import { geoEqualEarth, geoPath, type GeoPermissibleObjects, type GeoProjection } from 'd3-geo';
import { feature } from 'topojson-client';
import type { Topology, GeometryCollection } from 'topojson-specification';
import type { Feature, FeatureCollection, Geometry } from 'geojson';
import { atlasGeometryIso3 } from '@/lib/iso-numeric';

/**
 * Geometry side of the trial map — pure helpers (no React, no I/O) so the projection can be unit-
 * tested: TopoJSON → GeoJSON features keyed by ISO3, an Equal Earth projection fitted to the SVG
 * viewport, and a `project()` for the city layer. Equal Earth (Šavrič, Patterson & Jenny 2018) is
 * equal-area, so a country's visual weight is not inflated at high latitudes, and its outline is
 * familiar; it is fitted to a 960×480 viewBox and scaled by the browser.
 */

export const MAP_WIDTH = 960;
export const MAP_HEIGHT = 480;

export interface CountryFeature {
  /** ISO 3166-1 alpha-3, or null when Natural Earth has no code for the polygon (N. Cyprus, Somaliland). */
  iso3: string | null;
  /** Natural Earth short name (display fallback only). */
  name: string;
  /** SVG path data in viewBox coordinates. */
  d: string;
}

export interface WorldGeometry {
  countries: CountryFeature[];
  /** Outline of the globe (the projection's sphere). */
  sphere: string;
  project: (lng: number, lat: number) => [number, number] | null;
}

type CountriesTopology = Topology<{ countries: GeometryCollection<{ name: string }> }>;

/** Fitted Equal Earth projection for the given viewport (default 960×480, 8 px padding). */
export function fitProjection(collection: FeatureCollection | GeoPermissibleObjects, width = MAP_WIDTH, height = MAP_HEIGHT, pad = 8): GeoProjection {
  return geoEqualEarth().fitExtent(
    [
      [pad, pad],
      [width - pad, height - pad],
    ],
    collection as GeoPermissibleObjects,
  );
}

/** TopoJSON (world-atlas countries-110m) → projected SVG paths keyed by ISO3. */
export function buildWorldGeometry(topology: CountriesTopology, width = MAP_WIDTH, height = MAP_HEIGHT): WorldGeometry {
  const fc = feature(topology, topology.objects.countries) as FeatureCollection<Geometry, { name: string }>;
  // Fit on the sphere (not on the land bbox) so Antarctica's absence/presence never shifts the frame.
  const projection = fitProjection({ type: 'Sphere' }, width, height);
  // One decimal in a 960-unit viewBox is sub-pixel at any realistic display width and shrinks the
  // path data (shipped twice: HTML + RSC payload) by about 40 % versus the default 3 digits.
  const path = geoPath(projection).digits(1);
  const countries: CountryFeature[] = [];
  for (const f of fc.features as Array<Feature<Geometry, { name: string }>>) {
    const name = f.properties?.name ?? '';
    // Antarctica carries no trial sites and would dominate the lower band of the map.
    if (f.id === '010') continue;
    const d = path(f);
    if (!d) continue;
    countries.push({ iso3: atlasGeometryIso3(f.id as string | number | undefined, name), name, d });
  }
  const sphere = path({ type: 'Sphere' }) ?? '';
  const project = (lng: number, lat: number): [number, number] | null => {
    if (!Number.isFinite(lng) || !Number.isFinite(lat) || Math.abs(lat) > 90 || Math.abs(lng) > 180) return null;
    const p = projection([lng, lat]);
    return p ? [Math.round(p[0] * 10) / 10, Math.round(p[1] * 10) / 10] : null;
  };
  return { countries, sphere, project };
}