module ROTP
  DEFAULT_INTERVAL = 30
  class TOTP < OTP

    attr_reader :interval, :issuer

    # @option options [Integer] interval (30) the time interval in seconds for OTP
    #     This defaults to 30 which is standard.
    def initialize(s, options = {})
      @interval = options[:interval] || DEFAULT_INTERVAL
      @issuer = options[:issuer]
      super
    end

    # Accepts either a Unix timestamp integer or a Time object.
    # Time objects will be adjusted to UTC automatically
    # @param [Time/Integer] time the time to generate an OTP for
    # @option [Boolean] padding (true) Issue the number as a 0 padded string
    def at(time, padding=true)
      unless time.class == Time
        time = Time.at(time.to_i)
      end
      generate_otp(timecode(time), padding)
    end

    # Generate the current time OTP
    # @return [Integer] the OTP as an integer
    def now(padding=true)
      generate_otp(timecode(Time.now), padding)
    end

    # Verifies the OTP passed in against the current time OTP
    # @param [String/Integer] otp the OTP to check against
    def verify(otp, time = Time.now)
      super(otp, self.at(time))
    end

    # Verifies the OTP passed in against the current time OTP
    # and adjacent intervals up to +drift+.
    # @param [String] otp the OTP to check against
    # @param [Integer] drift the number of seconds that the client
    #     and server are allowed to drift apart
    def verify_with_drift(otp, drift, time = Time.now)
      time = time.to_i
      times = (time-drift..time+drift).step(interval).to_a
      times << time + drift if times.last < time + drift
      times.any? { |ti| verify(otp, ti) }
    end

    # Verifies the OTP passed in against the current time OTP
    # and adjacent intervals up to +drift+.  Excludes OTPs
    # from prior_time and earlier.  Returns time value of
    # matching OTP code for use in subsequent call.
    # @param [String] otp the OTP to check against
    # @param [Integer] drift the number of seconds that the client
    #     and server are allowed to drift apart
    # @param [Integer] time value of previous match
    def verify_with_drift_and_prior(otp, drift, prior_time = nil, time = Time.now)
      # calculate normalized bin start times based on drift
      first_bin = (time - drift).to_i / interval * interval
      last_bin = (time + drift).to_i / interval * interval

      # if prior_time was supplied, adjust first bin if necessary to exclude it
      if prior_time
        prior_bin = prior_time.to_i / interval * interval
        first_bin = prior_bin + interval if prior_bin >= first_bin
        # fail if we've already used the last available OTP code
        return if first_bin > last_bin
      end
      times = (first_bin..last_bin).step(interval).to_a
      times.find { |ti| verify(otp, ti) }
    end

    # Returns the provisioning URI for the OTP
    # This can then be encoded in a QR Code and used
    # to provision the Google Authenticator app
    # @param [String] name of the account
    # @return [String] provisioning URI
    def provisioning_uri(name)
      # The format of this URI is documented at:
      # https://github.com/google/google-authenticator/wiki/Key-Uri-Format
      # For compatibility the issuer appears both before that account name and also in the
      # query string.
      issuer_string = issuer.nil? ? "" : "#{URI.encode(issuer)}:"
      params = {
        secret: secret,
        period: interval == 30 ? nil : interval,
        issuer: issuer,
        digits: digits == DEFAULT_DIGITS ? nil : digits,
      }
      encode_params("otpauth://totp/#{issuer_string}#{URI.encode(name)}", params)
    end

    private

    def timecode(time)
      time.utc.to_i / interval
    end

  end
end