mainlandBoundsfunction

The box around a region as anyone would picture it: its largest outline, and whatever else is close enough to belong in the same glance.

A region's own bounds are its whole reach, which is the wrong frame for a picture of it. Kagoshima's run from the foot of Kyushu to the Amami islands, three hundred kilometres south, so the prefecture drew at a quarter of the size it could with two specks under it.

reach is how far a ring may sit from the mainland and still be framed with it, as a share of the mainland's own longer side - so a near island comes along and a far one is left out of the picture, which is where the map itself now draws it anyway.

import { mainlandBounds } from "@johnmorrisdotca/chizu";

function mainlandBounds(rings: readonly MapRing[], reach?: number): [number, number, number, number] | null

import WORLD from "@johnmorrisdotca/chizu/world";
import { mainlandBounds, mapOutlines } from "@johnmorrisdotca/chizu";

const index = WORLD.regions.findIndex((region) => region.code === "FR");
// France on the world reaches French Guiana; its mainland does not.
const mainland = mainlandBounds(mapOutlines(WORLD)[index]!.rings)!;
console.log(mainland[3] - mainland[1] < WORLD.regions[index]!.bbox[3] - WORLD.regions[index]!.bbox[1]);
// true

true