LibJS: Implement Sets using Maps

This implements ordered sets using Maps with a sentinel value, and
includes some extra set tests.
Fixes #11004.

Co-Authored-By: davidot <davidot@serenityos.org>

Userland/Libraries/LibJS/Tests/builtins/Set/Set.prototype.forEach.js

@@ -46,3 +46,106 @@ describe("normal behavior", () => {
         });
     });
 });
+
+describe("modification during iteration", () => {
+    test("adding items during forEach also get visited", () => {
+        const set = new Set([1, 2]);
+        const visited = [];
+        set.forEach(val => {
+            if (val <= 2) set.add(4 * val);
+
+            visited.push(val);
+        });
+        expect(set).toHaveSize(4);
+
+        expect(visited).toEqual([1, 2, 4, 8]);
+    });
+
+    test("removing an item before it is visited means it doesn't get visited", () => {
+        const set = new Set([1, 2, 3]);
+        const visited = [];
+        set.forEach(val => {
+            visited.push(val);
+            if (val === 1) {
+                expect(set.delete(2)).toBeTrue();
+            } else {
+                expect(val).toBe(3);
+                expect(set.delete(2)).toBeFalse();
+            }
+        });
+        expect(set).toHaveSize(2);
+        expect(visited).toEqual([1, 3]);
+    });
+
+    test("removing an item after it was visited and adding it again means it gets visited twice", () => {
+        const set = new Set([1, 2, 3]);
+        const visited = [];
+        set.forEach(val => {
+            visited.push(val);
+            if (val === 2) {
+                expect(set.delete(1)).toBeTrue();
+            } else if (val === 3) {
+                expect(set).toHaveSize(2);
+                set.add(1);
+                expect(set).toHaveSize(3);
+            }
+        });
+        expect(set).toHaveSize(3);
+        expect(visited).toEqual([1, 2, 3, 1]);
+    });
+
+    test("adding a new item and removing it before it gets visited means it never gets visited", () => {
+        const set = new Set([1, 2]);
+        const visited = [];
+        set.forEach(val => {
+            visited.push(val);
+            if (val === 1) {
+                set.add(3);
+                expect(set).toHaveSize(3);
+            } else if (val === 2) {
+                expect(set).toHaveSize(3);
+                expect(set.delete(3)).toBeTrue();
+                expect(set).toHaveSize(2);
+            }
+            expect(val).not.toBe(3);
+        });
+        expect(set).toHaveSize(2);
+        expect(visited).toEqual([1, 2]);
+    });
+
+    test("removing and adding in the same iterations", () => {
+        const set = new Set([1, 2, 3]);
+        const visited = [];
+        let first = true;
+        set.forEach(val => {
+            visited.push(val);
+            if (val === 1 && first) {
+                expect(set.delete(1)).toBeTrue();
+                set.add(1);
+            }
+
+            first = false;
+        });
+        expect(set).toHaveSize(3);
+
+        expect(visited).toEqual([1, 2, 3, 1]);
+    });
+
+    test("removing and readding the same item means it can get visited n times", () => {
+        let n = 3;
+
+        const set = new Set([1, 2]);
+
+        const visited = [];
+        set.forEach(val => {
+            visited.push(val);
+            if (n-- > 0) {
+                expect(set.delete(val)).toBeTrue();
+                set.add(val);
+            }
+        });
+
+        expect(set).toHaveSize(2);
+        expect(visited).toEqual([1, 2, 1, 2, 1]);
+    });
+});