import { createReadStream } from "fs";
import { createHash } from "crypto";
import { pipeline } from "stream/promises";
import { PassThrough } from "stream";

/**
 * Compute SHA-256 hash of one or more files by streaming them in order.
 * Memory usage: O(1) — reads in 64KB chunks regardless of total size.
 * For multipart archives, pass all parts sorted by part number.
 */
export async function hashParts(filePaths: string[]): Promise<string> {
  const hash = createHash("sha256");
  for (const filePath of filePaths) {
    await pipeline(
      createReadStream(filePath),
      new PassThrough({
        transform(chunk, _encoding, callback) {
          hash.update(chunk);
          callback();
        },
      })
    );
  }
  return hash.digest("hex");
}