fuzzyworld

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

calendarConflicts.ts

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3cbfe9ad 2026-06-22 Pavel Safronov1// Which calendars can actually appear together, for color assignment.
2//
3// Two calendars only need visually distinct colors if they can ever show up as
4// simultaneous answers — either two eras in the same region overlapping in
5// time, or two eras in *different* regions whose areas overlap geographically
6// and whose time ranges overlap. Most calendars in the dataset are nowhere
7// near each other (Japan and Iran never compete for a color), so the actual
8// "must be distinct" graph is sparse — far sparser than "every calendar must
9// differ from every other calendar," which is what forced near-collisions
10// once the dataset grew past ~20 calendars sharing one hue wheel.
11
12import type { CalendarRegion, Era, Geometry } from '../types';
13import { parseGregorianDate } from './julianDay';
14import { MODERN_DEFAULT_START_YEAR } from './resolve';
15
16interface BBox {
17 minLng: number;
18 minLat: number;
19 maxLng: number;
20 maxLat: number;
21}
22
23function expandBBox(box: BBox, lng: number, lat: number): void {
24 if (lng < box.minLng) box.minLng = lng;
25 if (lng > box.maxLng) box.maxLng = lng;
26 if (lat < box.minLat) box.minLat = lat;
27 if (lat > box.maxLat) box.maxLat = lat;
28}
29
30/** A conservative bounding box for a Polygon/MultiPolygon (over-approximates the shape). */
31function geometryBBox(geo: Geometry): BBox {
32 const box: BBox = { minLng: Infinity, minLat: Infinity, maxLng: -Infinity, maxLat: -Infinity };
33 const polygons = geo.type === 'Polygon' ? [geo.coordinates] : geo.coordinates;
34 for (const rings of polygons) {
35 for (const ring of rings) {
36 for (const [lng, lat] of ring) expandBBox(box, lng, lat);
37 }
38 }
39 return box;
40}
41
42function bboxesOverlap(a: BBox, b: BBox): boolean {
43 return a.minLng <= b.maxLng && b.minLng <= a.maxLng && a.minLat <= b.maxLat && b.minLat <= a.maxLat;
44}
45
46/** [start, end) in astronomical years; null start/end means open-ended. */
47interface TimeRange {
48 start: number;
49 end: number;
50}
51
52function eraTimeRange(era: Era): TimeRange {
53 return {
54 start: era.start ? parseGregorianDate(era.start).year : -Infinity,
55 end: era.end ? parseGregorianDate(era.end).year : Infinity,
56 };
57}
58
59function rangesOverlap(a: TimeRange, b: TimeRange): boolean {
60 return a.start < b.end && b.start < a.end;
61}
62
63/**
64 * Build the conflict graph: a map from each (normalized) calendar name to the
65 * set of other calendar names it can co-occur with, given the loaded region
66 * data plus the modern global Gregorian default (which can pair with anything
67 * still active from MODERN_DEFAULT_START_YEAR on, anywhere on Earth).
68 */
69export function buildConflictGraph(regions: CalendarRegion[]): Map<string, Set<string>> {
70 const graph = new Map<string, Set<string>>();
71 const addEdge = (a: string, b: string) => {
72 const ka = a.trim().toLowerCase();
73 const kb = b.trim().toLowerCase();
74 if (ka === kb) return;
75 if (!graph.has(ka)) graph.set(ka, new Set());
76 if (!graph.has(kb)) graph.set(kb, new Set());
77 graph.get(ka)!.add(kb);
78 graph.get(kb)!.add(ka);
79 };
80
81 const entries = regions.map((cr) => ({
82 bbox: geometryBBox(cr.region.geo),
83 eras: cr.eras.map((era) => ({ calendar: era.calendar, range: eraTimeRange(era) })),
84 }));
85
86 for (let i = 0; i < entries.length; i++) {
87 // Eras within the same region: any two overlapping in time conflict,
88 // regardless of geometry (they're the same place by definition).
89 const { eras } = entries[i];
90 for (let a = 0; a < eras.length; a++) {
91 for (let b = a + 1; b < eras.length; b++) {
92 if (rangesOverlap(eras[a].range, eras[b].range)) {
93 addEdge(eras[a].calendar, eras[b].calendar);
94 }
95 }
96 }
97 // Eras across different regions: only conflict if the regions'
98 // bounding boxes overlap (conservative — may flag a few pairs whose
99 // exact shapes don't actually touch, but never misses a real one).
100 for (let j = i + 1; j < entries.length; j++) {
101 if (!bboxesOverlap(entries[i].bbox, entries[j].bbox)) continue;
102 for (const eraA of entries[i].eras) {
103 for (const eraB of entries[j].eras) {
104 if (rangesOverlap(eraA.range, eraB.range)) {
105 addEdge(eraA.calendar, eraB.calendar);
106 }
107 }
108 }
109 }
110 }
111
112 // The modern global Gregorian default applies everywhere from
113 // MODERN_DEFAULT_START_YEAR on, so it conflicts with anything else still
114 // active in that window (mirrors resolve.ts's DEFAULT_MODERN_ERA).
115 const modernRange: TimeRange = { start: MODERN_DEFAULT_START_YEAR, end: Infinity };
116 for (const { eras } of entries) {
117 for (const era of eras) {
118 if (era.calendar.trim().toLowerCase() === 'gregorian') continue;
119 if (rangesOverlap(era.range, modernRange)) addEdge(era.calendar, 'Gregorian');
120 }
121 }
122
123 return graph;
124}