import { createHash, randomUUID } from "node:crypto";
import { createWriteStream } from "node:fs";
import { mkdir, rename, stat, unlink } from "node:fs/promises";
import { join } from "node:path";
import type { Readable } from "node:stream";

/**
 * `<64 hex sha256>` plus an optional short, lowercase, alphanumeric extension.
 * Everything else is rejected before any filesystem call — the name is the only
 * client-controlled thing that ever reaches a path.
 */
const NAME_PATTERN = /^[0-9a-f]{64}(\.[a-z0-9]{1,16})?$/;

export function isValidObjectName(name: string): boolean {
  return NAME_PATTERN.test(name);
}

export function hashOf(name: string): string {
  return name.slice(0, 64);
}

export type UploadOutcome =
  | { status: "created" }
  | { status: "exists" }
  | { status: "hash-mismatch"; actual: string }
  | { status: "too-large" };

export class UploadDir {
  private readonly downloadsDir: string;
  private readonly maxBytes: number;
  private readonly tmpDir: string;

  // No parameter properties: `node --test` runs this file under strip-only
  // type stripping, which rejects them.
  constructor(downloadsDir: string, maxBytes: number) {
    this.downloadsDir = downloadsDir;
    this.maxBytes = maxBytes;
    // Same filesystem as the destination, or the final rename stops being
    // atomic and a half-written file could be seen with its final name.
    this.tmpDir = join(downloadsDir, ".internaluploader-tmp");
  }

  async init(): Promise<void> {
    await mkdir(this.tmpDir, { recursive: true });
  }

  async exists(name: string): Promise<boolean> {
    try {
      await stat(join(this.downloadsDir, name));
      return true;
    } catch {
      return false;
    }
  }

  /**
   * Stream a request body to disk, verify its sha256 matches the declared
   * name, and only then rename it into the downloads directory. Idempotent:
   * a name that already exists is reported without touching anything.
   */
  async receive(name: string, body: Readable): Promise<UploadOutcome> {
    if (await this.exists(name)) {
      return { status: "exists" };
    }

    const tmpPath = join(this.tmpDir, randomUUID());
    const hash = createHash("sha256");
    let received = 0;

    try {
      await new Promise<void>((resolve, reject) => {
        const out = createWriteStream(tmpPath, { flags: "wx", mode: 0o644 });
        body.on("data", (chunk: Buffer) => {
          received += chunk.length;
          if (received > this.maxBytes) {
            body.destroy();
            out.destroy();
            reject(new TooLargeError());
            return;
          }
          hash.update(chunk);
        });
        body.on("error", reject);
        out.on("error", reject);
        out.on("finish", resolve);
        body.pipe(out);
      });

      const actual = hash.digest("hex");
      if (actual !== hashOf(name)) {
        await unlink(tmpPath);
        return { status: "hash-mismatch", actual };
      }

      // Lost race with a concurrent identical upload: same content, so the
      // file already on disk is exactly what this request carried.
      if (await this.exists(name)) {
        await unlink(tmpPath);
        return { status: "exists" };
      }

      await rename(tmpPath, join(this.downloadsDir, name));
      return { status: "created" };
    } catch (error) {
      await unlink(tmpPath).catch(() => {});
      if (error instanceof TooLargeError) {
        return { status: "too-large" };
      }
      throw error;
    }
  }
}

class TooLargeError extends Error {
  constructor() {
    super("upload exceeds the configured maximum");
  }
}