fix: solve fragment utils edge cases

Author: agviegas

packages/fragments/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@thatopen/fragments",
   "description": "Simple geometric system built on top of Three.js to display 3D BIM data efficiently.",
-  "version": "2.3.0-alpha.7",
+  "version": "2.3.0-alpha.8",
   "author": "That Open Company",
   "contributors": [
     "Antonio Gonzalez Viegas (https://github.com/agviegas)",

packages/fragments/src/fragment-utils.ts

@@ -1,7 +1,15 @@
 import { FragmentIdMap } from "./base-types";
 
 export class FragmentUtils {
-  static combine(maps: Iterable<FragmentIdMap>) {
+  static combine(maps: FragmentIdMap[]) {
+    if (maps.length === 0) {
+      return {};
+    }
+
+    if (maps.length === 1) {
+      return maps[0];
+    }
+
     const result: FragmentIdMap = {};
     for (const map of maps) {
       for (const fragID in map) {
@@ -16,10 +24,18 @@ export class FragmentUtils {
     return result;
   }
 
-  static intersect(maps: Iterable<FragmentIdMap>) {
+  static intersect(maps: FragmentIdMap[]) {
     // Strategy: count all fragIDs and all IDs
     // only stay with values whose count equals the maps count
 
+    if (maps.length === 0) {
+      return {};
+    }
+
+    if (maps.length === 1) {
+      return maps[0];
+    }
+
     const visitedIDs = new Map<
       string,
       { count: number; ids: Map<number, number> }