gistfile1.swift

	func pollutionFromStartDate(startDate: NSDate, toEndDate:NSDate) -> Double {
		if (readings.count == 0) {
			return 0
		}

		let totalPollution = 0
		if (startDate < readings.first()!.date) {
			totalPollution += readings.first()!.pollutionLevel * (readings.first()!.date - startDate)
		}

		if (endDate > readings.last().date) {
			totalPollution += readings.last()!.pollutionLevel * (endDate - readings.last()!.date)
		}
		return totalPollution
	}