rpg-dialogue-js/dist/tests/libs/plotboilerplate/src/ts/utils/algorithms/splitPolygonToNonIntersecting.ts

A simple roleplay game dialogue engine and editor.

/**
 * @requires Line
 * @requires Vertex
 * 
 * @author   Ikaros Kappler
 * @date     2020-12-04
 * @modified 2020-12-07 Ported from vanilla JS to typescript.
 * @version  1.0.1
 */

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


/**
 * Splitting a polygon at a self intersection results in two new polygons.
 */
type ISplit = [ Array<Vertex>, Array<Vertex> ];


/**
 * This function cuts a given self-intersecting polygon into non-self-intersecting 
 * sub polygons.
 *
 * The algorithm only works for non-self-overlapping polygons:
 * Please note that the union set of the sub polygons themselves will _not_ be disjunct if
 * the input polyon overlaps with itself!
 *
 * See this illustration for details:
 * ./demos/27-polygon-intersection-greinerhormann/splitting-polygon-types.png
 *
 * @name splitPolygonToNonIntersecting
 * @param {Array<Vertex>} vertices - The polygon as an array of vertices.
 * @param {number=10} maxDepth - Number of max recursive steps (default is 10).
 * @return {Array<Array<Vertex>>} A sequence of non-self-intersecting sub polygons.
 */
export const splitPolygonToNonIntersecting = (() => {

    /**
     * @param {Array<Vertex>} vertices
     * @param {number=10} maxDepth
     */  
    const splitPolygonToNonIntersecting = ( vertices:Array<Vertex>,
					    maxDepth?:number ) : Array<Array<Vertex>> => {
	if( typeof maxDepth === "undefined" )
	    maxDepth = 10;
	return _splitPolygonToNonIntersecting( vertices, maxDepth );
    };

    
    const _splitPolygonToNonIntersecting = ( vertices:Array<Vertex>,
					     maxDepth:number ) : Array<Array<Vertex>> =>  {
	if( maxDepth <= 0 ) {
	    // aborting at max depth
	    return [ vertices ];
	}

	if( vertices.length <= 3 ) {
	    // No intersections possible
	    return [ vertices ];
	}

	const n : number= vertices.length;
	const lineA : Line = new Line( new Vertex(), new Vertex() );
	const lineB : Line = new Line( new Vertex(), new Vertex() );
	for( var a = 0; a < vertices.length; a++ ) {
	    lineA.a.set( vertices[a] );
	    lineA.b.set( vertices[(a+1) % n] );
	    for( var b = 0; b < vertices.length; b++ ) {
		// Equal edges or neighbour edges intersect by definition.
		// We ignore them.
		if( a == b || a+1 == b || a == b+1
		    || (a==0&&b+1==vertices.length) || (b==0&&a+1==vertices.length) )
		    continue;
		lineB.a.set( vertices[b] );
		lineB.b.set( vertices[(b+1) % n] );

		const intersectionPoint : Vertex|undefined = lineA.intersection(lineB);
		if( intersectionPoint && lineA.hasPoint(intersectionPoint) && lineB.hasPoint(intersectionPoint) ) {

		    // Cut polygon into two here
		    const split : ISplit = splitPolygonAt( vertices, a, b, intersectionPoint );
		    
		    // Split has 2 elements.
		    const leftCleaned : Array<Array<Vertex>> =
			_splitPolygonToNonIntersecting( split[0], maxDepth-1 );
		    const rightCleaned : Array<Array<Vertex>> =
			_splitPolygonToNonIntersecting( split[1], maxDepth-1 );

		    return leftCleaned.concat( rightCleaned );
		}
	    }	
	}

	// No intersection found:
	//    just return a list with the original
	//    polygon as its only element.
	return [ vertices ];
    };


    // Pre: edgeIndexA < edgeIndexB && vertices.length >= 4
    const splitPolygonAt = ( vertices:Array<Vertex>,
			     edgeIndexA:number,
			     edgeIndexB:number,
			     intersectionPoint:Vertex ) : ISplit => {
	const first : Array<Vertex> = [];
	var second : Array<Vertex> = [];
	for( var i = 0; i < vertices.length; i++ ) {
	    if( i <= edgeIndexA )
		first.push( vertices[i] );

	    if( i == edgeIndexA )  {
		first.push( intersectionPoint );
		second.push( intersectionPoint ); // clone???
	    }

	    if( i > edgeIndexA && i <= edgeIndexB )
		second.push( vertices[i] );

	    if( i > edgeIndexB )
		first.push( vertices[i] );
	    
	}

	return [ first, second ];
    };

    return splitPolygonToNonIntersecting;
})();