ahbeng/NUSMods

import _ from 'lodash';
import axios from 'axios';

import type { Venue, VenueLocationMap } from '../types/venues';

const COVID_ZONE_URL =
  'https://raw.githubusercontent.com/nusmodifications/nusmods/master/website/src/data/covidZones.json';

export type CovidZone = {
  color: string;
  positions: [number, number][];
};
export type CovidZoneId = 'A' | 'B' | 'C' | 'D' | 'E' | 'Unknown';
export type CovidZones = Record<Exclude<CovidZoneId, 'Unknown'>, CovidZone>;

// Explicitly typed so it excludes MemoizedFunction typing for easier mocking
const getCovidZones: () => Promise<CovidZones> = _.memoize(async () => {
  const response = await axios.get<CovidZones>(COVID_ZONE_URL);
  return response.data;
});

/**
 * Performs the even-odd-rule Algorithm (a raycasting algorithm) to find out whether a point is in a given polygon.
 * This runs in O(n) where n is the number of edges of the polygon.
 *
 * Source: https://www.algorithms-and-technologies.com/point_in_polygon/javascript
 */
function pointInPolygon(polygon: [number, number][], point: [number, number]) {
  // A point is in a polygon if a line from the point to infinity crosses the polygon an odd number of times
  let odd = false;
  // For each edge (In this case for each point of the polygon and the previous one)
  for (let i = 0, j = polygon.length - 1; i < polygon.length; i++) {
    // If a line from the point into infinity crosses this edge
    if (
      polygon[i][1] > point[1] !== polygon[j][1] > point[1] && // One point needs to be above, one below our y coordinate
      // ...and the edge doesn't cross our Y coordinate before our x coordinate (but between our x coordinate and infinity)
      point[0] <
        ((polygon[j][0] - polygon[i][0]) * (point[1] - polygon[i][1])) /
          (polygon[j][1] - polygon[i][1]) +
          polygon[i][0]
    ) {
      // Invert odd
      odd = !odd;
    }
    j = i;
  }
  // If the number of crossings was odd, the point is in the polygon
  return odd;
}

const getLocationCovidZone = _.memoize(
  (zones: CovidZones, location: { x: number; y: number }): CovidZoneId => {
    // Object.entries doesn't respect Record keys, so we need to cast here
    const zoneEntries = Object.entries(zones) as [CovidZoneId, CovidZone][];
    const zone = zoneEntries.find(([, { positions }]) =>
      pointInPolygon(positions, [location.y, location.x]),
    );

    return zone?.[0] ?? 'Unknown';
  },
);

export function getVenueCovidZone(
  venues: VenueLocationMap,
  zones: CovidZones,
  venue: Venue,
): CovidZoneId {
  const venueInfo = venues[venue];
  if (!venueInfo?.location) {
    return 'Unknown';
  }

  return getLocationCovidZone(zones, venueInfo.location);
}

export default getCovidZones;