source: node_modules/d3-geo/src/projection/resample.js

import {cartesian} from "../cartesian.js";
import {abs, asin, atan2, cos, epsilon, radians, sqrt} from "../math.js";
import {transformer} from "../transform.js";

var maxDepth = 16, // maximum depth of subdivision
    cosMinDistance = cos(30 * radians); // cos(minimum angular distance)

export default function(project, delta2) {
  return +delta2 ? resample(project, delta2) : resampleNone(project);
}

function resampleNone(project) {
  return transformer({
    point: function(x, y) {
      x = project(x, y);
      this.stream.point(x[0], x[1]);
    }
  });
}

function resample(project, delta2) {

  function resampleLineTo(x0, y0, lambda0, a0, b0, c0, x1, y1, lambda1, a1, b1, c1, depth, stream) {
    var dx = x1 - x0,
        dy = y1 - y0,
        d2 = dx * dx + dy * dy;
    if (d2 > 4 * delta2 && depth--) {
      var a = a0 + a1,
          b = b0 + b1,
          c = c0 + c1,
          m = sqrt(a * a + b * b + c * c),
          phi2 = asin(c /= m),
          lambda2 = abs(abs(c) - 1) < epsilon || abs(lambda0 - lambda1) < epsilon ? (lambda0 + lambda1) / 2 : atan2(b, a),
          p = project(lambda2, phi2),
          x2 = p[0],
          y2 = p[1],
          dx2 = x2 - x0,
          dy2 = y2 - y0,
          dz = dy * dx2 - dx * dy2;
      if (dz * dz / d2 > delta2 // perpendicular projected distance
          || abs((dx * dx2 + dy * dy2) / d2 - 0.5) > 0.3 // midpoint close to an end
          || a0 * a1 + b0 * b1 + c0 * c1 < cosMinDistance) { // angular distance
        resampleLineTo(x0, y0, lambda0, a0, b0, c0, x2, y2, lambda2, a /= m, b /= m, c, depth, stream);
        stream.point(x2, y2);
        resampleLineTo(x2, y2, lambda2, a, b, c, x1, y1, lambda1, a1, b1, c1, depth, stream);
      }
    }
  }
  return function(stream) {
    var lambda00, x00, y00, a00, b00, c00, // first point
        lambda0, x0, y0, a0, b0, c0; // previous point

    var resampleStream = {
      point: point,
      lineStart: lineStart,
      lineEnd: lineEnd,
      polygonStart: function() { stream.polygonStart(); resampleStream.lineStart = ringStart; },
      polygonEnd: function() { stream.polygonEnd(); resampleStream.lineStart = lineStart; }
    };

    function point(x, y) {
      x = project(x, y);
      stream.point(x[0], x[1]);
    }

    function lineStart() {
      x0 = NaN;
      resampleStream.point = linePoint;
      stream.lineStart();
    }

    function linePoint(lambda, phi) {
      var c = cartesian([lambda, phi]), p = project(lambda, phi);
      resampleLineTo(x0, y0, lambda0, a0, b0, c0, x0 = p[0], y0 = p[1], lambda0 = lambda, a0 = c[0], b0 = c[1], c0 = c[2], maxDepth, stream);
      stream.point(x0, y0);
    }

    function lineEnd() {
      resampleStream.point = point;
      stream.lineEnd();
    }

    function ringStart() {
      lineStart();
      resampleStream.point = ringPoint;
      resampleStream.lineEnd = ringEnd;
    }

    function ringPoint(lambda, phi) {
      linePoint(lambda00 = lambda, phi), x00 = x0, y00 = y0, a00 = a0, b00 = b0, c00 = c0;
      resampleStream.point = linePoint;
    }

    function ringEnd() {
      resampleLineTo(x0, y0, lambda0, a0, b0, c0, x00, y00, lambda00, a00, b00, c00, maxDepth, stream);
      resampleStream.lineEnd = lineEnd;
      lineEnd();
    }

    return resampleStream;
  };
}