scoll indices based on ND sort name for artists

tests/music_service.test.ts

@@ -1,7 +1,57 @@
 import { v4 as uuid } from "uuid";
 
 import { anArtist } from "./builders";
-import { artistToArtistSummary } from "../src/music_service";
+import { artistToArtistSummary, slice2 } from "../src/music_service";
+
+
+describe("slice2", () => {
+  const things = ["a", "b", "c", "d", "e", "f", "g", "h", "i"];
+  
+  describe("when slice is a subset of the things", () => {
+    it("should return the page", () => {
+      expect(slice2({ _index: 3, _count: 4 })(things)).toEqual([
+        ["d", "e", "f", "g"],
+        things.length
+      ])
+    });
+  });
+
+  describe("when slice goes off the end of the things", () => {
+    it("should return the page", () => {
+      expect(slice2({ _index: 5, _count: 100 })(things)).toEqual([
+        ["f", "g", "h", "i"],
+        things.length
+      ])
+    });
+  });
+
+  describe("when no _count is provided", () => {
+    it("should return from the index", () => {
+      expect(slice2({ _index: 5 })(things)).toEqual([
+        ["f", "g", "h", "i"],
+        things.length
+      ])
+    });
+  });
+
+  describe("when no _index is provided", () => {
+    it("should assume from the start", () => {
+      expect(slice2({ _count: 3 })(things)).toEqual([
+        ["a", "b", "c"],
+        things.length
+      ])
+    });
+  });
+
+  describe("when no _index or _count is provided", () => {
+    it("should return all the things", () => {
+      expect(slice2()(things)).toEqual([
+        things,
+        things.length
+      ])
+    });
+  });
+});
 
 describe("artistToArtistSummary", () => {
   it("should map fields correctly", () => {