import type { AstronomyBody, Location } from "@deadhead/content";
import { Body, HelioVector, KM_PER_AU, RotateVector, Rotation_EQJ_ECL } from "astronomy-engine";
import type { GameTime } from "./clock";

/** A heliocentric point in the ecliptic plane, in million km. */
export interface Position {
  x: number;
  y: number;
}

const MILLION_KM_PER_AU = KM_PER_AU / 1e6;
const MS_PER_DAY = 86_400_000;
const J2000 = Date.UTC(2000, 0, 1, 12);

/** Fixed Earth→Moon offset (mean lunar distance); the Moon's orbit is ignored. */
const MOON_OFFSET_MILLION_KM = 0.384;

/**
 * Ceres is not in astronomy-engine, so it follows a circular orbit with its
 * real mean radius and period, starting from its approximate J2000 mean longitude.
 */
const CERES_RADIUS_AU = 2.7675;
const CERES_PERIOD_DAYS = 1680.5;
const CERES_J2000_LONGITUDE_DEG = 160;

const eqjToEcliptic = Rotation_EQJ_ECL();

function planetPosition(body: Body, gameTime: GameTime): Position {
  const ecliptic = RotateVector(eqjToEcliptic, HelioVector(body, new Date(gameTime)));
  return { x: ecliptic.x * MILLION_KM_PER_AU, y: ecliptic.y * MILLION_KM_PER_AU };
}

function ceresPosition(gameTime: GameTime): Position {
  const days = (gameTime - J2000) / MS_PER_DAY;
  const angle = ((CERES_J2000_LONGITUDE_DEG + (360 * days) / CERES_PERIOD_DAYS) * Math.PI) / 180;
  const radius = CERES_RADIUS_AU * MILLION_KM_PER_AU;
  return { x: radius * Math.cos(angle), y: radius * Math.sin(angle) };
}

export function bodyPosition(body: AstronomyBody, gameTime: GameTime): Position {
  switch (body) {
    case "earth":
      return planetPosition(Body.Earth, gameTime);
    case "moon": {
      const earth = planetPosition(Body.Earth, gameTime);
      return { x: earth.x + MOON_OFFSET_MILLION_KM, y: earth.y };
    }
    case "mars":
      return planetPosition(Body.Mars, gameTime);
    case "ceres":
      return ceresPosition(gameTime);
    case "jupiter":
      return planetPosition(Body.Jupiter, gameTime);
  }
}

/** A location's 2D position at a game time, in million km from the Sun. */
export function locationPosition(location: Location, gameTime: GameTime): Position {
  return bodyPosition(location.body, gameTime);
}