import { Colordx, colordx } from "@colordx/core"

// constants from Hillaire, section 4 table 1
const RAYLEIGH_SCATTER = [5.802e-6, 13.558e-6, 33.1e-6] // rgb
const MIE_SCATTER = 3.996e-6
const MIE_ABSORB = 4.44e-6
const OZONE_SCATTER = 0
const OZONE_ABSORB = [0.65e-6, 1.881e-6, 0.085e-6] // rgb

const GROUND_ALBEDO = 0.3

const RAYLEIGH_SCALE_HEIGHT = 8e3 // metres
const MIE_SCALE_HEIGHT = 1.2e3 // metres

const GROUND_RADIUS = 6_360e3 // radius of the earth, metres
const TOP_RADIUS = 6_460e3 // radius of the atmosphere, metres

type Vec3 = [number, number, number];

function dot(v1: Vec3, v2: Vec3) {
  return v1[0] * v2[0] + v1[1] * v2[1] + v1[2] * v2[2];
}

// ACES tonemapper (Knarkowicz)
function aces(colour: Colordx): Colordx {
	Object.keys(colour.toRgb()).forEach((_key, colour) => {
		const n = colour * (2.51 * colour + 0.03)
		const d = colour * (2.43 * colour + 0.59) + 0.14
		return Math.max(0, Math.min(1, n / d))
	})
	return colordx(colour)
}

function rayleighPhase(angle: number): number {
  return (3 * (1 + Math.cos(angle) ** 2)) / (16 * Math.PI);
}

function miePhase(angle: number): number {
  const g = 0.8;
  const scale = 3 / (8 * Math.PI);
  const num = (1 - g ** 2) * (1 + Math.cos(angle) ** 2);
  const denom =
    (2 + g ** 2) * (1 + g ** 2 - 2 * g * Math.cos(angle)) ** (3 / 2);
  return (scale * num) / denom;
}

// From "5.3.2 Intersecting Ray or Segment Against Sphere" (Real-Time Collision Detection)
function intersectSphere(rayOrigin: Vec3, direction: Vec3, radius: number): number | null {
  // Sphere center is at origin (= 0), so rayOrigin - sphereCenter = rayOrigin
  const b = dot(rayOrigin, direction);
  const c = dot(rayOrigin, rayOrigin) - (radius ** 2);
  const discr = (b ** 2) - c;
  if (discr < 0)
    // Ray misses sphere
    return null;
  const t = -b - Math.sqrt(discr);
  if (t < 0)
    // Ray inside sphere; Use far discriminant
    return -b + Math.sqrt(discr);
  return t;
}