homepage/tile-proxy/scripts/gen-regions.mjs

#!/usr/bin/env node
/**
 * Generate `src/regions.in` — the polygons the tile proxy treats as
 * "swisstopo-covered": Switzerland and Liechtenstein (swisstopo has
 * high-quality data for both). A tile overlapping any of them is served by
 * swisstopo; everything else by the global providers.
 *
 * Pipeline per source: largest exterior ring → Douglas–Peucker simplify →
 * push every vertex ~BUFFER_KM outward (so border tiles still prefer
 * swisstopo, which covers a margin past the border). Output is a Rust array of
 * rings `&[ &[[lng,lat],…], … ]` consumed via `include!`.
 *
 *   node scripts/gen-regions.mjs [geojson-url-or-path …]
 *   BUFFER_KM=2 SIMPLIFY_DEG=0.004 node scripts/gen-regions.mjs
 */
import { writeFileSync, readFileSync } from 'node:fs';
import { fileURLToPath } from 'node:url';
import { dirname, join } from 'node:path';

const SOURCES =
	process.argv.slice(2).length > 0
		? process.argv.slice(2)
		: [
				'https://raw.githubusercontent.com/georgique/world-geojson/develop/countries/switzerland.json',
				'https://raw.githubusercontent.com/georgique/world-geojson/develop/countries/liechtenstein.json'
			];
const BUFFER_KM = Number(process.env.BUFFER_KM ?? 2);
const SIMPLIFY_DEG = Number(process.env.SIMPLIFY_DEG ?? 0.004); // ≈ 440 m

const OUT = join(dirname(fileURLToPath(import.meta.url)), '..', 'src', 'regions.in');

async function load(src) {
	if (/^https?:/.test(src)) {
		const r = await fetch(src);
		if (!r.ok) throw new Error(`fetch ${src}: ${r.status}`);
		return r.json();
	}
	return JSON.parse(readFileSync(src, 'utf8'));
}

function exteriorRings(geojson) {
	const rings = [];
	const visit = (g) => {
		if (!g) return;
		if (g.type === 'FeatureCollection') g.features.forEach((f) => visit(f.geometry));
		else if (g.type === 'Feature') visit(g.geometry);
		else if (g.type === 'Polygon') rings.push(g.coordinates[0]);
		else if (g.type === 'MultiPolygon') g.coordinates.forEach((p) => rings.push(p[0]));
	};
	visit(geojson);
	return rings;
}

function signedArea(ring) {
	let a = 0;
	for (let i = 0, n = ring.length, j = n - 1; i < n; j = i++) {
		a += ring[j][0] * ring[i][1] - ring[i][0] * ring[j][1];
	}
	return a / 2;
}

function perpDist(p, a, b) {
	const dx = b[0] - a[0];
	const dy = b[1] - a[1];
	const L = dx * dx + dy * dy;
	if (L === 0) return Math.hypot(p[0] - a[0], p[1] - a[1]);
	let t = ((p[0] - a[0]) * dx + (p[1] - a[1]) * dy) / L;
	t = Math.max(0, Math.min(1, t));
	return Math.hypot(p[0] - (a[0] + t * dx), p[1] - (a[1] + t * dy));
}

function simplify(points, eps) {
	if (points.length < 3) return points;
	const keep = new Array(points.length).fill(false);
	keep[0] = keep[points.length - 1] = true;
	const stack = [[0, points.length - 1]];
	while (stack.length) {
		const [s, e] = stack.pop();
		let dmax = 0;
		let idx = -1;
		for (let i = s + 1; i < e; i++) {
			const d = perpDist(points[i], points[s], points[e]);
			if (d > dmax) {
				dmax = d;
				idx = i;
			}
		}
		if (dmax > eps && idx > 0) {
			keep[idx] = true;
			stack.push([s, idx], [idx, e]);
		}
	}
	return points.filter((_, i) => keep[i]);
}

function bufferOutward(ring, km, lat0) {
	const k = Math.cos((lat0 * Math.PI) / 180);
	const d = km / 111.32;
	const n = ring.length;
	const edgeNormal = (a, b) => {
		const dx = (b[0] - a[0]) * k;
		const dy = b[1] - a[1];
		const len = Math.hypot(dx, dy) || 1;
		return [dy / len, -dx / len];
	};
	const out = [];
	for (let i = 0; i < n; i++) {
		const prev = ring[(i - 1 + n) % n];
		const cur = ring[i];
		const next = ring[(i + 1) % n];
		const [ax, ay] = edgeNormal(prev, cur);
		const [bx, by] = edgeNormal(cur, next);
		let nx = ax + bx;
		let ny = ay + by;
		const len = Math.hypot(nx, ny) || 1;
		nx /= len;
		ny /= len;
		out.push([cur[0] + (nx * d) / k, cur[1] + ny * d]);
	}
	return out;
}

async function buildRing(src) {
	const gj = await load(src);
	let ring = exteriorRings(gj).sort((a, b) => Math.abs(signedArea(b)) - Math.abs(signedArea(a)))[0];
	if (!ring) throw new Error(`no polygon rings in ${src}`);
	if (ring.length > 1 && ring[0][0] === ring.at(-1)[0] && ring[0][1] === ring.at(-1)[1]) {
		ring = ring.slice(0, -1);
	}
	if (signedArea(ring) < 0) ring = ring.slice().reverse();
	ring = simplify(ring, SIMPLIFY_DEG);
	const lat0 = ring.reduce((s, p) => s + p[1], 0) / ring.length;
	return bufferOutward(ring, BUFFER_KM, lat0);
}

const rings = [];
for (const src of SOURCES) rings.push(await buildRing(src));

const ringLit = (ring) =>
	'\t&[\n' + ring.map((p) => `\t\t[${p[0].toFixed(5)}, ${p[1].toFixed(5)}]`).join(',\n') + '\n\t]';
const total = rings.reduce((s, r) => s + r.length, 0);
const all = rings.flat();
const lngs = all.map((p) => p[0]);
const lats = all.map((p) => p[1]);
writeFileSync(
	OUT,
	`// Auto-generated by scripts/gen-regions.mjs — do not edit by hand.\n` +
		`// swisstopo-covered regions (CH + LI), simplified (${SIMPLIFY_DEG}°) + ${BUFFER_KM} km buffer.\n` +
		`// ${rings.length} rings, ${total} points · lng [${Math.min(...lngs).toFixed(2)}, ${Math.max(...lngs).toFixed(2)}] · lat [${Math.min(...lats).toFixed(2)}, ${Math.max(...lats).toFixed(2)}]\n` +
		`&[\n${rings.map(ringLit).join(',\n')}\n]\n`
);
console.log(
	`wrote ${OUT}: ${rings.length} rings, ${total} points (${rings.map((r) => r.length).join(' + ')})`
);