From 208c9db4c5b68009f587f07dfc6616c2fbcd66e7 Mon Sep 17 00:00:00 2001
From: "John Paul E. Balandan, CPA" <paulbalandan@gmail.com>
Date: Sat, 7 Sep 2024 00:45:59 +0800
Subject: [PATCH] Add `keys` operation

---
 src/Nexus/Collection/Collection.php           |  9 +++++
 src/Nexus/Collection/CollectionInterface.php  |  2 ++
 src/Nexus/Collection/Operation/Keys.php       | 36 +++++++++++++++++++
 .../Collection/AbstractCollectionTestCase.php | 13 +++++++
 4 files changed, 60 insertions(+)
 create mode 100644 src/Nexus/Collection/Operation/Keys.php

diff --git a/src/Nexus/Collection/Collection.php b/src/Nexus/Collection/Collection.php
index d7a3d5b..e5d3a4f 100644
--- a/src/Nexus/Collection/Collection.php
+++ b/src/Nexus/Collection/Collection.php
@@ -98,6 +98,15 @@ public function getIterator(): \Traversable
         yield from $this->innerIterator->getIterator();
     }
 
+    public function keys(): CollectionInterface
+    {
+        return new self(static function (iterable $collection): iterable {
+            foreach ($collection as $key => $_) {
+                yield $key;
+            }
+        }, [$this]);
+    }
+
     public function values(): CollectionInterface
     {
         return new self(static function (iterable $collection): iterable {
diff --git a/src/Nexus/Collection/CollectionInterface.php b/src/Nexus/Collection/CollectionInterface.php
index 04ac70b..c3ef1bd 100644
--- a/src/Nexus/Collection/CollectionInterface.php
+++ b/src/Nexus/Collection/CollectionInterface.php
@@ -19,12 +19,14 @@
  *
  * @extends \IteratorAggregate<TKey, T>
  * @extends Operation\All<TKey, T>
+ * @extends Operation\Keys<TKey, T>
  * @extends Operation\Values<TKey, T>
  */
 interface CollectionInterface extends
     \Countable,
     \IteratorAggregate,
     Operation\All,
+    Operation\Keys,
     Operation\Values
 {
     /**
diff --git a/src/Nexus/Collection/Operation/Keys.php b/src/Nexus/Collection/Operation/Keys.php
new file mode 100644
index 0000000..e88245b
--- /dev/null
+++ b/src/Nexus/Collection/Operation/Keys.php
@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * This file is part of the Nexus framework.
+ *
+ * (c) John Paul E. Balandan, CPA <paulbalandan@gmail.com>
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Nexus\Collection\Operation;
+
+use Nexus\Collection\CollectionInterface;
+
+/**
+ * @template TKey
+ * @template T
+ */
+interface Keys
+{
+    /**
+     * Returns a new collection from keys of the original collection as
+     * the new values.
+     *
+     * ```
+     * Collection::wrap(['banana' => 1, 'apple' => 2])->keys();
+     * => Collection(['banana', 'apple'])
+     * ```
+     *
+     * @return CollectionInterface<int, TKey>
+     */
+    public function keys(): CollectionInterface;
+}
diff --git a/tests/Collection/AbstractCollectionTestCase.php b/tests/Collection/AbstractCollectionTestCase.php
index ec742f8..1b6f60f 100644
--- a/tests/Collection/AbstractCollectionTestCase.php
+++ b/tests/Collection/AbstractCollectionTestCase.php
@@ -55,6 +55,19 @@ public function testCount(): void
         self::assertCount(2, $this->collection([1, 2]));
     }
 
+    public function testKeys(): void
+    {
+        $collection = $this->collection(static function (): \Generator {
+            yield 'bananas' => 5;
+
+            yield 'apples' => 4;
+
+            yield 'oranges' => 7;
+        });
+
+        self::assertSame(['bananas', 'apples', 'oranges'], $collection->keys()->all());
+    }
+
     public function testValues(): void
     {
         $collection = $this->collection(static function (): \Generator {