refactor!: split also returns empty sets for sectors

makes it easier to reason about the result in relation to the sectors

src/split.ts

@@ -1,7 +1,7 @@
 import { pairwiseTraversal } from "./util";
 
 /**
- * Splits a set into multiple sets at sector points
+ * Splits a set into multiple sets at sector points.
  * 
  * @param source - Set to split at each sector point (inclusive)
  * @param sectors - Set of sector points
@@ -15,8 +15,8 @@ export function* split <T>(source: T[], sectors: T[], comparator: (a: T, b: T) =
       section.push(a)
     }
 
-    // only split if b is not null and section has elements OR source is exausted
-    if ((b !== null && section.length > 0) || a === source[source.length - 1]) {
+    // only split if b is not null OR source is exausted
+    if (b !== null || a === source[source.length - 1]) {
       yield section
       section = []
 

test/split.test.ts

@@ -16,7 +16,7 @@ describe("split", () => {
     });
 
     test(testNames.firstEmpty, () => {
-      expect([...split([], numbers, comparator)]).toEqual([]);
+      expect([...split([], numbers, comparator)]).toEqual(numbers.map(_ => []));
     });
 
     test(testNames.secondEmpty, () => {
@@ -49,7 +49,7 @@ describe("split", () => {
 
     test(testNames.noOverlap, () => {
       expect([...split(even, odd, comparator)]).toEqual(even.map((n) => [n]));
-      expect([...split(odd, even, comparator)]).toEqual(odd.map((n) => [n]));
+      expect([...split(odd, even, comparator)]).toEqual([[], ...odd.map((n) => [n])]);
     });
   });
 });