source: node_modules/d3-color/src/cubehelix.js

import define, {extend} from "./define.js";
import {Color, rgbConvert, Rgb, darker, brighter} from "./color.js";
import {degrees, radians} from "./math.js";

var A = -0.14861,
    B = +1.78277,
    C = -0.29227,
    D = -0.90649,
    E = +1.97294,
    ED = E * D,
    EB = E * B,
    BC_DA = B * C - D * A;

function cubehelixConvert(o) {
  if (o instanceof Cubehelix) return new Cubehelix(o.h, o.s, o.l, o.opacity);
  if (!(o instanceof Rgb)) o = rgbConvert(o);
  var r = o.r / 255,
      g = o.g / 255,
      b = o.b / 255,
      l = (BC_DA * b + ED * r - EB * g) / (BC_DA + ED - EB),
      bl = b - l,
      k = (E * (g - l) - C * bl) / D,
      s = Math.sqrt(k * k + bl * bl) / (E * l * (1 - l)), // NaN if l=0 or l=1
      h = s ? Math.atan2(k, bl) * degrees - 120 : NaN;
  return new Cubehelix(h < 0 ? h + 360 : h, s, l, o.opacity);
}

export default function cubehelix(h, s, l, opacity) {
  return arguments.length === 1 ? cubehelixConvert(h) : new Cubehelix(h, s, l, opacity == null ? 1 : opacity);
}

export function Cubehelix(h, s, l, opacity) {
  this.h = +h;
  this.s = +s;
  this.l = +l;
  this.opacity = +opacity;
}

define(Cubehelix, cubehelix, extend(Color, {
  brighter(k) {
    k = k == null ? brighter : Math.pow(brighter, k);
    return new Cubehelix(this.h, this.s, this.l * k, this.opacity);
  },
  darker(k) {
    k = k == null ? darker : Math.pow(darker, k);
    return new Cubehelix(this.h, this.s, this.l * k, this.opacity);
  },
  rgb() {
    var h = isNaN(this.h) ? 0 : (this.h + 120) * radians,
        l = +this.l,
        a = isNaN(this.s) ? 0 : this.s * l * (1 - l),
        cosh = Math.cos(h),
        sinh = Math.sin(h);
    return new Rgb(
      255 * (l + a * (A * cosh + B * sinh)),
      255 * (l + a * (C * cosh + D * sinh)),
      255 * (l + a * (E * cosh)),
      this.opacity
    );
  }
}));