module CALCULATION_PNSI
# A method for calculating the end date of committee scrutiny periods for Proposed Negative Statutory Instruments (PNSIs).

The calculation counts in parliamentary sitting days, requiring the laying date and the number of days to count.

The calculation is defined by the [European Union (Withdrawal) Act 2018 schedule 7 paragraph 17(10)](https://www.legislation.gov.uk/ukpga/2018/16/schedule/7/enacted#schedule-7-paragraph-17-10) (number of days to count) and [paragraph 17(11)](https://www.legislation.gov.uk/ukpga/2018/16/schedule/7/enacted#schedule-7-paragraph-17-11) (definition of sitting day).

  def pnsi_calculation( date, target_day_count )
## We start counting on the **first joint parliamentary sitting day following the laying of the instrument**.

If the date of laying is a joint parliamentary sitting day, we do not count that day.

If we find a day meeting that criteria ...

    if date.next_day.first_joint_parliamentary_sitting_day
... we set the date to start counting from as that day.

      date = date.next_day.first_joint_parliamentary_sitting_day
... we have found the start of the scrutiny period.

      @scrutiny_start_date = date
PNSIs are always before both Houses, so we'll get ready to start counting the sitting days in each House.

The first joint sitting day counts as day 1 in both Houses, so we count from 1 rather than from 0.

    	commons_day_count = 1
      lords_day_count = 1
## We look at subsequent days, ensuring that we've counted at least ten parliamentary sitting days in each House ...

      until ( ( commons_day_count >= target_day_count ) and ( lords_day_count >= target_day_count ) ) do
... continue to the **next day**.

        date = date.next_day
PNSIs use parliamentary sitting days, rather than naive sitting days.

If the Lords sat on the date we've found, we add another day to the Lords’ count.

        lords_day_count +=1 if date.is_lords_parliamentary_sitting_day?
If the Commons sat on the date we've found, we add another day to the Commons’ count.

        commons_day_count+=1 if date.is_commons_parliamentary_sitting_day?
If the calendar has no record of what type of day this is, we can't calculate the end date, ...

        if date.is_calendar_not_populated?
... this error message is displayed to users ...

          @error_message = "It's not currently possible to calculate an anticipated end date, as the likely end date occurs during a period for which sitting days are yet to be announced."
... and we stop looking through the calendar.

          break
        end
      end
If we didn't find any **future joint sitting date** in our calendar, we can't calculate the scrutiny period ...

    else
... and this error message is displayed.

      @error_message = "Unable to find a future joint sitting day. It's not currently possible to calculate an anticipated end date, as the likely end date occurs during a period for which sitting days are yet to be announced."
    end
Return the anticipated end date of the scrutiny period for display.

    date
  end
end