attendance-count.js

// This attendance counting function assumes the following:
// - You can attend multiple times in one day (and still be counted as one)
// - Attendance time is not counted, you can attend for several minutes and it still counts as one day

/** @typedef {string | number | Date} DateLike */

/**
 * @param {Array<{ start: DateLike, end: DateLike }>} attendances
 * @param {Date} startDate
 * @param {Date} endDate
 * @returns {{ total: number, attended: number, absent: number }}
 */
function countAttendances (attendances, startDate, endDate) {
	let attended = 0;
	let total = 0;

	let days = new Set();
	
	let date = new Date(startDate);

	for (let idx = 0, len = attendances.length; idx < len; idx++) {
		let attendance = attendances[idx];

		let start = new Date(attendance.start);
		let end = new Date(attendance.end);

		// if it starts after the end date, or ends before the start date,
		// they're outside the period we're looking for
		if (start > endDate || end < startDate) {
			continue;
		}

		// if it starts before the start date, cap it
		if (start < startDate) {
			start.setTime(startDate.getTime());
		}
		
		// if it ends after the end date, cap it
		if (end > endDate) {
			end.setTime(endDate.getTime());
		}

		// normalize the hours as we only want to count the days
		start.setHours(0, 0, 0, 0);

		while (start <= end) {
			let time = start.getTime();

			if (!days.has(time)) {
				attended++;
				days.add(time);
			}

			start.setDate(start.getDate() + 1);
		}
	}

	// count the number of days in a given period
	date.setHours(0, 0, 0, 0);
	while (date <= endDate) {
		total++;
		date.setDate(date.getDate() + 1);
	}

	return {
		total: total,
		attended: attended,
		absent: total - attended,
	};
}