/**
 * @module storage/blobStorage
 * @description Content-addressed blob store. Backs VFS file content with
 * SHA-256 addressing (automatic deduplication), reference counting, zstd
 * compression where it pays (MIME-aware: already-compressed media is stored
 * raw), and an LRU read cache.
 *
 * The store persists through a narrow key-value bridge
 * ({@link BlobStoreAdapter}) provided by `BaseStorage`, which roots all keys
 * under the `_cas/` storage area. Key naming is an explicit type contract:
 * `blob:<hash>` keys hold binary bytes, `blob-meta:<hash>` keys hold JSON
 * metadata — the key format decides how bytes are encoded, never content
 * sniffing.
 */

import { createHash } from 'crypto'
import { unwrapBinaryData } from './binaryDataCodec.js'

/**
 * @description Key-value bridge the blob store persists through. Implemented
 * by `BaseStorage.initializeBlobStorage()` over the adapter's raw object
 * primitives.
 */
export interface BlobStoreAdapter {
  /** Read the bytes stored under `key`, or `undefined` when absent. */
  get(key: string): Promise<Buffer | undefined>
  /** Persist `data` under `key` (overwrites). */
  put(key: string, data: Buffer): Promise<void>
  /** Delete the value under `key`. Missing keys are ignored. */
  delete(key: string): Promise<void>
  /** List all keys starting with `prefix`. */
  list(prefix: string): Promise<string[]>
}

/**
 * @description Metadata persisted alongside each blob (under
 * `blob-meta:<hash>`).
 */
export interface BlobMetadata {
  /** SHA-256 content hash (the blob's identity). */
  hash: string
  /** Original (uncompressed) size in bytes. */
  size: number
  /** Stored size in bytes (after compression, if any). */
  compressedSize: number
  /** Compression applied to the stored bytes. */
  compression: 'none' | 'zstd'
  /** Creation timestamp (epoch ms). */
  createdAt: number
  /** Number of logical references to this blob (deduplicated writes). */
  refCount: number
}

/**
 * @description Options for {@link BlobStorage.write}.
 */
export interface BlobWriteOptions {
  /**
   * Compression strategy. `'auto'` (default) compresses payloads above 1 KB
   * with zstd unless the MIME type says the bytes are already compressed.
   * Explicit `'none'`/`'zstd'` is honoured as asserted by the caller.
   */
  compression?: 'none' | 'zstd' | 'auto'
  /**
   * Content type of the payload (e.g. `image/jpeg`, `video/mp4`).
   *
   * When set on an `auto`-compression write, the store skips zstd for MIME
   * types that are already heavily compressed (JPEG, PNG, WebP, MP4, WebM,
   * MP3, ZIP, PDF, etc.). zstd over these formats wastes CPU and rarely
   * shaves more than a single-digit percent — usually it actually grows the
   * payload because the entropy is already maximised by the format itself.
   *
   * Has no effect when `compression` is `'none'` or `'zstd'` explicitly —
   * the caller is asserting the choice and the store honours it.
   */
  mimeType?: string
}

/**
 * MIME types whose payload is already heavily compressed. zstd over these is
 * almost always a CPU-only loss — the bytes are already near entropy-maximal,
 * so the output is the same size or slightly larger plus the cost of running
 * the compressor. Used by `BlobStorage.selectCompression()` in `auto` mode.
 *
 * Conservative denylist (well-known formats only). Anything not in this set
 * goes through the size heuristic. False negatives (compressing something we
 * should have skipped) waste CPU; false positives (skipping something we
 * could have compressed) waste a few percent of bytes. The denylist favours
 * CPU-cycle safety because the formats listed here are the ones where
 * gzip/zstd is reliably a net loss.
 */
const ALREADY_COMPRESSED_MIME_TYPES = new Set<string>([
  // Images
  'image/jpeg', 'image/jpg', 'image/png', 'image/gif', 'image/webp',
  'image/avif', 'image/heic', 'image/heif', 'image/jp2',
  // Video
  'video/mp4', 'video/webm', 'video/x-matroska', 'video/quicktime',
  'video/x-msvideo', 'video/mpeg', 'video/3gpp', 'video/x-ms-wmv',
  // Audio
  'audio/mpeg', 'audio/mp4', 'audio/aac', 'audio/ogg', 'audio/webm',
  'audio/opus', 'audio/flac', 'audio/x-ms-wma',
  // Archives
  'application/zip', 'application/gzip', 'application/x-gzip',
  'application/x-bzip2', 'application/x-7z-compressed',
  'application/x-rar-compressed', 'application/x-xz', 'application/x-zstd',
  'application/x-compress', 'application/vnd.rar',
  // Documents with internal compression
  'application/pdf', 'application/epub+zip',
  // Office formats (zip-based)
  'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
  'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
  'application/vnd.openxmlformats-officedocument.presentationml.presentation',
  'application/vnd.oasis.opendocument.text',
  'application/vnd.oasis.opendocument.spreadsheet',
  'application/vnd.oasis.opendocument.presentation'
])

/**
 * @description True when the MIME type names a payload format known to be
 * already heavily compressed. Strips any `;charset=…` / `;boundary=…`
 * parameters and lowercases the bare type/subtype before lookup, so
 * `'IMAGE/JPEG; charset=binary'` and `'image/jpeg'` resolve identically.
 * @param mimeType - MIME type string, or `undefined`.
 * @returns Whether `auto` compression should skip zstd for this payload.
 */
export function isAlreadyCompressedMimeType(mimeType: string | undefined): boolean {
  if (!mimeType) return false
  const bare = mimeType.split(';', 1)[0].trim().toLowerCase()
  return ALREADY_COMPRESSED_MIME_TYPES.has(bare)
}

/**
 * LRU cache entry.
 */
interface CacheEntry {
  data: Buffer
  metadata: BlobMetadata
  lastAccess: number
  size: number
}

/**
 * @description Content-addressed, deduplicating, reference-counted blob
 * store with MIME-aware zstd compression and an LRU read cache. See the
 * module doc for the persistence contract.
 *
 * @example
 * const hash = await blobStorage.write(buffer, { mimeType: 'image/png' })
 * const bytes = await blobStorage.read(hash) // verified against the hash
 * await blobStorage.delete(hash)             // decrements refCount first
 */
export class BlobStorage {
  private adapter: BlobStoreAdapter
  private cache: Map<string, CacheEntry>
  private cacheMaxSize: number
  private currentCacheSize: number

  // Compression (lazily loaded)
  private zstdCompress?: (data: Buffer) => Promise<Buffer>
  private zstdDecompress?: (data: Buffer) => Promise<Buffer>
  private compressionReady = false

  // Configuration
  private readonly CACHE_MAX_SIZE = 100 * 1024 * 1024 // 100MB default
  private readonly COMPRESSION_THRESHOLD = 1024 // 1KB - don't compress smaller

  /**
   * @param adapter - Key-value bridge to persist through.
   * @param options - `cacheMaxSize` bounds the LRU read cache (bytes,
   *   default 100 MB).
   */
  constructor(adapter: BlobStoreAdapter, options?: { cacheMaxSize?: number }) {
    this.adapter = adapter
    this.cache = new Map()
    this.cacheMaxSize = options?.cacheMaxSize ?? this.CACHE_MAX_SIZE
    this.currentCacheSize = 0
  }

  /**
   * Lazy-load the zstd compression module. Falls back to uncompressed
   * storage when the optional dependency is unavailable.
   */
  private async ensureCompressionReady(): Promise<void> {
    if (this.compressionReady) return
    try {
      // Dynamic import to avoid loading if not needed
      // @ts-ignore - Optional dependency, gracefully handled if missing
      const zstd = await import('@mongodb-js/zstd')
      this.zstdCompress = async (data: Buffer) => {
        return Buffer.from(await zstd.compress(data, 3)) // Level 3 = fast
      }
      this.zstdDecompress = async (data: Buffer) => {
        return Buffer.from(await zstd.decompress(data))
      }
    } catch (error) {
      console.warn('zstd compression not available, falling back to uncompressed')
      this.zstdCompress = undefined
      this.zstdDecompress = undefined
    }
    this.compressionReady = true
  }

  /**
   * @description Compute the SHA-256 content hash of `data`.
   * @param data - Bytes to hash.
   * @returns Hex-encoded SHA-256 hash.
   */
  static hash(data: Buffer): string {
    return createHash('sha256').update(data).digest('hex')
  }

  /**
   * @description Write a blob. Content-addressed: the SHA-256 hash of the
   * bytes is the storage key, so identical payloads deduplicate (the
   * existing blob's reference count is incremented instead of rewriting).
   *
   * @param data - Blob bytes.
   * @param options - Compression strategy and MIME hint (see
   *   {@link BlobWriteOptions}).
   * @returns The blob's SHA-256 hash.
   */
  async write(data: Buffer, options: BlobWriteOptions = {}): Promise<string> {
    const hash = BlobStorage.hash(data)

    // Deduplication: identical content already stored — just add a reference.
    if (await this.has(hash)) {
      await this.incrementRefCount(hash)
      return hash
    }

    await this.ensureCompressionReady()

    // Determine compression strategy
    const compression = this.selectCompression(data, options)

    // Compress if needed
    let finalData = data
    let compressedSize = data.length

    if (compression === 'zstd' && this.zstdCompress) {
      finalData = await this.zstdCompress(data)
      compressedSize = finalData.length
    }

    // Record the ACTUAL compression state, not the intended one — prevents
    // corruption if compression failed to initialize.
    const actualCompression = finalData === data ? 'none' : compression
    const metadata: BlobMetadata = {
      hash,
      size: data.length,
      compressedSize,
      compression: actualCompression,
      createdAt: Date.now(),
      refCount: 1
    }

    await this.adapter.put(`blob:${hash}`, finalData)
    await this.adapter.put(`blob-meta:${hash}`, Buffer.from(JSON.stringify(metadata)))

    // Write-through cache (caches the ORIGINAL bytes, not the compressed form)
    this.addToCache(hash, data, metadata)

    return hash
  }

  /**
   * @description Read a blob: LRU cache first, then storage with
   * decompression and integrity verification (the bytes are re-hashed and
   * compared against the requested hash).
   *
   * @param hash - The blob's SHA-256 hash.
   * @returns The original (decompressed) blob bytes.
   * @throws Error when the blob is missing or fails integrity verification.
   */
  async read(hash: string): Promise<Buffer> {
    // Check cache first
    const cached = this.getFromCache(hash)
    if (cached) {
      return cached.data
    }

    const metadataBuffer = await this.adapter.get(`blob-meta:${hash}`)
    if (!metadataBuffer) {
      throw new Error(`Blob metadata not found: ${hash}`)
    }
    // Unwrap before parsing (defense-in-depth): metadata should come back as
    // JSON bytes, but an adapter might return the wrapped binary format.
    const metadata: BlobMetadata = JSON.parse(unwrapBinaryData(metadataBuffer).toString())

    const data = await this.adapter.get(`blob:${hash}`)
    if (!data) {
      throw new Error(`Blob not found: ${hash}`)
    }

    // Decompress if needed
    let finalData = data
    if (metadata.compression === 'zstd') {
      if (!this.zstdDecompress) {
        await this.ensureCompressionReady()
      }
      if (!this.zstdDecompress) {
        throw new Error('zstd decompression not available')
      }
      finalData = await this.zstdDecompress(data)
    }

    // Defense-in-depth unwrap: even though the bridge unwraps, verify it
    // happened and re-unwrap if needed. Hash verification must run on the
    // original content bytes.
    const unwrappedData = unwrapBinaryData(finalData)

    // Integrity verification (always on — a content-addressed store that
    // returns bytes not matching the address is corruption, not a result)
    if (BlobStorage.hash(unwrappedData) !== hash) {
      throw new Error(`Blob integrity check failed: ${hash}`)
    }

    this.addToCache(hash, unwrappedData, metadata)

    return unwrappedData
  }

  /**
   * @description Whether a blob with this hash exists (cache or storage).
   * @param hash - The blob's SHA-256 hash.
   * @returns True when the blob exists.
   */
  async has(hash: string): Promise<boolean> {
    if (this.cache.has(hash)) {
      return true
    }
    const exists = await this.adapter.get(`blob:${hash}`)
    return exists !== undefined
  }

  /**
   * @description Drop one reference to the blob. The stored bytes and
   * metadata are physically deleted only when the reference count reaches
   * zero — deduplicated content shared by other writers survives.
   *
   * @param hash - The blob's SHA-256 hash.
   */
  async delete(hash: string): Promise<void> {
    const refCount = await this.decrementRefCount(hash)

    // Only delete if no references remain
    if (refCount > 0) {
      return
    }

    await this.adapter.delete(`blob:${hash}`)
    await this.adapter.delete(`blob-meta:${hash}`)
    this.removeFromCache(hash)
  }

  /**
   * @description Read a blob's metadata without reading its bytes.
   * @param hash - The blob's SHA-256 hash.
   * @returns The metadata, or `undefined` when the blob does not exist.
   */
  async getMetadata(hash: string): Promise<BlobMetadata | undefined> {
    const data = await this.adapter.get(`blob-meta:${hash}`)
    if (data) {
      return JSON.parse(unwrapBinaryData(data).toString())
    }
    return undefined
  }

  // ========== PRIVATE METHODS ==========

  /**
   * Select the compression strategy for a write (see
   * {@link BlobWriteOptions.compression}).
   */
  private selectCompression(
    data: Buffer,
    options: BlobWriteOptions
  ): 'none' | 'zstd' {
    if (options.compression === 'none') {
      return 'none'
    }

    if (options.compression === 'zstd') {
      return this.zstdCompress ? 'zstd' : 'none'
    }

    // Auto mode
    if (data.length < this.COMPRESSION_THRESHOLD) {
      return 'none' // Too small to benefit
    }

    // Content-type policy: skip already-compressed media. zstd over
    // JPEG / MP4 / ZIP etc. is a CPU loss for no measurable byte savings,
    // and on hot save paths (image / video uploads) it's the difference
    // between fast and slow. Applies only to `auto`; explicit `'zstd'` is
    // honoured because the caller is asserting the choice.
    if (isAlreadyCompressedMimeType(options.mimeType)) {
      return 'none'
    }

    return this.zstdCompress ? 'zstd' : 'none'
  }

  /**
   * Increment the reference count for an existing blob.
   */
  private async incrementRefCount(hash: string): Promise<number> {
    const metadata = await this.getMetadata(hash)
    if (!metadata) {
      throw new Error(`Cannot increment ref count, blob not found: ${hash}`)
    }

    metadata.refCount++
    await this.adapter.put(`blob-meta:${hash}`, Buffer.from(JSON.stringify(metadata)))
    return metadata.refCount
  }

  /**
   * Decrement the reference count for a blob (floored at zero).
   */
  private async decrementRefCount(hash: string): Promise<number> {
    const metadata = await this.getMetadata(hash)
    if (!metadata) {
      return 0
    }

    metadata.refCount = Math.max(0, metadata.refCount - 1)
    await this.adapter.put(`blob-meta:${hash}`, Buffer.from(JSON.stringify(metadata)))
    return metadata.refCount
  }

  /**
   * Add a blob to the LRU cache (evicting least-recently-used entries to
   * stay under the size bound).
   */
  private addToCache(hash: string, data: Buffer, metadata: BlobMetadata): void {
    if (data.length > this.cacheMaxSize) {
      return // Blob too large for cache
    }

    while (
      this.currentCacheSize + data.length > this.cacheMaxSize &&
      this.cache.size > 0
    ) {
      this.evictLRU()
    }

    this.cache.set(hash, {
      data,
      metadata,
      lastAccess: Date.now(),
      size: data.length
    })

    this.currentCacheSize += data.length
  }

  /**
   * Get a blob from the cache, refreshing its LRU position.
   */
  private getFromCache(hash: string): CacheEntry | undefined {
    const entry = this.cache.get(hash)
    if (entry) {
      entry.lastAccess = Date.now() // Update LRU
    }
    return entry
  }

  /**
   * Remove a blob from the cache.
   */
  private removeFromCache(hash: string): void {
    const entry = this.cache.get(hash)
    if (entry) {
      this.cache.delete(hash)
      this.currentCacheSize -= entry.size
    }
  }

  /**
   * Evict the least-recently-used cache entry.
   */
  private evictLRU(): void {
    let oldestHash: string | null = null
    let oldestTime = Infinity

    for (const [hash, entry] of this.cache.entries()) {
      if (entry.lastAccess < oldestTime) {
        oldestTime = entry.lastAccess
        oldestHash = hash
      }
    }

    if (oldestHash) {
      this.removeFromCache(oldestHash)
    }
  }
}