Implemented iterable/flattenDeep.

Fixes #8.

src/iterable/flattenDeep.js

@@ -0,0 +1,27 @@
+"use strict";
+
+/**
+ * Yields the elements in the iterable at the given depth.
+ * Flattening with depth of 1 is the same as doing a plain `flatten()`.
+ *
+ * @this {Iterable}
+ * @param depth The depth that the generator will flatten to.
+ * @example Basic Usage
+ *
+ * ```javascript
+ * [[[1],[2]]]::flattenDeep(2) // yields [1,2]
+ * ```
+*/
+
+export function * flattenDeep <T> (
+    depth : number,
+) : Iterable {
+    if ( depth < 0 ) {
+        yield this;
+        return;
+    }
+
+    for ( const item of this ) {
+        yield * item::flattenDeep(depth - 1);
+    }
+};

test/spec/iterable/flattenDeepSpec.js

@@ -0,0 +1,32 @@
+"use strict";
+
+import { flattenDeep } from "../../../src/iterable/flattenDeep";
+
+describe("flattenDeep()", function () {
+    it("should throw when trying to flatten a non iterable item", function () {
+        void function () {
+            [...[1, 2, 3]::flattenDeep(1)]
+        }.should.throw();
+    })
+
+    it("should flatten a simple nested array", function () {
+        [...[[1], [2]]::flattenDeep(1)].should.deep.equal([1, 2]);
+    });
+
+    it("should flattenDeep to 1 level deep", function () {
+        [...[[[[1]], [2, [3]]]]::flattenDeep(1)].should.deep.equal([[[1]], [2, [3]]]);
+    });
+
+    it("should flattenDeep to 2 levels deep", function () {
+        [...[[[[1]], [2, [3]]]]::flattenDeep(2)].should.deep.equal([[1], 2, [3]]);
+    });
+
+    it("should flattenDeep to 3 levels deep", function () {
+        [...[[[[1]], [[2], [3]]]]::flattenDeep(3)].should.deep.equal([1, 2, 3]);
+    });
+
+    it("should flattenDeep set with nested array", function () {
+        const set = new Set(["a", [1, 2, 3]]);
+        [...[set]::flattenDeep(2)].should.deep.equal(["a", 1, 2, 3]);
+    });
+});