source: node_modules/d3-delaunay/src/delaunay.js@ e4c61dd

import Delaunator from "delaunator";
import Path from "./path.js";
import Polygon from "./polygon.js";
import Voronoi from "./voronoi.js";

const tau = 2 * Math.PI, pow = Math.pow;

function pointX(p) {
  return p[0];
}

function pointY(p) {
  return p[1];
}

// A triangulation is collinear if all its triangles have a non-null area
function collinear(d) {
  const {triangles, coords} = d;
  for (let i = 0; i < triangles.length; i += 3) {
    const a = 2 * triangles[i],
          b = 2 * triangles[i + 1],
          c = 2 * triangles[i + 2],
          cross = (coords[c] - coords[a]) * (coords[b + 1] - coords[a + 1])
                - (coords[b] - coords[a]) * (coords[c + 1] - coords[a + 1]);
    if (cross > 1e-10) return false;
  }
  return true;
}

function jitter(x, y, r) {
  return [x + Math.sin(x + y) * r, y + Math.cos(x - y) * r];
}

export default class Delaunay {
  static from(points, fx = pointX, fy = pointY, that) {
    return new Delaunay("length" in points
        ? flatArray(points, fx, fy, that)
        : Float64Array.from(flatIterable(points, fx, fy, that)));
  }
  constructor(points) {
    this._delaunator = new Delaunator(points);
    this.inedges = new Int32Array(points.length / 2);
    this._hullIndex = new Int32Array(points.length / 2);
    this.points = this._delaunator.coords;
    this._init();
  }
  update() {
    this._delaunator.update();
    this._init();
    return this;
  }
  _init() {
    const d = this._delaunator, points = this.points;

    // check for collinear
    if (d.hull && d.hull.length > 2 && collinear(d)) {
      this.collinear = Int32Array.from({length: points.length/2}, (_,i) => i)
        .sort((i, j) => points[2 * i] - points[2 * j] || points[2 * i + 1] - points[2 * j + 1]); // for exact neighbors
      const e = this.collinear[0], f = this.collinear[this.collinear.length - 1],
        bounds = [ points[2 * e], points[2 * e + 1], points[2 * f], points[2 * f + 1] ],
        r = 1e-8 * Math.hypot(bounds[3] - bounds[1], bounds[2] - bounds[0]);
      for (let i = 0, n = points.length / 2; i < n; ++i) {
        const p = jitter(points[2 * i], points[2 * i + 1], r);
        points[2 * i] = p[0];
        points[2 * i + 1] = p[1];
      }
      this._delaunator = new Delaunator(points);
    } else {
      delete this.collinear;
    }

    const halfedges = this.halfedges = this._delaunator.halfedges;
    const hull = this.hull = this._delaunator.hull;
    const triangles = this.triangles = this._delaunator.triangles;
    const inedges = this.inedges.fill(-1);
    const hullIndex = this._hullIndex.fill(-1);

    // Compute an index from each point to an (arbitrary) incoming halfedge
    // Used to give the first neighbor of each point; for this reason,
    // on the hull we give priority to exterior halfedges
    for (let e = 0, n = halfedges.length; e < n; ++e) {
      const p = triangles[e % 3 === 2 ? e - 2 : e + 1];
      if (halfedges[e] === -1 || inedges[p] === -1) inedges[p] = e;
    }
    for (let i = 0, n = hull.length; i < n; ++i) {
      hullIndex[hull[i]] = i;
    }

    // degenerate case: 1 or 2 (distinct) points
    if (hull.length <= 2 && hull.length > 0) {
      this.triangles = new Int32Array(3).fill(-1);
      this.halfedges = new Int32Array(3).fill(-1);
      this.triangles[0] = hull[0];
      inedges[hull[0]] = 1;
      if (hull.length === 2) {
        inedges[hull[1]] = 0;
        this.triangles[1] = hull[1];
        this.triangles[2] = hull[1];
      }
    }
  }
  voronoi(bounds) {
    return new Voronoi(this, bounds);
  }
  *neighbors(i) {
    const {inedges, hull, _hullIndex, halfedges, triangles, collinear} = this;

    // degenerate case with several collinear points
    if (collinear) {
      const l = collinear.indexOf(i);
      if (l > 0) yield collinear[l - 1];
      if (l < collinear.length - 1) yield collinear[l + 1];
      return;
    }

    const e0 = inedges[i];
    if (e0 === -1) return; // coincident point
    let e = e0, p0 = -1;
    do {
      yield p0 = triangles[e];
      e = e % 3 === 2 ? e - 2 : e + 1;
      if (triangles[e] !== i) return; // bad triangulation
      e = halfedges[e];
      if (e === -1) {
        const p = hull[(_hullIndex[i] + 1) % hull.length];
        if (p !== p0) yield p;
        return;
      }
    } while (e !== e0);
  }
  find(x, y, i = 0) {
    if ((x = +x, x !== x) || (y = +y, y !== y)) return -1;
    const i0 = i;
    let c;
    while ((c = this._step(i, x, y)) >= 0 && c !== i && c !== i0) i = c;
    return c;
  }
  _step(i, x, y) {
    const {inedges, hull, _hullIndex, halfedges, triangles, points} = this;
    if (inedges[i] === -1 || !points.length) return (i + 1) % (points.length >> 1);
    let c = i;
    let dc = pow(x - points[i * 2], 2) + pow(y - points[i * 2 + 1], 2);
    const e0 = inedges[i];
    let e = e0;
    do {
      let t = triangles[e];
      const dt = pow(x - points[t * 2], 2) + pow(y - points[t * 2 + 1], 2);
      if (dt < dc) dc = dt, c = t;
      e = e % 3 === 2 ? e - 2 : e + 1;
      if (triangles[e] !== i) break; // bad triangulation
      e = halfedges[e];
      if (e === -1) {
        e = hull[(_hullIndex[i] + 1) % hull.length];
        if (e !== t) {
          if (pow(x - points[e * 2], 2) + pow(y - points[e * 2 + 1], 2) < dc) return e;
        }
        break;
      }
    } while (e !== e0);
    return c;
  }
  render(context) {
    const buffer = context == null ? context = new Path : undefined;
    const {points, halfedges, triangles} = this;
    for (let i = 0, n = halfedges.length; i < n; ++i) {
      const j = halfedges[i];
      if (j < i) continue;
      const ti = triangles[i] * 2;
      const tj = triangles[j] * 2;
      context.moveTo(points[ti], points[ti + 1]);
      context.lineTo(points[tj], points[tj + 1]);
    }
    this.renderHull(context);
    return buffer && buffer.value();
  }
  renderPoints(context, r) {
    if (r === undefined && (!context || typeof context.moveTo !== "function")) r = context, context = null;
    r = r == undefined ? 2 : +r;
    const buffer = context == null ? context = new Path : undefined;
    const {points} = this;
    for (let i = 0, n = points.length; i < n; i += 2) {
      const x = points[i], y = points[i + 1];
      context.moveTo(x + r, y);
      context.arc(x, y, r, 0, tau);
    }
    return buffer && buffer.value();
  }
  renderHull(context) {
    const buffer = context == null ? context = new Path : undefined;
    const {hull, points} = this;
    const h = hull[0] * 2, n = hull.length;
    context.moveTo(points[h], points[h + 1]);
    for (let i = 1; i < n; ++i) {
      const h = 2 * hull[i];
      context.lineTo(points[h], points[h + 1]);
    }
    context.closePath();
    return buffer && buffer.value();
  }
  hullPolygon() {
    const polygon = new Polygon;
    this.renderHull(polygon);
    return polygon.value();
  }
  renderTriangle(i, context) {
    const buffer = context == null ? context = new Path : undefined;
    const {points, triangles} = this;
    const t0 = triangles[i *= 3] * 2;
    const t1 = triangles[i + 1] * 2;
    const t2 = triangles[i + 2] * 2;
    context.moveTo(points[t0], points[t0 + 1]);
    context.lineTo(points[t1], points[t1 + 1]);
    context.lineTo(points[t2], points[t2 + 1]);
    context.closePath();
    return buffer && buffer.value();
  }
  *trianglePolygons() {
    const {triangles} = this;
    for (let i = 0, n = triangles.length / 3; i < n; ++i) {
      yield this.trianglePolygon(i);
    }
  }
  trianglePolygon(i) {
    const polygon = new Polygon;
    this.renderTriangle(i, polygon);
    return polygon.value();
  }
}

function flatArray(points, fx, fy, that) {
  const n = points.length;
  const array = new Float64Array(n * 2);
  for (let i = 0; i < n; ++i) {
    const p = points[i];
    array[i * 2] = fx.call(that, p, i, points);
    array[i * 2 + 1] = fy.call(that, p, i, points);
  }
  return array;
}

function* flatIterable(points, fx, fy, that) {
  let i = 0;
  for (const p of points) {
    yield fx.call(that, p, i, points);
    yield fy.call(that, p, i, points);
    ++i;
  }
}