src/path/centroid.js

import {sqrt} from "../math.js";

// TODO Enforce positive area for exterior, negative area for interior?

var X0 = 0,
    Y0 = 0,
    Z0 = 0,
    X1 = 0,
    Y1 = 0,
    Z1 = 0,
    X2 = 0,
    Y2 = 0,
    Z2 = 0,
    x00,
    y00,
    x0,
    y0;

var centroidStream = {
  point: centroidPoint,
  lineStart: centroidLineStart,
  lineEnd: centroidLineEnd,
  polygonStart: function() {
    centroidStream.lineStart = centroidRingStart;
    centroidStream.lineEnd = centroidRingEnd;
  },
  polygonEnd: function() {
    centroidStream.point = centroidPoint;
    centroidStream.lineStart = centroidLineStart;
    centroidStream.lineEnd = centroidLineEnd;
  },
  result: function() {
    var centroid = Z2 ? [X2 / Z2, Y2 / Z2]
        : Z1 ? [X1 / Z1, Y1 / Z1]
        : Z0 ? [X0 / Z0, Y0 / Z0]
        : [NaN, NaN];
    X0 = Y0 = Z0 =
    X1 = Y1 = Z1 =
    X2 = Y2 = Z2 = 0;
    return centroid;
  }
};

function centroidPoint(x, y) {
  X0 += x;
  Y0 += y;
  ++Z0;
}

function centroidLineStart() {
  centroidStream.point = centroidPointFirstLine;
}

function centroidPointFirstLine(x, y) {
  centroidStream.point = centroidPointLine;
  centroidPoint(x0 = x, y0 = y);
}

function centroidPointLine(x, y) {
  var dx = x - x0, dy = y - y0, z = sqrt(dx * dx + dy * dy);
  X1 += z * (x0 + x) / 2;
  Y1 += z * (y0 + y) / 2;
  Z1 += z;
  centroidPoint(x0 = x, y0 = y);
}

function centroidLineEnd() {
  centroidStream.point = centroidPoint;
}

function centroidRingStart() {
  centroidStream.point = centroidPointFirstRing;
}

function centroidRingEnd() {
  centroidPointRing(x00, y00);
}

function centroidPointFirstRing(x, y) {
  centroidStream.point = centroidPointRing;
  centroidPoint(x00 = x0 = x, y00 = y0 = y);
}

function centroidPointRing(x, y) {
  var dx = x - x0,
      dy = y - y0,
      z = sqrt(dx * dx + dy * dy);

  X1 += z * (x0 + x) / 2;
  Y1 += z * (y0 + y) / 2;
  Z1 += z;

  z = y0 * x - x0 * y;
  X2 += z * (x0 + x);
  Y2 += z * (y0 + y);
  Z2 += z * 3;
  centroidPoint(x0 = x, y0 = y);
}

export default centroidStream;