/**
 * The admin login file: format, password hashing, and atomic read/write.
 *
 * Plain Node on purpose (no "server-only", no path aliases, no TS-only syntax) so that
 * scripts/create-admin.mts can run it directly with `node` to create the file outside the
 * app. The app uses it through admin-auth.ts.
 */
import { randomBytes, scrypt as scryptCb, timingSafeEqual, type ScryptOptions } from "node:crypto";
import { link, mkdir, readFile, rename, unlink, writeFile } from "node:fs/promises";
import path from "node:path";
import { z } from "zod";
import { adminFile } from "./paths.ts";

// Runtime data, not a build input: the ignore comment keeps it out of Next's output file
// tracing (see camera-registry.ts).
/** ADMIN_AUTH_FILE if set, otherwise admin.json in the data folder (see lib/paths.ts). */
export function adminFilePath(env: Record<string, string | undefined> = process.env): string {
  return adminFile(env);
}

export const usernameSchema = z
  .string()
  .regex(/^[A-Za-z0-9._@-]{1,64}$/, "1–64 letters, digits, or . _ @ -");

export const passwordSchema = z
  .string()
  .min(12, "At least 12 characters")
  .max(256, "At most 256 characters");

// scrypt cost: N=2^16, r=8, p=1 needs 64 MiB and ~100–200 ms per hash, which makes guessing
// against a stolen file expensive while keeping a login quick.
const COST = { N: 2 ** 16, r: 8, p: 1 };
const KEY_LENGTH = 64;
const SALT_LENGTH = 16;
// Refuse absurd parameters from a tampered file rather than exhausting memory.
const MAX_N = 2 ** 20;

function scrypt(
  password: string,
  salt: Buffer,
  keylen: number,
  { N, r, p }: { N: number; r: number; p: number },
) {
  // scrypt needs 128·N·r bytes; allow twice that so Node's default 32 MiB cap doesn't refuse.
  const options: ScryptOptions = { N, r, p, maxmem: 256 * N * r };
  return new Promise<Buffer>((resolve, reject) =>
    scryptCb(password, salt, keylen, options, (err, key) => (err ? reject(err) : resolve(key))),
  );
}

/** `scrypt$N$r$p$salt$hash`, with salt and hash in base64. */
export async function hashPassword(password: string): Promise<string> {
  const salt = randomBytes(SALT_LENGTH);
  const key = await scrypt(password, salt, KEY_LENGTH, COST);
  const { N, r, p } = COST;
  return ["scrypt", N, r, p, salt.toString("base64"), key.toString("base64")].join("$");
}

const HASH_PATTERN = /^scrypt\$(\d+)\$(\d+)\$(\d+)\$([A-Za-z0-9+/=]+)\$([A-Za-z0-9+/=]+)$/;

function parseHash(encoded: string) {
  const m = HASH_PATTERN.exec(encoded);
  if (!m) return null;
  const [N, r, p] = [m[1], m[2], m[3]].map(Number);
  const salt = Buffer.from(m[4], "base64");
  const key = Buffer.from(m[5], "base64");
  const powerOfTwo = N > 1 && (N & (N - 1)) === 0;
  if (!powerOfTwo || N > MAX_N || r < 1 || r > 32 || p < 1 || p > 16) return null;
  if (salt.length < 8 || key.length < 32) return null;
  return { N, r, p, salt, key };
}

export function isValidPasswordHash(encoded: string): boolean {
  return parseHash(encoded) !== null;
}

/** Recomputes the hash with its stored parameters and compares in constant time. */
export async function verifyPassword(password: string, encoded: string): Promise<boolean> {
  const parsed = parseHash(encoded);
  if (!parsed) return false;
  const { N, r, p, salt, key } = parsed;
  const candidate = await scrypt(password, salt, key.length, { N, r, p });
  return timingSafeEqual(candidate, key);
}

export const adminFileSchema = z.object({
  version: z.literal(1),
  username: usernameSchema,
  passwordHash: z.string().refine(isValidPasswordHash, "Not a scrypt$N$r$p$salt$hash string"),
});

export type AdminFile = z.infer<typeof adminFileSchema>;

export class AdminFileError extends Error {
  name = "AdminFileError";
}

/**
 * The admin file, or null if it doesn't exist. A file that exists but is unreadable or
 * malformed throws: a typo must never silently turn authentication off.
 */
export async function readAdminFile(file: string): Promise<AdminFile | null> {
  let text: string;
  try {
    text = await readFile(file, "utf8");
  } catch (err) {
    if ((err as NodeJS.ErrnoException).code === "ENOENT") return null;
    throw new AdminFileError(`Can't read admin file ${file}: ${(err as Error).message}`);
  }
  let json: unknown;
  try {
    json = JSON.parse(text);
  } catch {
    throw new AdminFileError(`Admin file ${file} is not valid JSON`);
  }
  const parsed = adminFileSchema.safeParse(json);
  if (!parsed.success) {
    const problems = parsed.error.issues.map((i) => `${i.path.join(".") || "file"}: ${i.message}`);
    throw new AdminFileError(`Admin file ${file} is malformed: ${problems.join("; ")}`);
  }
  return parsed.data;
}

/**
 * Writes the file owner-only (0600) via a temp file, so a crash never leaves it half
 * written. Without `overwrite`, fails with EEXIST if an admin already exists, atomically.
 */
export async function writeAdminFile(
  file: string,
  admin: AdminFile,
  { overwrite = false }: { overwrite?: boolean } = {},
): Promise<void> {
  const contents = `${JSON.stringify(adminFileSchema.parse(admin), null, 2)}\n`;
  await mkdir(path.dirname(file), { recursive: true, mode: 0o700 });
  const tmp = `${file}.${process.pid}.${randomBytes(4).toString("hex")}.tmp`;
  await writeFile(tmp, contents, { mode: 0o600 });
  try {
    // link() refuses to replace an existing file; rename() replaces it.
    await (overwrite ? rename(tmp, file) : link(tmp, file));
  } finally {
    if (!overwrite) await unlink(tmp).catch(() => {});
  }
}

/** Builds a validated admin record, hashing the password. */
export async function createAdminRecord(username: string, password: string): Promise<AdminFile> {
  return {
    version: 1,
    username: usernameSchema.parse(username),
    passwordHash: await hashPassword(passwordSchema.parse(password)),
  };
}