dataMerge

/**
 * Merge all sessions belonging to the same user into a single row.
 *
 * Rules:
 * - Sum duration for the same user.
 * - Combine equipment, remove duplicates, then sort alphabetically.
 * - Keep output order by the user's earliest occurrence in input.
 * - Do not mutate input rows.
 *
 * @param {Array<{user: number, duration: number, equipment: Array<string>}>} sessions
 * @returns {Array<{user: number, duration: number, equipment: Array<string>}>>}
 */
export default function mergeData(sessions) {
	const mergedByUser = new Map();

	for (let i = 0; i < sessions.length; i++) {
		const session = sessions[i];
		const existing = mergedByUser.get(session.user);

		if (!existing) {
			mergedByUser.set(session.user, {
				user: session.user,
				duration: session.duration,
				equipmentSet: new Set(session.equipment),
				firstIndex: i,
			});
			continue;
		}

		existing.duration += session.duration;
		for (const item of session.equipment) {
			existing.equipmentSet.add(item);
		}
	}

	return Array.from(mergedByUser.values())
		.sort((a, b) => a.firstIndex - b.firstIndex)
		.map((row) => ({
			user: row.user,
			duration: row.duration,
			equipment: Array.from(row.equipmentSet).sort(),
		}));
}

// Example:
// mergeData([
//   { user: 8, duration: 50, equipment: ['bench'] },
//   { user: 7, duration: 150, equipment: ['dumbbell'] },
//   { user: 1, duration: 10, equipment: ['barbell'] },
//   { user: 7, duration: 100, equipment: ['bike', 'kettlebell'] },
//   { user: 7, duration: 200, equipment: ['bike'] },
//   { user: 2, duration: 200, equipment: ['treadmill'] },
//   { user: 2, duration: 200, equipment: ['bike'] },
// ]);
// => [
//   { user: 8, duration: 50, equipment: ['bench'] },
//   { user: 7, duration: 450, equipment: ['bike', 'dumbbell', 'kettlebell'] },
//   { user: 1, duration: 10, equipment: ['barbell'] },
//   { user: 2, duration: 400, equipment: ['bike', 'treadmill'] },
// ]