feat(iterables): Sort by - descending

Add version of sort which reverses output.

re #4

Author: danielrbradley

src/iterables.ts

@@ -285,6 +285,26 @@ export function sortBy<T, Key>(a: any, b?: any): any {
   return partial ? exec : exec(a)
 }
 
+export function sortByDescending<T, Key>(
+  selector: (item: T) => Key
+): (source: Iterable<T>) => Iterable<T>
+export function sortByDescending<T, Key>(
+  source: Iterable<T>,
+  selector: (item: T) => Key
+): Iterable<T>
+export function sortByDescending<T, Key>(a: any, b?: any): any {
+  const partial = typeof a === 'function'
+  const selector: (item: T) => Key = partial ? a : b
+  function exec(source: Iterable<T>): Iterable<T> {
+    const copy = Array.from(source)
+    copy.sort((a: T, b: T) => {
+      return selector(a) > selector(b) ? -1 : 1
+    })
+    return copy
+  }
+  return partial ? exec : exec(a)
+}
+
 export function sumBy<T>(selector: (item: T) => number): (source: Iterable<T>) => number
 export function sumBy<T>(source: Iterable<T>, selector: (item: T) => number): number
 export function sumBy<T>(a: any, b?: any): any {

test/iterable.test.ts

@@ -530,6 +530,36 @@ describe('sortBy', () => {
   })
 })
 
+describe('sortByDescending', () => {
+  test('piped', () => {
+    expect(
+      pipe(
+        (function*() {
+          yield { name: 'amy', age: 21 }
+          yield { name: 'bob', age: 2 }
+          yield { name: 'cat', age: 18 }
+        })()
+      )
+        .then(Iterables.sortByDescending(x => x.age))
+        .then(Iterables.toArray).result
+    ).toEqual([{ name: 'amy', age: 21 }, { name: 'cat', age: 18 }, { name: 'bob', age: 2 }])
+  })
+  test('invoke', () => {
+    expect(
+      Iterables.toArray(
+        Iterables.sortByDescending(
+          (function*() {
+            yield { name: 'amy', age: 21 }
+            yield { name: 'bob', age: 2 }
+            yield { name: 'cat', age: 18 }
+          })(),
+          x => x.age
+        )
+      )
+    ).toEqual([{ name: 'amy', age: 21 }, { name: 'cat', age: 18 }, { name: 'bob', age: 2 }])
+  })
+})
+
 describe('sumBy', () => {
   it('sums ages', () => {
     expect(