implemented a lot of features

lib/yenc/yenc_file.ts

@@ -0,0 +1,86 @@
+import { crc32Hex, type DecodedSegment } from './decode.js'
+import { YencCrcError, YencError } from './errors.js'
+import { inferTotalParts } from './meta.js'
+
+/**
+ * Assembles the decoded segments of one file into a single buffer. Segments
+ * may be added in any order.
+ */
+export class YencFile {
+  readonly name: string
+  readonly size: number
+  readonly totalParts: number
+
+  private readonly buffer: Buffer
+  private readonly received = new Set<number>()
+  private bytesWritten = 0
+  private fileCrc?: string
+
+  constructor(name: string, size: number, totalParts = 1) {
+    this.name = name
+    this.size = size
+    this.totalParts = totalParts
+    this.buffer = Buffer.alloc(size)
+  }
+
+  /** Creates a file sized from the metadata of any of its segments. */
+  static fromSegment(segment: DecodedSegment): YencFile {
+    const { begin } = segment.meta
+    // Many posters leave total= out; it follows from the part size.
+    return new YencFile(begin.name, begin.size, inferTotalParts(segment.meta))
+  }
+
+  add(segment: DecodedSegment): void {
+    const { meta, data, offset } = segment
+    // Only the size is compared: obfuscating posters can give every segment
+    // of one file a different random name.
+    if (meta.begin.size !== this.size) {
+      throw new YencError(
+        `Segment of "${meta.begin.name}" (${meta.begin.size} bytes) does not belong to "${this.name}" (${this.size} bytes)`
+      )
+    }
+    if (offset + data.length > this.size) {
+      throw new YencError(`Segment at offset ${offset} overruns file size ${this.size}`)
+    }
+
+    const partNumber = meta.begin.part ?? 1
+    if (this.received.has(partNumber)) return
+
+    data.copy(this.buffer, offset)
+    this.received.add(partNumber)
+    this.bytesWritten += data.length
+    if (meta.end.crc32) this.fileCrc = meta.end.crc32
+  }
+
+  get partsReceived(): number {
+    return this.received.size
+  }
+
+  get isComplete(): boolean {
+    return this.received.size === this.totalParts && this.bytesWritten === this.size
+  }
+
+  /** Part numbers (1-based) that have not been added yet. */
+  missingParts(): number[] {
+    const missing: number[] = []
+    for (let part = 1; part <= this.totalParts; part++) {
+      if (!this.received.has(part)) missing.push(part)
+    }
+    return missing
+  }
+
+  /**
+   * Returns the assembled file. Throws if parts are missing or, when the poster
+   * included a whole-file crc32, if it does not match.
+   */
+  getBuffer(): Buffer {
+    if (!this.isComplete) {
+      throw new YencError(`File "${this.name}" is incomplete, missing parts: ${this.missingParts().join(', ')}`)
+    }
+    if (this.fileCrc) {
+      const actual = crc32Hex(this.buffer)
+      if (actual !== this.fileCrc) throw new YencCrcError(this.fileCrc, actual)
+    }
+    return this.buffer
+  }
+}