function base64UrlEncode(buffer) {

  const bytes = new Uint8Array(buffer)

  let binary = ''

  for (const byte of bytes) {

    binary += String.fromCharCode(byte)

  }

  return btoa(binary).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')

}

function base64UrlDecode(str) {

  const padded = str.replace(/-/g, '+').replace(/_/g, '/') + '=='.slice(0, (4 - (str.length % 4)) % 4)

  const binary = atob(padded)

  const bytes = new Uint8Array(binary.length)

  for (let i = 0; i < binary.length; i++) {

    bytes[i] = binary.charCodeAt(i)

  }

  return bytes.buffer

}

async function getHmacKey(secret) {

  return crypto.subtle.importKey(

    'raw',

    new TextEncoder().encode(secret),

    { name: 'HMAC', hash: 'SHA-256' },

    false,

    ['sign', 'verify']

  )

}

export async function createToken(secret) {

  const nonce = base64UrlEncode(crypto.getRandomValues(new Uint8Array(16)))

  const issuedAt = Date.now()

  const payload = base64UrlEncode(new TextEncoder().encode(JSON.stringify({ nonce, issuedAt })))

  const key = await getHmacKey(secret)

  const signature = await crypto.subtle.sign('HMAC', key, new TextEncoder().encode(payload))

  return { token: payload + '.' + base64UrlEncode(signature), nonce, issuedAt }

}

export async function verifyToken(tokenString, secret) {

  if (typeof tokenString !== 'string') return null

  const parts = tokenString.split('.')

  if (parts.length !== 2) return null

  const [payloadB64, signatureB64] = parts

  try {

    const key = await getHmacKey(secret)

    const signatureBuffer = base64UrlDecode(signatureB64)

    const valid = await crypto.subtle.verify(

      'HMAC',

      key,

      signatureBuffer,

      new TextEncoder().encode(payloadB64)

    )

    if (!valid) return null

    const payloadJson = new TextDecoder().decode(base64UrlDecode(payloadB64))

    const payload = JSON.parse(payloadJson)

    if (typeof payload.nonce !== 'string' || typeof payload.issuedAt !== 'number') return null

    return payload

  } catch {

    return null

  }

}