source: node_modules/d3-shape/src/curve/monotone.js

function sign(x) {
  return x < 0 ? -1 : 1;
}

// Calculate the slopes of the tangents (Hermite-type interpolation) based on
// the following paper: Steffen, M. 1990. A Simple Method for Monotonic
// Interpolation in One Dimension. Astronomy and Astrophysics, Vol. 239, NO.
// NOV(II), P. 443, 1990.
function slope3(that, x2, y2) {
  var h0 = that._x1 - that._x0,
      h1 = x2 - that._x1,
      s0 = (that._y1 - that._y0) / (h0 || h1 < 0 && -0),
      s1 = (y2 - that._y1) / (h1 || h0 < 0 && -0),
      p = (s0 * h1 + s1 * h0) / (h0 + h1);
  return (sign(s0) + sign(s1)) * Math.min(Math.abs(s0), Math.abs(s1), 0.5 * Math.abs(p)) || 0;
}

// Calculate a one-sided slope.
function slope2(that, t) {
  var h = that._x1 - that._x0;
  return h ? (3 * (that._y1 - that._y0) / h - t) / 2 : t;
}

// According to https://en.wikipedia.org/wiki/Cubic_Hermite_spline#Representations
// "you can express cubic Hermite interpolation in terms of cubic Bézier curves
// with respect to the four values p0, p0 + m0 / 3, p1 - m1 / 3, p1".
function point(that, t0, t1) {
  var x0 = that._x0,
      y0 = that._y0,
      x1 = that._x1,
      y1 = that._y1,
      dx = (x1 - x0) / 3;
  that._context.bezierCurveTo(x0 + dx, y0 + dx * t0, x1 - dx, y1 - dx * t1, x1, y1);
}

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

MonotoneX.prototype = {
  areaStart: function() {
    this._line = 0;
  },
  areaEnd: function() {
    this._line = NaN;
  },
  lineStart: function() {
    this._x0 = this._x1 =
    this._y0 = this._y1 =
    this._t0 = NaN;
    this._point = 0;
  },
  lineEnd: function() {
    switch (this._point) {
      case 2: this._context.lineTo(this._x1, this._y1); break;
      case 3: point(this, this._t0, slope2(this, this._t0)); break;
    }
    if (this._line || (this._line !== 0 && this._point === 1)) this._context.closePath();
    this._line = 1 - this._line;
  },
  point: function(x, y) {
    var t1 = NaN;

    x = +x, y = +y;
    if (x === this._x1 && y === this._y1) return; // Ignore coincident points.
    switch (this._point) {
      case 0: this._point = 1; this._line ? this._context.lineTo(x, y) : this._context.moveTo(x, y); break;
      case 1: this._point = 2; break;
      case 2: this._point = 3; point(this, slope2(this, t1 = slope3(this, x, y)), t1); break;
      default: point(this, this._t0, t1 = slope3(this, x, y)); break;
    }

    this._x0 = this._x1, this._x1 = x;
    this._y0 = this._y1, this._y1 = y;
    this._t0 = t1;
  }
}

function MonotoneY(context) {
  this._context = new ReflectContext(context);
}

(MonotoneY.prototype = Object.create(MonotoneX.prototype)).point = function(x, y) {
  MonotoneX.prototype.point.call(this, y, x);
};

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

ReflectContext.prototype = {
  moveTo: function(x, y) { this._context.moveTo(y, x); },
  closePath: function() { this._context.closePath(); },
  lineTo: function(x, y) { this._context.lineTo(y, x); },
  bezierCurveTo: function(x1, y1, x2, y2, x, y) { this._context.bezierCurveTo(y1, x1, y2, x2, y, x); }
};

export function monotoneX(context) {
  return new MonotoneX(context);
}

export function monotoneY(context) {
  return new MonotoneY(context);
}