require 'net/http'

module Preview
  class HttpFetcher
    class FetchFailed < StandardError; end
    class FetchTimeout < FetchFailed; end
    class ResponseTooLarge < FetchFailed; end

    MAX_REDIRECTS = 5
    DEFAULT_MAX_BYTES = 5.megabytes

    Response = Data.define(:body, :content_type, :url)

    def self.fetch(raw_url, max_bytes: DEFAULT_MAX_BYTES, redirects: MAX_REDIRECTS)
      uri, addresses = UrlSafety.validate(raw_url)
      response = request(uri, addresses.first, max_bytes)

      if response.is_a?(Net::HTTPRedirection)
        raise FetchFailed, 'redirect が多すぎます．' if redirects.zero?

        location = response['location']
        raise FetchFailed, 'redirect 先が不正です．' if location.blank?

        return fetch(URI.join(uri, location).to_s,
                     max_bytes:,
                     redirects: redirects - 1)
      end

      unless response.is_a?(Net::HTTPSuccess)
        raise FetchFailed, "外部サーバーが HTTP #{ response.code } を返しました．"
      end

      Response.new(response.body, response['content-type'].to_s, uri.to_s)
    rescue Net::OpenTimeout, Net::ReadTimeout, Timeout::Error => e
      raise FetchTimeout, e.message
    rescue SocketError, SystemCallError, OpenSSL::SSL::SSLError, EOFError => e
      raise FetchFailed, e.message
    end

    def self.request(uri, ip_address, max_bytes)
      http = Net::HTTP.new(uri.host, uri.port)
      http.ipaddr = ip_address
      http.use_ssl = uri.scheme == 'https'
      http.open_timeout = 5
      http.read_timeout = 8
      http.write_timeout = 5

      request = Net::HTTP::Get.new(uri)
      request['User-Agent'] = 'BTRC-Hub thumbnail preview'
      request['Accept'] = 'text/html,image/*;q=0.9,*/*;q=0.1'

      http.request(request) do |response|
        length = response['content-length'].to_i
        raise ResponseTooLarge, '外部データが大きすぎます．' if length > max_bytes

        body = +''
        response.read_body do |chunk|
          body << chunk
          raise ResponseTooLarge, '外部データが大きすぎます．' if body.bytesize > max_bytes
        end
        response.instance_variable_set(:@body, body)
        response.instance_variable_set(:@read, true)
        return response
      end
    end

    private_class_method :request
  end
end