Source: core/math/shapes/Polygon.js

var Point = require('../Point'),
    CONST = require('../../const');

/**
 * @class
 * @memberof PIXI
 * @param points {PIXI.Point[]|number[]|...PIXI.Point|...number} This can be an array of Points that form the polygon,
 *      a flat array of numbers that will be interpreted as [x,y, x,y, ...], or the arguments passed can be
 *      all the points of the polygon e.g. `new PIXI.Polygon(new PIXI.Point(), new PIXI.Point(), ...)`, or the
 *      arguments passed can be flat x,y values e.g. `new Polygon(x,y, x,y, x,y, ...)` where `x` and `y` are
 *      Numbers.
 */
function Polygon(points_)
{
    // prevents an argument assignment deopt
    // see section 3.1: https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments
    var points = points_;

    //if points isn't an array, use arguments as the array
    if (!Array.isArray(points))
    {
        // prevents an argument leak deopt
        // see section 3.2: https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments
        points = new Array(arguments.length);

        for (var a = 0; a < points.length; ++a) {
            points[a] = arguments[a];
        }
    }

    // if this is an array of points, convert it to a flat array of numbers
    if (points[0] instanceof Point)
    {
        var p = [];
        for (var i = 0, il = points.length; i < il; i++)
        {
            p.push(points[i].x, points[i].y);
        }

        points = p;
    }

    this.closed = true;

    /**
     * An array of the points of this polygon
     *
     * @member {number[]}
     */
    this.points = points;

    /**
     * The type of the object, mainly used to avoid `instanceof` checks
     *
     * @member {number}
     */
    this.type = CONST.SHAPES.POLY;
}

Polygon.prototype.constructor = Polygon;
module.exports = Polygon;

/**
 * Creates a clone of this polygon
 *
 * @return {PIXI.Polygon} a copy of the polygon
 */
Polygon.prototype.clone = function ()
{
    return new Polygon(this.points.slice());
};

/**
 * Checks whether the x and y coordinates passed to this function are contained within this polygon
 *
 * @param x {number} The X coordinate of the point to test
 * @param y {number} The Y coordinate of the point to test
 * @return {boolean} Whether the x/y coordinates are within this polygon
 */
Polygon.prototype.contains = function (x, y)
{
    var inside = false;

    // use some raycasting to test hits
    // https://github.com/substack/point-in-polygon/blob/master/index.js
    var length = this.points.length / 2;

    for (var i = 0, j = length - 1; i < length; j = i++)
    {
        var xi = this.points[i * 2], yi = this.points[i * 2 + 1],
            xj = this.points[j * 2], yj = this.points[j * 2 + 1],
            intersect = ((yi > y) !== (yj > y)) && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);

        if (intersect)
        {
            inside = !inside;
        }
    }

    return inside;
};