when-am-i / files / src / utils / lanes.ts

lanes.ts

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`ba77fd91` 2026-06-19 Pavel Safronov	1	// Pack time intervals into non-overlapping horizontal lanes — the trick the
	2	// Wikipedia band-member timelines use to show concurrent roles as parallel
	3	// stripes instead of overpainting. Greedy: sweep by start time, reuse the first
	4	// lane whose last interval has already ended, else open a new lane.
	5
	6	export interface Laned<T> {
	7	item: T;
	8	lane: number;
	9	}
	10
	11	export function assignLanes<T>(
	12	items: T[],
	13	startOf: (item: T) => number,
	14	endOf: (item: T) => number,
	15	): { lanes: Laned<T>[]; count: number } {
	16	const ordered = items
	17	.map((item, i) => ({ item, i }))
	18	.sort((a, b) => {
	19	const sa = startOf(a.item);
	20	const sb = startOf(b.item);
	21	if (sa < sb) return -1;
	22	if (sa > sb) return 1;
	23	return a.i - b.i; // stable for equal starts
	24	});
	25
	26	const laneEnds: number[] = [];
	27	const lanes: Laned<T>[] = [];
	28	for (const { item } of ordered) {
	29	const start = startOf(item);
	30	// Half-open intervals: a lane is free if its last interval ended at or
	31	// before this one starts.
	32	let lane = laneEnds.findIndex((end) => end <= start);
	33	if (lane === -1) {
	34	lane = laneEnds.length;
	35	laneEnds.push(endOf(item));
	36	} else {
	37	laneEnds[lane] = endOf(item);
	38	}
	39	lanes.push({ item, lane });
	40	}
	41
	42	return { lanes, count: laneEnds.length };
	43	}