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

import {Adder} from "d3-array";
import {abs} from "../math.js";
import noop from "../noop.js";

var areaSum = new Adder(),
    areaRingSum = new Adder(),
    x00,
    y00,
    x0,
    y0;

var areaStream = {
  point: noop,
  lineStart: noop,
  lineEnd: noop,
  polygonStart: function() {
    areaStream.lineStart = areaRingStart;
    areaStream.lineEnd = areaRingEnd;
  },
  polygonEnd: function() {
    areaStream.lineStart = areaStream.lineEnd = areaStream.point = noop;
    areaSum.add(abs(areaRingSum));
    areaRingSum = new Adder();
  },
  result: function() {
    var area = areaSum / 2;
    areaSum = new Adder();
    return area;
  }
};

function areaRingStart() {
  areaStream.point = areaPointFirst;
}

function areaPointFirst(x, y) {
  areaStream.point = areaPoint;
  x00 = x0 = x, y00 = y0 = y;
}

function areaPoint(x, y) {
  areaRingSum.add(y0 * x - x0 * y);
  x0 = x, y0 = y;
}

function areaRingEnd() {
  areaPoint(x00, y00);
}

export default areaStream;