d3-geo-polygon/src/clip/rejoin.js

Clipping and geometric operations for spherical polygons.

import pointEqual from "../pointEqual.js";

function Intersection(point, points, other, entry) {
  this.x = point;
  this.z = points;
  this.o = other; // another intersection
  this.e = entry; // is an entry?
  this.v = false; // visited
  this.n = this.p = null; // next & previous
}

// A generalized polygon clipping algorithm: given a polygon that has been cut
// into its visible line segments, and rejoins the segments by interpolating
// along the clip edge.
export default function(segments, compareIntersection, startInside, interpolate, stream) {
  const subject = [];
  const clip = [];

  segments.forEach((segment) => {
    let n;
    if ((n = segment.length - 1) <= 0) return;
    let p0 = segment[0];
    const p1 = segment[n];
    
    // If the first and last points of a segment are coincident, then treat as a
    // closed ring. TODO if all rings are closed, then the winding order of the
    // exterior ring should be checked.
    if (pointEqual(p0, p1)) {
      stream.lineStart();
      for (let i = 0; i < n; ++i) stream.point((p0 = segment[i])[0], p0[1]);
      stream.lineEnd();
      return;
    }
    
    let x;
    subject.push(x = new Intersection(p0, segment, null, true));
    clip.push(x.o = new Intersection(p0, null, x, false));
    subject.push(x = new Intersection(p1, segment, null, false));
    clip.push(x.o = new Intersection(p1, null, x, true));
  });

  if (!subject.length) return;

  clip.sort(compareIntersection);
  link(subject);
  link(clip);

  for (let i = 0, n = clip.length; i < n; ++i) {
    clip[i].e = startInside = !startInside;
  }

  let start = subject[0],
      points,
      point;

  // eslint-disable-next-line no-constant-condition
  while (1) {
    // Find first unvisited intersection.
    let current = start,
        isSubject = true;
    while (current.v) if ((current = current.n) === start) return;
    points = current.z;
    stream.lineStart();
    do {
      current.v = current.o.v = true;
      if (current.e) {
        if (isSubject) {
          for (let i = 0, n = points.length; i < n; ++i) stream.point((point = points[i])[0], point[1]);
        } else {
          interpolate(current.x, current.n.x, 1, stream);
        }
        current = current.n;
      } else {
        if (isSubject) {
          points = current.p.z;
          for (let i = points.length - 1; i >= 0; --i) stream.point((point = points[i])[0], point[1]);
        } else {
          interpolate(current.x, current.p.x, -1, stream);
        }
        current = current.p;
      }
      current = current.o;
      points = current.z;
      isSubject = !isSubject;
    } while (!current.v);
    stream.lineEnd();
  }
}

function link(array) {
  const n = array.length;
  if (!n) return;
  let i = 0,
      a = array[0],
      b;
  while (++i < n) {
    a.n = b = array[i];
    b.p = a;
    a = b;
  }
  a.n = b = array[0];
  b.p = a;
}