Create Firebase Token

class CreateFirebaseToken
  FIREBASE_URL = 'https://identitytoolkit.googleapis.com/v1/accounts:signInWithCustomToken?key=#{API_KEY}'
  URL = 'https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit'
  SERVICE_ACCOUNT_EMAIL = 'firebase-adminsdk-j4rgm@auth-rails-bdc20.iam.gserviceaccount.com'
  PRIVATE_KEY = OpenSSL::PKey::RSA.new(File.read('/PATH/Downloads/firebase.key'))

  class << self
    def call
      @payload = create_payload
      token = encode_payload
      res = RestClient.post(FIREBASE_URL, {token: token}, content_type: :json)
      JSON.parse(res.body)['idToken']
    end

    def create_payload
      {
        iss: SERVICE_ACCOUNT_EMAIL,
        sub: SERVICE_ACCOUNT_EMAIL,
        aud: URL,
        iat: now_seconds,
        exp: now_seconds+(60*60),
        uid: SecureRandom.hex(6),
        claims: {
          premium_account: false
        }
      }
    end

    private

    def encode_payload
      JWT.encode(@payload, PRIVATE_KEY, 'RS256')
    end

    def self.now_seconds
      Time.now.to_i
    end
  end
end