // Check if destination already exists
    if (!options?.overwrite) {
      try {
        await this.pathResolver.resolve(dest)
        throw new VFSError(VFSErrorCode.EEXIST, `File exists: ${dest}`, dest, 'copy')
      } catch (err: any) {
        if (err.code !== VFSErrorCode.ENOENT) throw err
      }
    }

    // Copy the entity
    if (srcEntity.metadata.vfsType === 'file') {
      await this.copyFile(srcEntity, dest, options)
    } else if (srcEntity.metadata.vfsType === 'directory') {
      await this.copyDirectory(src, dest, options)
    }
  }

  private async copyFile(srcEntity: Entity, destPath: string, options?: CopyOptions): Promise<void> {
    // Create new entity with same content but different path. Preserve the source
    // entity's subtype when it has one (so a vfs-file stays vfs-file); fall back
    // to 'vfs-file' for the rare case of a VFS entity without subtype (pre-7.30
    // legacy data path that hits the copy operation).
    const newEntity = await this.brain.add({
      type: srcEntity.type,
      subtype: srcEntity.subtype ?? 'vfs-file',
      data: srcEntity.data,
      vector: options?.preserveVector ? srcEntity.vector : undefined,
      metadata: {
        ...srcEntity.metadata,
        path: destPath,
        name: this.getBasename(destPath),
        created: Date.now(),
        modified: Date.now(),
        copiedFrom: srcEntity.metadata.path
      }
    })

    // Add to parent directory
    const parentPath = this.getParentPath(destPath)
    if (parentPath && parentPath !== '/') {
      const parentId = await this.pathResolver.resolve(parentPath)
      await this.brain.relate({
        from: parentId,
        to: newEntity,
        type: VerbType.Contains,
        subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
        metadata: { isVFS: true }  // Mark as VFS relationship
      })
    }

    // Update path cache
    await this.pathResolver.createPath(destPath, newEntity)

    // Copy relationships if requested
    if (options?.preserveRelationships) {
      const relations = await this.brain.related({ from: srcEntity.id })
      for (const relation of relations) {
        if (relation.type !== VerbType.Contains) {
          // Skip relationship without Contains type
          // Future: implement proper relation copying
        }
      }
    }
  }

  /**
   * Copy a directory recursively
   *
   * Optimized for cloud storage using batch operations
   * - Uses gatherDescendants() for efficient graph traversal + batch fetch
   * - Uses addMany() for batch entity creation
   * - Uses relateMany() for batch relationship creation
   *
   * Performance improvement: 3-6x faster on cloud storage (GCS, S3, R2, Azure)
   */
  private async copyDirectory(srcPath: string, destPath: string, options?: CopyOptions): Promise<void> {
    // Shallow copy - just create directory
    if (options?.deepCopy === false) {
      await this.mkdir(destPath, { recursive: true })
      return
    }

    // OPTIMIZED: Batch fetch all source entities in ONE call
    const srcEntityId = await this.pathResolver.resolve(srcPath)
    const descendants = await this.gatherDescendants(srcEntityId, Infinity)
    const srcEntity = await this.getEntityById(srcEntityId)
    const allEntities = [srcEntity, ...descendants]

    // Build path mapping: srcPath -> destPath
    const pathMap = new Map<string, string>()
    const idMap = new Map<string, string>()  // old ID -> new ID

    for (const entity of allEntities) {
      const relativePath = entity.metadata.path.substring(srcPath.length)
      const newPath = destPath + relativePath
      pathMap.set(entity.metadata.path, newPath)
    }

    // Phase 1: Create all directories first (maintain hierarchy)
    // Sort by path length to ensure parents are created before children
    const directories = allEntities
      .filter(e => e.metadata.vfsType === 'directory')
      .sort((a, b) => a.metadata.path.length - b.metadata.path.length)

    for (const dir of directories) {
      const newPath = pathMap.get(dir.metadata.path)!
      await this.mkdir(newPath)  // mkdir is relatively fast
      const newId = await this.pathResolver.resolve(newPath)
      idMap.set(dir.id, newId)
    }

    // Phase 2: Batch-create all files using addMany
    const files = allEntities.filter(e => e.metadata.vfsType === 'file')

    if (files.length > 0) {
      const items = files.map(srcFile => {
        const newPath = pathMap.get(srcFile.metadata.path)!

        return {
          type: srcFile.type,
          data: srcFile.data,
          vector: options?.preserveVector ? srcFile.vector : undefined,
          metadata: {
            ...srcFile.metadata,
            path: newPath,
            name: this.getBasename(newPath),
            parent: undefined,  // Will be set via relationship
            created: Date.now(),
            modified: Date.now(),
            copiedFrom: srcFile.metadata.path
          }
        }
      })

      const result = await this.brain.addMany({ items, continueOnError: false })

      // Build ID mapping for new files
      for (let i = 0; i < files.length; i++) {
        idMap.set(files[i].id, result.successful[i])
      }

      // Phase 3: Batch-create parent relationships using relateMany
      const relations = files.map((srcFile, i) => {
        const newPath = pathMap.get(srcFile.metadata.path)!
        const parentPath = this.getParentPath(newPath)

        // Find parent ID from directories we created
        let parentId: string
        if (parentPath === '/') {
          parentId = VirtualFileSystem.VFS_ROOT_ID
        } else {
          // Find the source directory that maps to this parent path
          const srcParentDir = directories.find(d => pathMap.get(d.metadata.path) === parentPath)
          parentId = srcParentDir ? idMap.get(srcParentDir.id)! : VirtualFileSystem.VFS_ROOT_ID
        }

        return {
          from: parentId,
          to: result.successful[i],
          type: VerbType.Contains,
          subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
          metadata: { isVFS: true }
        }
      })

      await this.brain.relateMany({ items: relations })

      // Phase 4: Update path resolver cache for all new files
      for (let i = 0; i < files.length; i++) {
        const newPath = pathMap.get(files[i].metadata.path)!
        await this.pathResolver.createPath(newPath, result.successful[i])
      }
    }
  }

  /**
   * Move a file or directory to a new path.
   *
   * Implemented as an in-place {@link rename} rather than copy-then-delete: on the
   * content-addressed blob store a copy would share the source content hash, so
   * deleting the source would orphan the destination. Renaming preserves the blob
   * and entity id and handles directory descendants.
   *
   * @param src - The source VFS path to move from.
   * @param dest - The destination VFS path to move to; must not already exist.
   * @returns A promise that resolves once the move is complete.
   * @throws {VFSError} ENOENT when `src` does not exist.
   * @throws {VFSError} EEXIST when `dest` already exists.
   */
  async move(src: string, dest: string): Promise<void> {
    await this.ensureInitialized()

    // A move is a RENAME (in-place path change), NOT copy + delete. The old
    // copy+delete path was broken on the content-addressed blob store: copy()
    // makes the destination reference the SAME content-hash as the source, then
    // unlink(src) deletes that shared blob — orphaning the destination
    // ("Blob metadata not found" on the next read). rename() updates the path
    // in place (preserving the blob, keeping the same entity id) and already
    // handles both files and directories, including child path updates.
    await this.rename(src, dest)
  }

  /**
   * Create a symbolic link at `path` that points to `target` (POSIX `symlink`).
   *
   * The link is stored as a dedicated `vfs-symlink` entity whose
   * `metadata.symlinkTarget` records the target path; it is linked into its parent
   * directory with a `Contains` edge.
   *
   * @param target - The path the symlink should resolve to.
   * @param path - The VFS path at which to create the symlink; must not already exist.
   * @returns A promise that resolves once the symlink is created.
   * @throws {VFSError} EEXIST when `path` already exists.
   */
  async symlink(target: string, path: string): Promise<void> {
    await this.ensureInitialized()

    // Check if symlink already exists
    try {
      await this.pathResolver.resolve(path)
      throw new VFSError(VFSErrorCode.EEXIST, `File exists: ${path}`, path, 'symlink')
    } catch (err: any) {
      if (err.code !== VFSErrorCode.ENOENT) throw err
    }

    // Parse path to get parent and name
    const parentPath = this.getParentPath(path)
    const name = this.getBasename(path)

    // Ensure parent directory exists
    const parentId = await this.ensureDirectory(parentPath)

    // Create symlink entity
    const metadata: VFSMetadata = {
      path,
      name,
      parent: parentId,
      vfsType: 'symlink',
      isVFS: true,            // Infrastructure-bypass marker for strict-mode enforcement
      isVFSEntity: true,
      symlinkTarget: target,
      size: 0,
      permissions: 0o777,
      owner: 'user',
      group: 'users',
      accessed: Date.now(),
      modified: Date.now()
    }

    const entity = await this.brain.add({
      data: `symlink:${target}`,
      type: NounType.File,  // Symlinks are special files
      subtype: 'vfs-symlink',  // Distinct from 'vfs-file' so consumers can find symlinks (7.30.1+)
      metadata
    })

    // Create parent-child relationship
    await this.brain.relate({
      from: parentId,
      to: entity,
      type: VerbType.Contains,
      subtype: 'vfs-contains',  // Standard subtype for VFS containment edges (7.30+)
      metadata: { isVFS: true }  // Mark as VFS relationship
    })

    // Update path resolver cache
    await this.pathResolver.createPath(path, entity)
  }

  /**
   * Read the target of a symbolic link (POSIX `readlink`).
   *
   * @param path - The VFS path of the symlink.
   * @returns The stored target path, or an empty string when no target is recorded.
   * @throws {VFSError} EINVAL when `path` is not a symbolic link.
   */
  async readlink(path: string): Promise<string> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Verify it's a symlink
    if (entity.metadata.vfsType !== 'symlink') {
      throw new VFSError(VFSErrorCode.EINVAL, `Not a symbolic link: ${path}`, path, 'readlink')
    }

    return entity.metadata.symlinkTarget || ''
  }

  /**
   * Resolve a path to its canonical form, following symbolic links (POSIX `realpath`).
   *
   * Symlinks are followed iteratively until a non-symlink target is reached, up to a
   * fixed depth limit that guards against link cycles.
   *
   * @param path - The VFS path to resolve.
   * @returns The fully resolved (non-symlink) path.
   * @throws {VFSError} ENOENT when the path (or a link in the chain) does not exist.
   * @throws {VFSError} ELOOP when too many symbolic links are encountered.
   */
  async realpath(path: string): Promise<string> {
    await this.ensureInitialized()

    // Resolve symlinks recursively
    let currentPath = path
    let depth = 0
    const maxDepth = 20 // Prevent infinite loops

    while (depth < maxDepth) {
      try {
        const entityId = await this.pathResolver.resolve(currentPath)
        const entity = await this.getEntityById(entityId)

        if (entity.metadata.vfsType === 'symlink') {
          // Follow the symlink
          currentPath = entity.metadata.symlinkTarget || ''
          depth++
        } else {
          // Not a symlink, we have the real path
          return currentPath
        }
      } catch (err) {
        throw new VFSError(VFSErrorCode.ENOENT, `No such file or directory: ${path}`, path, 'realpath')
      }
    }

    throw new VFSError(VFSErrorCode.ELOOP, `Too many symbolic links: ${path}`, path, 'realpath')
  }

  /**
   * Read a single extended attribute of a file or directory (POSIX `getxattr`).
   *
   * @param path - The VFS path to read.
   * @param name - The extended-attribute key to fetch.
   * @returns The attribute value, or `undefined` when the attribute is not set.
   */
  async getxattr(path: string, name: string): Promise<any> {
    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)
    return entity.metadata.attributes?.[name]
  }

  /**
   * Set a single extended attribute on a file or directory (POSIX `setxattr`).
   *
   * @param path - The VFS path to update.
   * @param name - The extended-attribute key to set.
   * @param value - The value to store under `name`.
   * @returns A promise that resolves once the attribute is persisted.
   */
  async setxattr(path: string, name: string, value: any): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Create extended attributes object
    const xattrs = entity.metadata.attributes || {}
    xattrs[name] = value

    // Update entity metadata
    await this.brain.update({
      id: entityId,
      metadata: {
        ...entity.metadata,
        attributes: xattrs
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * List the extended-attribute names set on a file or directory (POSIX `listxattr`).
   *
   * @param path - The VFS path to inspect.
   * @returns The list of extended-attribute keys (empty when none are set).
   */
  async listxattr(path: string): Promise<string[]> {
    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)
    return Object.keys(entity.metadata.attributes || {})
  }

  /**
   * Remove a single extended attribute from a file or directory (POSIX `removexattr`).
   *
   * @param path - The VFS path to update.
   * @param name - The extended-attribute key to remove.
   * @returns A promise that resolves once the attribute is removed.
   */
  async removexattr(path: string, name: string): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Remove from extended attributes
    const xattrs = { ...entity.metadata.attributes }
    delete xattrs[name]

    // Update entity metadata
    await this.brain.update({
      ...entity,
      id: entityId,
      metadata: {
        ...entity.metadata,
        attributes: xattrs
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * List the paths related to a file or directory via graph edges (both directions).
   *
   * Walks outgoing and incoming relationships and maps each connected entity back to
   * its VFS path, recording the relationship type and direction.
   *
   * @param path - The VFS path whose relationships to list.
   * @param options - Reserved relationship-filtering options.
   * @returns Related entries, each with the related `path`, `relationship` type, and
   *   `direction` (`'from'` for outgoing edges, `'to'` for incoming).
   */
  async getRelated(path: string, options?: RelatedOptions): Promise<Array<{
    path: string
    relationship: string
    direction: 'from' | 'to'
  }>> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const results: Array<{ path: string, relationship: string, direction: 'from' | 'to' }> = []

    // Use proper Brainy relationship API to get all relationships
    const [fromRelations, toRelations] = await Promise.all([
      this.brain.related({ from: entityId }),
      this.brain.related({ to: entityId })
    ])

    // Add outgoing relationships
    for (const rel of fromRelations) {
      const targetEntity = await this.brain.get(rel.to)
      if (targetEntity && targetEntity.metadata?.path) {
        results.push({
          path: targetEntity.metadata.path,
          relationship: rel.type || 'related',
          direction: 'from'
        })
      }
    }

    // Add incoming relationships
    for (const rel of toRelations) {
      const sourceEntity = await this.brain.get(rel.from)
      if (sourceEntity && sourceEntity.metadata?.path) {
        results.push({
          path: sourceEntity.metadata.path,
          relationship: rel.type || 'related',
          direction: 'to'
        })
      }
    }

    return results
  }

  /**
   * Get the non-hierarchical relationships of a file or directory.
   *
   * Returns both outgoing and incoming edges as {@link Relation} objects, excluding
   * the `Contains` edges that model the directory tree itself.
   *
   * @param path - The VFS path whose relationships to return.
   * @returns The list of semantic relationships connected to the path.
   */
  async getRelationships(path: string): Promise<Relation[]> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const relationships: Relation[] = []

    // Use proper Brainy relationship API
    const [fromRelations, toRelations] = await Promise.all([
      this.brain.related({ from: entityId }),
      this.brain.related({ to: entityId })
    ])

    // Process outgoing relationships (excluding Contains for parent-child)
    for (const rel of fromRelations) {
      if (rel.type !== VerbType.Contains) {  // Skip filesystem hierarchy
        const targetEntity = await this.brain.get(rel.to)
        if (targetEntity && targetEntity.metadata?.path) {
          relationships.push({
            id: rel.id || crypto.randomUUID(),
            from: entityId,
            to: rel.to,
            type: rel.type,
            createdAt: rel.createdAt || Date.now()
          })
        }
      }
    }

    // Process incoming relationships (excluding Contains for parent-child)
    for (const rel of toRelations) {
      if (rel.type !== VerbType.Contains) {  // Skip filesystem hierarchy
        const sourceEntity = await this.brain.get(rel.from)
        if (sourceEntity && sourceEntity.metadata?.path) {
          relationships.push({
            id: rel.id || crypto.randomUUID(),
            from: rel.from,
            to: entityId,
            type: rel.type,
            createdAt: rel.createdAt || Date.now()
          })
        }
      }
    }

    return relationships
  }

  /**
   * Create a graph relationship between two VFS paths.
   *
   * @param from - The source VFS path.
   * @param to - The target VFS path.
   * @param type - The relationship (verb) type to create; coerced to a {@link VerbType}.
   * @returns A promise that resolves once the relationship is created.
   */
  async addRelationship(from: string, to: string, type: string): Promise<void> {
    await this.ensureInitialized()

    const fromEntityId = await this.pathResolver.resolve(from)
    const toEntityId = await this.pathResolver.resolve(to)

    // Create relationship using brain
    await this.brain.relate({
      from: fromEntityId,
      to: toEntityId,
      type: type as VerbType,  // Convert string to VerbType
      metadata: { isVFS: true }  // Mark as VFS relationship
    })

    // Invalidate caches for both paths
    this.invalidateCaches(from)
    this.invalidateCaches(to)
  }

  /**
   * Remove a graph relationship between two VFS paths.
   *
   * Deletes outgoing edges from `from` to `to`; when `type` is given, only edges of
   * that relationship type are removed.
   *
   * @param from - The source VFS path.
   * @param to - The target VFS path.
   * @param type - Optional relationship type to match; when omitted, all matching
   *   edges to `to` are removed.
   * @returns A promise that resolves once the relationship(s) are removed.
   */
  async removeRelationship(from: string, to: string, type?: string): Promise<void> {
    await this.ensureInitialized()

    const fromEntityId = await this.pathResolver.resolve(from)
    const toEntityId = await this.pathResolver.resolve(to)

    // Find and delete the relationship
    const relations = await this.brain.related({ from: fromEntityId })
    for (const relation of relations) {
      if (relation.to === toEntityId && (!type || relation.type === type)) {
        // Delete the relationship using brain.unrelate
        if (relation.id) {
          await this.brain.unrelate(relation.id)
        }
      }
    }

    // Invalidate caches
    this.invalidateCaches(from)
    this.invalidateCaches(to)
  }

  /**
   * Get the todo items attached to a file or directory.
   *
   * @param path - The VFS path whose todos to read.
   * @returns The stored todo list, or `undefined` when none are attached.
   */
  async getTodos(path: string): Promise<VFSMetadata['todos']> {
    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)
    return entity.metadata.todos
  }

  /**
   * Replace the entire todo list attached to a file or directory.
   *
   * @param path - The VFS path to update.
   * @param todos - The full set of todo items to store (overwrites any existing list).
   * @returns A promise that resolves once the todos are persisted.
   */
  async setTodos(path: string, todos: VFSTodo[]): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Update todos in metadata
    await this.brain.update({
      ...entity,
      id: entityId,
      metadata: {
        ...entity.metadata,
        todos,
        modified: Date.now()
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * Append a single todo item to a file or directory.
   *
   * Generates an id when one is not supplied and applies default `priority`
   * (`'medium'`) and `status` (`'pending'`).
   *
   * @param path - The VFS path to attach the todo to.
   * @param todo - The todo to add; `id`, `priority`, and `status` are optional.
   * @returns A promise that resolves once the todo is persisted.
   */
  async addTodo(path: string, todo: VFSTodo): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Get existing todos
    const todos = entity.metadata.todos || []

    // Add new todo with ID if not provided
    const newTodo: VFSTodo = {
      id: todo.id || crypto.randomUUID(),
      task: todo.task,
      priority: todo.priority || 'medium',
      status: todo.status || 'pending',
      assignee: todo.assignee,
      due: todo.due
    }

    todos.push(newTodo)

    // Update entity metadata
    await this.brain.update({
      id: entityId,
      metadata: {
        ...entity.metadata,
        todos
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }

  /**
   * Get metadata for a file or directory
   */
  async getMetadata(path: string): Promise<VFSMetadata | undefined> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    return entity.metadata
  }

  /**
   * Set custom metadata for a file or directory
   * Merges with existing metadata
   */
  async setMetadata(path: string, metadata: Partial<VFSMetadata>): Promise<void> {
    await this.ensureInitialized()

    const entityId = await this.pathResolver.resolve(path)
    const entity = await this.getEntityById(entityId)

    // Merge with existing metadata
    await this.brain.update({
      id: entityId,
      metadata: {
        ...entity.metadata,
        ...metadata,
        modified: Date.now()
      }
    })

    // Invalidate caches
    this.invalidateCaches(path)
  }


  /**
   * Set the current user for tracking who makes changes
   */
  setUser(username: string): void {
    this.currentUser = username || 'system'
  }

  /**
   * Get the current user
   */
  getCurrentUser(): string {
    return this.currentUser
  }



  /**
   * Search for entities with filters
   */
  async searchEntities(query: {
    type?: string
    name?: string
    where?: Record<string, any>
    limit?: number
  }): Promise<Array<{
    id: string
    path: string
    type: string
    metadata: any
  }>> {
    await this.ensureInitialized()

    // Build query for brain.find()
    const searchQuery: any = {
      where: {
        ...query.where,
        vfsType: 'entity'
      },
      limit: query.limit || 100
    }

    if (query.type) {
      searchQuery.where.entityType = query.type
    }

    if (query.name) {
      searchQuery.query = query.name
    }

    const results = await this.brain.find(searchQuery)

    return results.map(result => ({
      id: result.id,
      path: result.entity?.metadata?.path || '',
      type: result.entity?.metadata?.type || result.entity?.metadata?.entityType || 'unknown',
      metadata: result.entity?.metadata || {}
    }))
  }

  /**
   * Sort bulk operations to prevent race conditions
   *
   * Strategy:
   * 1. mkdir operations first, sorted by path depth (shallowest first)
   * 2. Other operations (write, delete, update) after, in original order
   *
   * This ensures parent directories exist before files are written,
   * preventing duplicate entity creation from concurrent mkdir calls.
   */
  private sortBulkOperations(operations: Array<{
    type: 'write' | 'delete' | 'mkdir' | 'update'
    path: string
    data?: Buffer | string
    options?: any
  }>): Array<typeof operations[number]> {
    const mkdirOps: typeof operations = []
    const otherOps: typeof operations = []

    for (const op of operations) {
      if (op.type === 'mkdir') {
        mkdirOps.push(op)
      } else {
        otherOps.push(op)
      }
    }

    // Sort mkdir by path depth (shallowest first)
    mkdirOps.sort((a, b) => {
      const depthA = (a.path.match(/\//g) || []).length
      const depthB = (b.path.match(/\//g) || []).length
      return depthA !== depthB ? depthA - depthB : a.path.localeCompare(b.path)
    })

    return [...mkdirOps, ...otherOps]
  }

  /**
   * Bulk write operations for performance
   *
   * Prevents race condition by processing mkdir operations
   * sequentially before parallel batch processing of other operations.
   */
  async bulkWrite(operations: Array<{
    type: 'write' | 'delete' | 'mkdir' | 'update'
    path: string
    data?: Buffer | string
    options?: any
  }>): Promise<{
    successful: number
    failed: Array<{ operation: any, error: string }>
  }> {
    await this.ensureInitialized()

    const result = {
      successful: 0,
      failed: [] as Array<{ operation: any, error: string }>
    }

    // Sort operations: mkdirs first (by depth), then others
    const sortedOps = this.sortBulkOperations(operations)

    // Separate mkdir operations for sequential processing
    const mkdirOps = sortedOps.filter(op => op.type === 'mkdir')
    const otherOps = sortedOps.filter(op => op.type !== 'mkdir')

    // Phase 1: Process mkdir operations SEQUENTIALLY
    // This prevents the race condition where parallel mkdir calls
    // create duplicate directory entities due to mutex timing window
    for (const op of mkdirOps) {
      try {
        await this.mkdir(op.path, op.options)
        result.successful++
      } catch (error: any) {
        result.failed.push({
          operation: op,
          error: error.message || 'Unknown error'
        })
      }
    }

    // Phase 2: Process other operations in parallel batches
    // These can safely run in parallel since parent directories now exist
    const batchSize = 10
    for (let i = 0; i < otherOps.length; i += batchSize) {
      const batch = otherOps.slice(i, i + batchSize)

      // Process batch in parallel
      const promises = batch.map(async (op) => {
        try {
          switch (op.type) {
            case 'write':
              await this.writeFile(op.path, op.data || '', op.options)
              break
            case 'delete':
              await this.unlink(op.path)
              break
            case 'update': {
              // Update only metadata without changing content
              const entityId = await this.pathResolver.resolve(op.path)
              await this.brain.update({
                id: entityId,
                metadata: op.options?.metadata
              })
              break
            }
          }
          result.successful++
        } catch (error: any) {
          result.failed.push({
            operation: op,
            error: error.message || 'Unknown error'
          })
        }
      })

      await Promise.all(promises)
    }

    return result
  }

  /**
   * Calculate disk usage for a path (POSIX du command)
   * Returns total bytes used by files in directory tree
   *
   * @param path - Path to calculate usage for
   * @param options - Options including maxDepth for safety
   */
  async du(path: string = '/', options?: {
    maxDepth?: number
    humanReadable?: boolean
  }): Promise<{
    bytes: number
    files: number
    directories: number
    formatted?: string
  }> {
    await this.ensureInitialized()

    const maxDepth = options?.maxDepth ?? 100 // Safety limit
    let totalBytes = 0
    let fileCount = 0
    let dirCount = 0

    const traverse = async (currentPath: string, depth: number) => {
      if (depth > maxDepth) {
        throw new Error(`Maximum depth ${maxDepth} exceeded. Use maxDepth option to increase limit.`)
      }

      try {
        const entityId = await this.pathResolver.resolve(currentPath)
        const entity = await this.getEntityById(entityId)

        if (entity.metadata.vfsType === 'directory') {
          dirCount++
          const children = await this.readdir(currentPath)
          for (const child of children) {
            const childPath = currentPath === '/' ? `/${child}` : `${currentPath}/${child}`
            await traverse(childPath, depth + 1)
          }
        } else if (entity.metadata.vfsType === 'file') {
          fileCount++
          totalBytes += entity.metadata.size || 0
        }
      } catch (error) {
        // Skip inaccessible paths
      }
    }

    await traverse(path, 0)

    const result: any = {
      bytes: totalBytes,
      files: fileCount,
      directories: dirCount
    }

    if (options?.humanReadable) {
      const units = ['B', 'KB', 'MB', 'GB', 'TB']
      let size = totalBytes
      let unitIndex = 0
      while (size >= 1024 && unitIndex < units.length - 1) {
        size /= 1024
        unitIndex++
      }
      result.formatted = `${size.toFixed(2)} ${units[unitIndex]}`
    }

    return result
  }

  /**
   * Check file access permissions (POSIX access command)
   * Verifies if path exists and is accessible with specified mode
   *
   * @param path - Path to check
   * @param mode - Access mode: 'r' (read), 'w' (write), 'x' (execute), or 'f' (exists only)
   */
  async access(path: string, mode: 'r' | 'w' | 'x' | 'f' = 'f'): Promise<boolean> {
    await this.ensureInitialized()

    try {
      const entityId = await this.pathResolver.resolve(path)
      const entity = await this.getEntityById(entityId)

      // Path exists
      if (mode === 'f') {
        return true
      }

      // Check permissions based on mode
      const permissions = entity.metadata.permissions || 0o644

      switch (mode) {
        case 'r':
          // Check read permission (owner, group, or other)
          return (permissions & 0o444) !== 0
        case 'w':
          // Check write permission
          return (permissions & 0o222) !== 0
        case 'x':
          // Check execute permission (only meaningful for directories)
          return entity.metadata.vfsType === 'directory' || (permissions & 0o111) !== 0
        default:
          return false
      }
    } catch (error) {
      // Path doesn't exist or not accessible
      return false
    }
  }

  /**
   * Find files matching patterns (Unix find command)
   * Pattern-based file search (complements semantic search())
   *
   * @param path - Starting path for search
   * @param options - Search options including pattern matching
   */
  async find(path: string = '/', options?: {
    name?: string | RegExp
    type?: 'file' | 'directory' | 'both'
    maxDepth?: number
    minSize?: number
    maxSize?: number
    modified?: { after?: Date, before?: Date }
    limit?: number
  }): Promise<Array<{
    path: string
    type: 'file' | 'directory'
    size?: number
    modified?: Date
  }>> {
    await this.ensureInitialized()

    const maxDepth = options?.maxDepth ?? 100 // Safety limit
    const limit = options?.limit ?? 1000 // Prevent unbounded results
    const results: Array<{
      path: string
      type: 'file' | 'directory'
      size?: number
      modified?: Date
    }> = []

    const namePattern = options?.name
    const nameRegex = namePattern instanceof RegExp
      ? namePattern
      : namePattern
        ? new RegExp(namePattern.replace(/\*/g, '.*').replace(/\?/g, '.'))
        : null

    const traverse = async (currentPath: string, depth: number) => {
      if (depth > maxDepth || results.length >= limit) {
        return
      }

      try {
        const entityId = await this.pathResolver.resolve(currentPath)
        const entity = await this.getEntityById(entityId)

        const vfsType = entity.metadata.vfsType
        const fileName = currentPath.split('/').pop() || ''

        // Check if this file matches criteria
        let matches = true

        // Type filter
        if (options?.type && options.type !== 'both') {
          matches = matches && vfsType === options.type
        }

        // Name pattern filter
        if (nameRegex) {
          matches = matches && nameRegex.test(fileName)
        }

        // Size filters (files only)
        if (vfsType === 'file') {
          const size = entity.metadata.size || 0
          if (options?.minSize !== undefined) {
            matches = matches && size >= options.minSize
          }
          if (options?.maxSize !== undefined) {
            matches = matches && size <= options.maxSize
          }
        }

        // Modified time filter
        if (options?.modified && entity.metadata.modified) {
          const modifiedTime = new Date(entity.metadata.modified)
          if (options.modified.after) {
            matches = matches && modifiedTime >= options.modified.after
          }
          if (options.modified.before) {
            matches = matches && modifiedTime <= options.modified.before
          }
        }

        // Add to results if matches
        if (matches && currentPath !== path) {
          results.push({
            path: currentPath,
            type: vfsType as 'file' | 'directory',
            size: entity.metadata.size,
            modified: entity.metadata.modified ? new Date(entity.metadata.modified) : undefined
          })
        }

        // Recurse into directories
        if (vfsType === 'directory' && results.length < limit) {
          const children = await this.readdir(currentPath)
          for (const child of children) {
            if (results.length >= limit) break
            const childPath = currentPath === '/' ? `/${child}` : `${currentPath}/${child}`
            await traverse(childPath, depth + 1)
          }
        }
      } catch (error) {
        // Skip inaccessible paths
      }
    }

    await traverse(path, 0)
    return results
  }

  /**
   * Open a file as a Node-compatible readable stream.
   *
   * @param path - The VFS path of the file to read.
   * @param options - Stream options (e.g. byte range, chunk size).
   * @returns A readable stream that yields the file's bytes.
   */
  createReadStream(path: string, options?: ReadStreamOptions): NodeJS.ReadableStream {
    // Lazy import to avoid circular dependencies
    const { VFSReadStream } = require('./streams/VFSReadStream.js')
    return new VFSReadStream(this, path, options)
  }

  /**
   * Open a file as a Node-compatible writable stream.
   *
   * @param path - The VFS path of the file to write.
   * @param options - Stream options controlling how buffered data is flushed.
   * @returns A writable stream whose contents are persisted to the file on finish.
   */
  createWriteStream(path: string, options?: WriteStreamOptions): NodeJS.WritableStream {
    // Lazy import to avoid circular dependencies
    const { VFSWriteStream } = require('./streams/VFSWriteStream.js')
    return new VFSWriteStream(this, path, options)
  }

  /**
   * Watch a path for changes and invoke a listener on filesystem events.
   *
   * @param path - The VFS path to watch.
   * @param listener - Called with the event type (`'rename'` or `'change'`) and the path.
   * @returns A handle whose `close()` method deregisters the listener.
   */
  watch(path: string, listener: WatchListener): { close(): void } {
    if (!this.watchers.has(path)) {
      this.watchers.set(path, new Set())
    }
    this.watchers.get(path)!.add(listener)

    return {
      close: () => {
        const watchers = this.watchers.get(path)
        if (watchers) {
          watchers.delete(listener)
          if (watchers.size === 0) {
            this.watchers.delete(path)
          }
        }
      }
    }
  }

  // ============= Import/Export Operations =============

  /**
   * Import a single file from the real filesystem into VFS
   */
  async importFile(sourcePath: string, targetPath: string): Promise<void> {
    const fs = await import('fs/promises')
    const pathModule = await import('path')

    // Read file from local filesystem
    const content = await fs.readFile(sourcePath)
    const stats = await fs.stat(sourcePath)

    // Ensure parent directory exists in VFS
    const parentPath = pathModule.dirname(targetPath)
    if (parentPath !== '/' && parentPath !== '.') {
      try {
        await this.mkdir(parentPath, { recursive: true })
      } catch (error: any) {
        if (error.code !== 'EEXIST') throw error
      }
    }

    // Write to VFS with metadata from source
    await this.writeFile(targetPath, content, {
      metadata: {
        imported: true,
        importedFrom: sourcePath,
        sourceSize: stats.size,
        sourceMtime: stats.mtime.getTime(),
        sourceMode: stats.mode
      }
    })
  }

  /**
   * Import a directory from the real filesystem into VFS
   */
  async importDirectory(sourcePath: string, options?: any): Promise<any> {
    const { DirectoryImporter } = await import('./importers/DirectoryImporter.js')
    const importer = new DirectoryImporter(this, this.brain)
    return await importer.import(sourcePath, options)
  }

  /**
   * Import a directory with progress tracking
   */
  async *importStream(sourcePath: string, options?: any): AsyncGenerator<any> {
    const { DirectoryImporter } = await import('./importers/DirectoryImporter.js')
    const importer = new DirectoryImporter(this, this.brain)
    yield* importer.importStream(sourcePath, options)
  }

  /**
   * Register a change listener for a path (Node `fs.watchFile`-style convenience).
   *
   * Delegates to {@link watch} without returning the watcher handle; remove the
   * listener with {@link unwatchFile}.
   *
   * @param path - The VFS path to watch.
   * @param listener - Called with the event type and path on each change.
   */
  watchFile(path: string, listener: WatchListener): void {
    this.watch(path, listener)
  }

  /**
   * Stop watching a path, removing all listeners registered for it.
   *
   * @param path - The VFS path to stop watching.
   */
  unwatchFile(path: string): void {
    this.watchers.delete(path)
  }

  /**
   * Resolve a path and return its backing {@link VFSEntity}.
   *
   * @param path - The VFS path to resolve.
   * @returns The entity (with normalized VFS metadata) for the path.
   * @throws {VFSError} ENOENT when the path cannot be resolved.
   */
  async getEntity(path: string): Promise<VFSEntity> {
    const entityId = await this.pathResolver.resolve(path)
    return this.getEntityById(entityId)
  }

  /**
   * Resolve a path to its normalized form
   * Returns the normalized absolute path (e.g., '/foo/bar/file.txt')
   */
  async resolvePath(path: string, from?: string): Promise<string> {
    // Handle relative paths
    if (!path.startsWith('/') && from) {
      path = `${from}/${path}`
    }

    // Normalize path: remove multiple slashes, trailing slashes
    return path.replace(/\/+/g, '/').replace(/\/$/, '') || '/'
  }

  /**
   * Resolve a path to its entity ID
   * Returns the UUID of the entity representing this path
   */
  async resolvePathToId(path: string, from?: string): Promise<string> {
    // Handle relative paths
    if (!path.startsWith('/') && from) {
      path = `${from}/${path}`
    }

    // Normalize path
    const normalizedPath = path.replace(/\/+/g, '/').replace(/\/$/, '') || '/'

    // Special case for root
    if (normalizedPath === '/') {
      return this.rootEntityId!
    }

    // Resolve the path to an entity ID
    return await this.pathResolver.resolve(normalizedPath)
  }
}