/**
 * @requires Line
 * @requires Vertex
 *
 * @author   Ikaros Kappler
 * @date     2020-12-04
 * @modified 2020-12-09 Ported from vanilla JS to Typescript.
 */

import { Line } from "../../Line";
import { Vertex } from "../../Vertex";
import { XYCoords } from "../../interfaces";

/**
 * Collect all self-intersection points of the given polygon.
 *
 * If the given polygon (vertices) is not self intersecting then the returned array is empty.
 *
 * @name findPolygonSelfIntersections
 * @param {Array<Vertex>} vertices - The vertices that form the polygon.
 * @return Array<Vertex>
 */
export const findPolygonSelfIntersections = ( vertices:Array<XYCoords> ) : Array<Vertex> => {
    const pointList : Array<Vertex> = [];
    const lineA : Line = new Line( new Vertex(), new Vertex() );
    const lineB : Line = new Line( new Vertex(), new Vertex() );
    const n : number = vertices.length;
    for( var a = 0; a < n; a++ ) {
	lineA.a.set( vertices[a] );
	lineA.b.set( vertices[(a+1)%n] );
	for( var b = 0; b < n; b++ ) {
	    // Same edge or neighbour edges intersect by definition.
	    // Skip them.
	    if( a == b || a+1 == b || a == b+1 || (a==0&&b+1==n) || (b==0&&a+1==n) )
		continue;
	    lineB.a.set( vertices[b] );
	    lineB.b.set( vertices[(b+1)%n] );

	    // Commpute the intersection point of the (infinite) lines.
	    // The point is undefined if and only if both line are parallel (co-linear).
	    const intersectionPoint : Vertex|undefined = lineA.intersection(lineB);

	    // Check if the intersection point is on both (finite) edges of the polygon.
	    if( intersectionPoint
		&& lineA.hasPoint(intersectionPoint) && lineB.hasPoint(intersectionPoint) ) {
		pointList.push( intersectionPoint );
	    }
	}	
    }
    return pointList;
};