module AuthorizationEndpoint
  class Discover
    # Ultra-orthodox pattern matching allowed values in HTTP Link header `rel` parameter
    # https://tools.ietf.org/html/rfc8288#section-3.3
    REGEXP_REG_REL_TYPE_PATTERN = '[a-z\d][a-z\d\-\.]*'.freeze

    # Liberal pattern matching a string of text between angle brackets
    # https://tools.ietf.org/html/rfc5988#section-5.1
    REGEXP_TARGET_URI_PATTERN = /^<(.*)>;/

    # Ultra-orthodox pattern matching HTTP Link header `rel` parameter including an `authorization_endpoint` value
    # https://www.w3.org/TR/indieauth/#x4-discovery
    REGEXP_AUTHORIZATION_ENDPOINT_REL_PATTERN = /(?:;|\s)rel="?(?:#{REGEXP_REG_REL_TYPE_PATTERN}+\s)?authorization_endpoint(?:\s#{REGEXP_REG_REL_TYPE_PATTERN})?"?/

    def initialize(response)
      raise ArgumentError, "response must be an HTTP::Response (given #{response.class.name})" unless response.is_a?(HTTP::Response)

      @response = response
    end

    def endpoint
      return unless endpoint_from_http_request

      @endpoint ||= Absolutely.to_absolute_uri(base: @response.uri.to_s, relative: endpoint_from_http_request)
    rescue Absolutely::InvalidURIError => error
      raise InvalidURIError, error
    end

    private

    def endpoint_from_body
      return unless @response.mime_type == 'text/html'

      doc = Nokogiri::HTML(@response.body.to_s)

      # Search response body for first `link` element with valid `rel` attribute
      link_element = doc.css('link[rel~="authorization_endpoint"][href]').shift

      return link_element['href'] if link_element
    end

    def endpoint_from_headers
      link_headers = @response.headers.get('link')

      return unless link_headers

      # Split Link headers with multiple values, flatten the resulting array, and strip whitespace
      link_headers = link_headers.map { |header| header.split(',') }.flatten.map(&:strip)

      authorization_endpoint_header = link_headers.find { |header| header.match?(REGEXP_AUTHORIZATION_ENDPOINT_REL_PATTERN) }

      return unless authorization_endpoint_header

      endpoint_match_data = authorization_endpoint_header.match(REGEXP_TARGET_URI_PATTERN)

      return endpoint_match_data[1] if endpoint_match_data
    end

    def endpoint_from_http_request
      @endpoint_from_http_request ||= endpoint_from_headers || endpoint_from_body || nil
    end
  end
end