From 077d05f7c4be31f884e798448d2b805566b7d2d1 Mon Sep 17 00:00:00 2001
From: ssube <seansube@gmail.com>
Date: Sat, 1 Aug 2020 10:01:00 -0500
Subject: [PATCH] feat(list): add filter many helper

---
 src/List.ts            | 19 +++++++++++++++++++
 test/utils/TestList.ts | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)
 create mode 100644 test/utils/TestList.ts

diff --git a/src/List.ts b/src/List.ts
index d668bfda..d59b92c1 100644
--- a/src/List.ts
+++ b/src/List.ts
@@ -63,3 +63,22 @@ export function isList<TVal>(list: TVal | ReadonlyArray<TVal>): list is Readonly
 export function isList<TVal>(list: TVal | ReadonlyArray<TVal>): list is ReadonlyArray<TVal> {
   return Array.isArray(list);
 }
+
+export function isEmpty(val: Optional<Array<unknown> | ReadonlyArray<unknown>>): boolean {
+  return !Array.isArray(val) || val.length === 0;
+}
+
+export function filterMany<T1>(cb: (a: T1) => boolean, l1: Array<T1>): Array<T1>;
+export function filterMany<T1, T2>(cb: (a: T1, b: T2) => boolean, l1: Array<T1>, l2: Array<T2>): [Array<T1>, Array<T2>];
+export function filterMany<T1, T2, T3>(cb: (a: T1, b: T2) => boolean, l1: Array<T1>, l2: Array<T2>, l3: Array<T3>): [Array<T1>, Array<T2>, Array<T3>];
+export function filterMany<T1, T2, T3, T4>(cb: (a: T1, b: T2) => boolean, l1: Array<T1>, l2: Array<T2>, l3: Array<T3>, l4: Array<T4>): [Array<T1>, Array<T2>, Array<T3>, Array<T4>];
+export function filterMany(cb: (...d: Array<unknown>) => boolean, ...l: Array<Array<unknown>>): Array<Array<unknown>> {
+  const results = [];
+  for (let i = 0; i < l[0].length; ++i) {
+    const slice = l.map((li) => li[i]);
+    if (cb(...slice)) {
+      results.push(slice);
+    }
+  }
+  return results;
+}
diff --git a/test/utils/TestList.ts b/test/utils/TestList.ts
new file mode 100644
index 00000000..0ffd10cf
--- /dev/null
+++ b/test/utils/TestList.ts
@@ -0,0 +1,35 @@
+import { expect } from 'chai';
+import { match, stub } from 'sinon';
+
+import { filterMany } from '../../src/List';
+
+describe('list utils', async () => {
+  describe('filter many helper', async () => {
+    it('should call the predicate with an item from each list', async () => {
+      const cb = stub().returns(true);
+      const results = filterMany(cb, [1], ['a']);
+
+      expect(results.length).to.equal(1);
+      expect(cb).to.have.callCount(1);
+      expect(cb).to.have.been.calledWithMatch(match.number, match.string);
+    });
+
+    it('should call the predicate for each slice', async () => {
+      const data = [1, 2, 3, 4, 5];
+      const cb = stub().returns(true);
+      const results = filterMany(cb, data);
+
+      expect(results.length).to.equal(data.length);
+      expect(cb).to.have.callCount(data.length);
+    });
+
+    it('should keep slices that passed the predicate', async () => {
+      const data = [1, 2, 3, 4, 5];
+      const cb = stub().returns(false);
+      const results = filterMany(cb, data);
+
+      expect(results.length).to.equal(0);
+      expect(cb).to.have.callCount(data.length);
+    });
+  });
+});