Add mapKeys operation

src/Nexus/Collection/Collection.php

@@ -158,6 +158,22 @@ public function map(\Closure $predicate): self
         }, [$this]);
     }
 
+    /**
+     * @template UKey
+     *
+     * @param (\Closure(TKey): UKey) $predicate
+     *
+     * @return self<UKey, T>
+     */
+    public function mapKeys(\Closure $predicate): self
+    {
+        return new self(static function (iterable $collection) use ($predicate): iterable {
+            foreach ($collection as $key => $item) {
+                yield $predicate($key) => $item;
+            }
+        }, [$this]);
+    }
+
     /**
      * @return self<int, T>
      */

src/Nexus/Collection/CollectionInterface.php

@@ -24,6 +24,7 @@
  * @extends Operation\FilterWithKey<TKey, T>
  * @extends Operation\Keys<TKey, T>
  * @extends Operation\Map<TKey, T>
+ * @extends Operation\MapKeys<TKey, T>
  * @extends Operation\Values<TKey, T>
  */
 interface CollectionInterface extends
@@ -35,6 +36,7 @@ interface CollectionInterface extends
     Operation\FilterWithKey,
     Operation\Keys,
     Operation\Map,
+    Operation\MapKeys,
     Operation\Values
 {
     /**

src/Nexus/Collection/Operation/MapKeys.php

@@ -0,0 +1,35 @@
+<?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 MapKeys
+{
+    /**
+     * Returns a new collection consisting of items transformed by the
+     * mapping function `$predicate` that accepts the keys as inputs.
+     *
+     * @template UKey
+     *
+     * @param (\Closure(TKey): UKey) $predicate
+     *
+     * @return CollectionInterface<UKey, T>
+     */
+    public function mapKeys(\Closure $predicate): CollectionInterface;
+}

tests/Collection/AbstractCollectionTestCase.php

@@ -109,6 +109,16 @@ public function testMap(): void
         );
     }
 
+    public function testMapKeys(): void
+    {
+        self::assertSame(
+            [7 => 5, 6 => 4, 5 => 6],
+            $this->collection(['bananas' => 5, 'apples' => 4, 'limes' => 6])
+                ->mapKeys(static fn(string $key): int => \strlen($key))
+                ->all(true),
+        );
+    }
+
     public function testValues(): void
     {
         $collection = $this->collection(static function (): \Generator {

tests/Collection/data/collection.php

@@ -45,3 +45,4 @@
 
 assertType('Nexus\Collection\Collection<int, int>', Collection::wrap([10, 11])->map(static fn(int $item): int => $item ** 2));
 assertType('Nexus\Collection\Collection<int, string>', Collection::wrap([1])->map(static fn(int $item): string => $item > 1 ? 'Yes' : 'No'));
+assertType('Nexus\Collection\Collection<int, int>', Collection::wrap(['apples' => 2])->mapKeys(static fn(string $key): int => \strlen($key)));