tests: Add tests for `SkipList.atOffset()`

src/tests/frontend/specs/skiplist.js

@@ -15,4 +15,40 @@ describe('skiplist.js', function () {
     skiplist.push({key: 'foo'});
     expect(() => skiplist.push({key: 'foo'})).to.throwError();
   });
+
+  it('atOffset() returns last entry that touches offset', async function () {
+    const skiplist = new SkipList();
+    const entries = [];
+    let nextId = 0;
+    const makeEntry = (width) => {
+      const entry = {key: `id${nextId++}`, width};
+      entries.push(entry);
+      return entry;
+    };
+
+    skiplist.push(makeEntry(5));
+    expect(skiplist.atOffset(4)).to.be(entries[0]);
+    expect(skiplist.atOffset(5)).to.be(entries[0]);
+    expect(() => skiplist.atOffset(6)).to.throwError();
+
+    skiplist.push(makeEntry(0));
+    expect(skiplist.atOffset(4)).to.be(entries[0]);
+    expect(skiplist.atOffset(5)).to.be(entries[1]);
+    expect(() => skiplist.atOffset(6)).to.throwError();
+
+    skiplist.push(makeEntry(0));
+    expect(skiplist.atOffset(4)).to.be(entries[0]);
+    expect(skiplist.atOffset(5)).to.be(entries[2]);
+    expect(() => skiplist.atOffset(6)).to.throwError();
+
+    skiplist.splice(2, 0, [makeEntry(0)]);
+    expect(skiplist.atOffset(4)).to.be(entries[0]);
+    expect(skiplist.atOffset(5)).to.be(entries[2]);
+    expect(() => skiplist.atOffset(6)).to.throwError();
+
+    skiplist.push(makeEntry(3));
+    expect(skiplist.atOffset(4)).to.be(entries[0]);
+    expect(skiplist.atOffset(5)).to.be(entries[4]);
+    expect(skiplist.atOffset(6)).to.be(entries[4]);
+  });
 });