import { createHmac, timingSafeEqual } from 'node:crypto'

/**
 * Lightweight JWT implementation using HMAC-SHA256.
 * No external dependencies — uses Node.js crypto.
 *
 * Token format: base64url(header).base64url(payload).base64url(signature)
 * Used for V2 worker authentication (session ingress / SSE / CCR).
 */

interface JwtPayload {
  session_id: string
  role: string
  iat: number
  exp: number
}

function base64url(data: string | Buffer): string {
  return Buffer.from(data as unknown as ArrayLike<number>)
    .toString('base64')
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
    .replace(/=+$/, '')
}

function base64urlDecode(str: string): string {
  const padded = str.replace(/-/g, '+').replace(/_/g, '/')
  return Buffer.from(padded, 'base64').toString('utf-8')
}

function getSigningKey(): string {
  const key = process.env.RCS_API_KEYS?.split(',').filter(Boolean)[0]
  if (!key) throw new Error('No API key configured for JWT signing')
  return key
}

/** Generate a JWT for worker authentication. */
export function generateWorkerJwt(
  sessionId: string,
  expiresInSeconds: number,
): string {
  const header = { alg: 'HS256', typ: 'JWT' }
  const payload: JwtPayload = {
    session_id: sessionId,
    role: 'worker',
    iat: Math.floor(Date.now() / 1000),
    exp: Math.floor(Date.now() / 1000) + expiresInSeconds,
  }

  const headerB64 = base64url(JSON.stringify(header))
  const payloadB64 = base64url(JSON.stringify(payload))
  const signingInput = `${headerB64}.${payloadB64}`

  const signature = createHmac('sha256', getSigningKey())
    .update(signingInput)
    .digest()

  return `${signingInput}.${base64url(signature)}`
}

/**
 * Verify a JWT and return its payload, or null if invalid/expired.
 * Uses timing-safe comparison to prevent timing attacks.
 */
export function verifyWorkerJwt(token: string): JwtPayload | null {
  const parts = token.split('.')
  if (parts.length !== 3) return null

  const [headerB64, payloadB64, signatureB64] = parts

  // Verify signature
  const signingInput = `${headerB64}.${payloadB64}`
  const expectedSig = createHmac('sha256', getSigningKey())
    .update(signingInput)
    .digest()
  const actualSig = Buffer.from(
    signatureB64.replace(/-/g, '+').replace(/_/g, '/'),
    'base64',
  )

  if (expectedSig.length !== actualSig.length) return null
  if (!timingSafeEqual(expectedSig, actualSig)) return null

  // Decode payload
  try {
    const payload: JwtPayload = JSON.parse(base64urlDecode(payloadB64))
    if (payload.exp < Math.floor(Date.now() / 1000)) return null
    return payload
  } catch {
    return null
  }
}