Self-intersecting output when cutting a multipolygon with a multiline #205 (#240)

* Update the `cut` method to support multipolygons.
Include corresponding regression test.
* Add `createFromArray` method to handle multiple polygons

Author: alexbol99

index.d.ts

@@ -545,6 +545,7 @@ declare namespace Flatten {
         isEmpty(): boolean;
         isValid(): boolean;
         area(): number;
+        createFromArray(polygons: Polygon): Polygon
         addFace(args: Array<Point> | Array<Segment | Arc> | Circle | Box): Face;
         addFace(edge_from: Edge, edge_to: Edge): Face;
         deleteFace(face: Face): boolean;

src/classes/polygon.js

@@ -116,6 +116,17 @@ export class Polygon {
         return polygon;
     }
 
+    /**
+     * Create a polygon from an array of polygons. Each polygon is an outer face with all containing inner faces
+     * @param polygons
+     * @returns {Polygon}
+     */
+    createFromArray(polygons) {
+        const newPolygon = new Polygon();
+        polygons.forEach(polygon => [...polygon.faces].forEach(face => newPolygon.addFace(face.shapes)));
+        return newPolygon;
+    }
+
     /**
      * Return true is polygon has no edges or faces
      * @returns {boolean}
@@ -156,7 +167,7 @@ export class Polygon {
     }
 
     /**
-     * Add new face to polygon. Returns added face
+     * Add a new face to polygon. Returns added face
      * @param {Point[]|Segment[]|Arc[]|Circle|Box} args -  new face may be create with one of the following ways: <br/>
      * 1) array of points that describe closed path (edges are segments) <br/>
      * 2) array of shapes (segments and arcs) which describe closed path <br/>
@@ -293,11 +304,25 @@ export class Polygon {
 
     /**
      * Cut polygon with multiline and return a new polygon
-     * @param {Multiline} multiline
+     * The cutting is done by intersection of multiline with edges of the polygon.
+     * @param multiline
      * @returns {Polygon}
      */
     cut(multiline) {
+        const polygons = this.splitToIslands();
+        const result = polygons.flatMap(polygon => polygon._cutSingleIsland(multiline))
+            .filter(polygon => polygon.isValid() && polygon.isEmpty() === false);
+        return this.createFromArray(result);
+    }
+
+    /**
+     * Cut polygon with multiline and return a new polygon
+     * @param {Multiline} multiline
+     * @returns {Polygon}
+     */
+    _cutSingleIsland(inputMultiline) {
         let newPoly = this.clone()
+        const multiline = inputMultiline.clone();
 
         // smart intersections
         let intersections = {

test/classes/polygon.js

@@ -1108,6 +1108,25 @@ describe('#Flatten.Polygon', function() {
             console.error('Cut fail.')
         }
     })
+    it('Self-intersecting output when cutting a multipolygon with a multiline #205', () => {
+        const input = new Polygon([[
+            {"ps":{"x":0,"y":0,"name":"point"},"pe":{"x":0,"y":50,"name":"point"},"name":"segment"},
+            {"ps":{"x":0,"y":50,"name":"point"},"pe":{"x":100,"y":50,"name":"point"},"name":"segment"},
+            {"ps":{"x":100,"y":50,"name":"point"},"pe":{"x":100,"y":0,"name":"point"},"name":"segment"},
+            {"ps":{"x":100,"y":0,"name":"point"},"pe":{"x":0,"y":0,"name":"point"},"name":"segment"}
+        ],[
+            {"ps":{"x":0,"y":50,"name":"point"},"pe":{"x":0,"y":100,"name":"point"},"name":"segment"},
+            {"ps":{"x":0,"y":100,"name":"point"},"pe":{"x":100,"y":100,"name":"point"},"name":"segment"},
+            {"ps":{"x":100,"y":100,"name":"point"},"pe":{"x":100,"y":50,"name":"point"},"name":"segment"},
+            {"ps":{"x":100,"y":50,"name":"point"},"pe":{"x":0,"y":50,"name":"point"},"name":"segment"}
+        ]])
+        const cutline = new Multiline([
+            new Segment(point(50,110), point(50, -10))
+        ])
+        const result = input.cut(cutline)
+        expect(result.faces.size).to.equal(4)
+        expect(result.edges.size).to.equal(16)
+    })
     describe('#Intersections', function () {
         it('Can perform intersection between polygons', function () {
             const poly1 = new Polygon(