node_modules/d3-shape/src/curve/natural.js

function Natural(context) {
  this._context = context;
}

Natural.prototype = {
  areaStart: function() {
    this._line = 0;
  },
  areaEnd: function() {
    this._line = NaN;
  },
  lineStart: function() {
    this._x = [];
    this._y = [];
  },
  lineEnd: function() {
    var x = this._x,
        y = this._y,
        n = x.length;

    if (n) {
      this._line ? this._context.lineTo(x[0], y[0]) : this._context.moveTo(x[0], y[0]);
      if (n === 2) {
        this._context.lineTo(x[1], y[1]);
      } else {
        var px = controlPoints(x),
            py = controlPoints(y);
        for (var i0 = 0, i1 = 1; i1 < n; ++i0, ++i1) {
          this._context.bezierCurveTo(px[0][i0], py[0][i0], px[1][i0], py[1][i0], x[i1], y[i1]);
        }
      }
    }

    if (this._line || (this._line !== 0 && n === 1)) this._context.closePath();
    this._line = 1 - this._line;
    this._x = this._y = null;
  },
  point: function(x, y) {
    this._x.push(+x);
    this._y.push(+y);
  }
};

// See https://www.particleincell.com/2012/bezier-splines/ for derivation.
function controlPoints(x) {
  var i,
      n = x.length - 1,
      m,
      a = new Array(n),
      b = new Array(n),
      r = new Array(n);
  a[0] = 0, b[0] = 2, r[0] = x[0] + 2 * x[1];
  for (i = 1; i < n - 1; ++i) a[i] = 1, b[i] = 4, r[i] = 4 * x[i] + 2 * x[i + 1];
  a[n - 1] = 2, b[n - 1] = 7, r[n - 1] = 8 * x[n - 1] + x[n];
  for (i = 1; i < n; ++i) m = a[i] / b[i - 1], b[i] -= m, r[i] -= m * r[i - 1];
  a[n - 1] = r[n - 1] / b[n - 1];
  for (i = n - 2; i >= 0; --i) a[i] = (r[i] - a[i + 1]) / b[i];
  b[n - 1] = (x[n] + a[n - 1]) / 2;
  for (i = 0; i < n - 1; ++i) b[i] = 2 * x[i + 1] - a[i + 1];
  return [a, b];
}

export default function(context) {
  return new Natural(context);
}