source: node_modules/d3-geo/src/length.js@ e4c61dd

import {Adder} from "d3-array";
import {abs, atan2, cos, radians, sin, sqrt} from "./math.js";
import noop from "./noop.js";
import stream from "./stream.js";

var lengthSum,
    lambda0,
    sinPhi0,
    cosPhi0;

var lengthStream = {
  sphere: noop,
  point: noop,
  lineStart: lengthLineStart,
  lineEnd: noop,
  polygonStart: noop,
  polygonEnd: noop
};

function lengthLineStart() {
  lengthStream.point = lengthPointFirst;
  lengthStream.lineEnd = lengthLineEnd;
}

function lengthLineEnd() {
  lengthStream.point = lengthStream.lineEnd = noop;
}

function lengthPointFirst(lambda, phi) {
  lambda *= radians, phi *= radians;
  lambda0 = lambda, sinPhi0 = sin(phi), cosPhi0 = cos(phi);
  lengthStream.point = lengthPoint;
}

function lengthPoint(lambda, phi) {
  lambda *= radians, phi *= radians;
  var sinPhi = sin(phi),
      cosPhi = cos(phi),
      delta = abs(lambda - lambda0),
      cosDelta = cos(delta),
      sinDelta = sin(delta),
      x = cosPhi * sinDelta,
      y = cosPhi0 * sinPhi - sinPhi0 * cosPhi * cosDelta,
      z = sinPhi0 * sinPhi + cosPhi0 * cosPhi * cosDelta;
  lengthSum.add(atan2(sqrt(x * x + y * y), z));
  lambda0 = lambda, sinPhi0 = sinPhi, cosPhi0 = cosPhi;
}

export default function(object) {
  lengthSum = new Adder();
  stream(object, lengthStream);
  return +lengthSum;
}