/**
 * mobile-lan-cert.ts
 *
 * Self-signed ECDSA P-256 certificate for the "Telefon bağlantısı" LAN proxy
 * (electron/mobile-lan-proxy.ts). No dependency: the X.509 structure is built
 * by hand and signed with node:crypto.
 *
 * The phone never trusts a CA for this server. It pins the certificate by
 * `fp` = SHA-256 of the certificate's DER bytes, base64url without padding
 * (43 characters), which it learns from the pairing QR / rendezvous record.
 *
 * The private key lives only in the injected vault (MobileVault → Electron
 * safeStorage → Windows DPAPI). Existing vault fields are preserved.
 */

import crypto from 'node:crypto'

export const MOBILE_LAN_CERT_DAYS = 3650
/** Regenerate when less than this is left; the phone must then pair again. */
const RENEW_BEFORE_MS = 30 * 24 * 3600e3
const FINGERPRINT_RE = /^[A-Za-z0-9_-]{43}$/

export interface MobileLanIdentity {
  /** PEM certificate. */
  cert: string
  /** PEM PKCS#8 private key. Never leaves the main process. */
  key: string
  /** SHA-256 of the DER certificate, base64url, 43 characters. */
  fingerprint: string
}

export interface MobileLanVault {
  load: () => Promise<Record<string, any>>
  save: (value: Record<string, any>) => Promise<void>
}

function der(tag: number, ...parts: Buffer[]): Buffer {
  const body = Buffer.concat(parts)
  let length: Buffer

  if (body.length < 128) {
    length = Buffer.from([body.length])
  } else {
    const bytes: number[] = []

    for (let n = body.length; n > 0; n >>= 8) {
      bytes.unshift(n & 255)
    }

    length = Buffer.from([0x80 | bytes.length, ...bytes])
  }

  return Buffer.concat([Buffer.from([tag]), length, body])
}

const seq = (...parts: Buffer[]) => der(0x30, ...parts)
const oid = (hex: string) => der(0x06, Buffer.from(hex, 'hex'))
const int = (value: Buffer) => der(0x02, value[0] & 0x80 ? Buffer.concat([Buffer.from([0]), value]) : value)

const utcTime = (date: Date) =>
  der(0x17, Buffer.from(`${date.toISOString().replace(/[-:T]/g, '').slice(2, 14)}Z`, 'ascii'))

const ECDSA_SHA256 = seq(oid('2a8648ce3d040302'))
const name = (cn: string) => seq(der(0x31, seq(oid('550403'), der(0x0c, Buffer.from(cn, 'utf8')))))

export function fingerprintOfDer(certDer: Uint8Array): string {
  return crypto.createHash('sha256').update(certDer).digest('base64url')
}

/** Fingerprint of a PEM certificate; throws when the PEM is not a certificate. */
export function fingerprintOfPem(certPem: string): string {
  return fingerprintOfDer(new crypto.X509Certificate(certPem).raw)
}

export function createSelfSignedCertificate(
  options: { commonName?: string; days?: number; now?: Date } = {}
): MobileLanIdentity {
  const commonName = options.commonName ?? 'AITURK IDE'
  const days = options.days ?? MOBILE_LAN_CERT_DAYS
  const now = options.now ?? new Date()
  const { publicKey, privateKey } = crypto.generateKeyPairSync('ec', { namedCurve: 'prime256v1' })
  const serial = crypto.randomBytes(16)

  serial[0] = (serial[0] & 0x7f) | 0x01

  const notBefore = new Date(now.getTime() - 24 * 3600e3)
  const notAfter = new Date(now.getTime() + days * 24 * 3600e3)

  if (notAfter.getUTCFullYear() >= 2050 || notBefore.getUTCFullYear() < 1950) {
    // UTCTime cannot carry these years; refuse rather than emit a wrong date.
    throw new Error('mobile-lan-cert:validity-out-of-range')
  }

  const tbs = seq(
    der(0xa0, int(Buffer.from([2]))), // v3
    int(serial),
    ECDSA_SHA256,
    name(commonName),
    seq(utcTime(notBefore), utcTime(notAfter)),
    name(commonName),
    publicKey.export({ type: 'spki', format: 'der' }) as Buffer
  )

  const signature = crypto.sign('sha256', tbs, privateKey)
  const certDer = seq(tbs, ECDSA_SHA256, der(0x03, Buffer.from([0]), signature))
  const body = (certDer.toString('base64').match(/.{1,64}/g) ?? []).join('\n')

  return {
    cert: `-----BEGIN CERTIFICATE-----\n${body}\n-----END CERTIFICATE-----\n`,
    key: privateKey.export({ type: 'pkcs8', format: 'pem' }) as string,
    fingerprint: fingerprintOfDer(certDer)
  }
}

/** True when the stored pair is a usable, unexpired, matching identity. */
export function isUsableIdentity(value: unknown, now: Date = new Date()): value is MobileLanIdentity {
  const row = value as MobileLanIdentity | null

  if (!row || typeof row.cert !== 'string' || typeof row.key !== 'string' || !FINGERPRINT_RE.test(row.fingerprint)) {
    return false
  }

  try {
    const x509 = new crypto.X509Certificate(row.cert)

    return (
      fingerprintOfDer(x509.raw) === row.fingerprint &&
      x509.checkPrivateKey(crypto.createPrivateKey(row.key)) &&
      new Date(x509.validTo).getTime() - now.getTime() > RENEW_BEFORE_MS
    )
  } catch {
    return false
  }
}

/**
 * Load the LAN identity from the vault, or create and store one. Stored under
 * the vault's `lanTls` field; every other field is written back untouched.
 * `created` tells the caller that previously paired phones no longer match.
 */
export async function loadOrCreateLanIdentity(
  vault: MobileLanVault,
  options: { now?: () => Date; create?: typeof createSelfSignedCertificate } = {}
): Promise<MobileLanIdentity & { created: boolean }> {
  const now = options.now ? options.now() : new Date()
  const secrets = (await vault.load()) ?? {}

  if (isUsableIdentity(secrets.lanTls, now)) {
    const { cert, key, fingerprint } = secrets.lanTls

    return { cert, key, fingerprint, created: false }
  }

  const identity = (options.create ?? createSelfSignedCertificate)({ now })

  await vault.save({ ...secrets, lanTls: identity })

  return { ...identity, created: true }
}