Add reductions operation

src/Nexus/Collection/Collection.php

@@ -369,6 +369,27 @@ public function partition(\Closure $predicate): self
         }, [$this]);
     }
 
+    /**
+     * @template TAcc
+     *
+     * @return self<int, TAcc>
+     */
+    public function reductions(\Closure $predicate, mixed $initial = null): self
+    {
+        return new self(
+            static function (iterable $collection) use ($predicate, $initial): iterable {
+                $accumulator = $initial;
+
+                foreach ($collection as $key => $item) {
+                    $accumulator = $predicate($accumulator, $item, $key);
+
+                    yield $accumulator;
+                }
+            },
+            [$this],
+        );
+    }
+
     /**
      * @return self<TKey, T>
      */

src/Nexus/Collection/CollectionInterface.php

@@ -37,6 +37,7 @@
  * @extends Operation\MapKeys<TKey, T>
  * @extends Operation\MapWithKey<TKey, T>
  * @extends Operation\Partition<TKey, T>
+ * @extends Operation\Reductions<TKey, T>
  * @extends Operation\Reject<TKey, T>
  * @extends Operation\Slice<TKey, T>
  * @extends Operation\Take<TKey, T>
@@ -65,6 +66,7 @@ interface CollectionInterface extends
     Operation\MapKeys,
     Operation\MapWithKey,
     Operation\Partition,
+    Operation\Reductions,
     Operation\Reject,
     Operation\Slice,
     Operation\Take,

src/Nexus/Collection/Operation/Reductions.php

@@ -0,0 +1,41 @@
+<?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 Reductions
+{
+    /**
+     * Returns a new collection consisting of the intermediate values
+     * of reducing the collection using a `$predicate`.
+     *
+     * The reduction `$predicate` is passed an `$initial` accumulator
+     * and the current collection value and returns a new accumulator.
+     *
+     * Reductions returns a list of every accumulator along the way.
+     *
+     * @template TAcc
+     *
+     * @param (\Closure(TAcc, T, TKey): TAcc) $predicate
+     * @param TAcc                            $initial
+     *
+     * @return CollectionInterface<int, TAcc>
+     */
+    public function reductions(\Closure $predicate, mixed $initial = null): CollectionInterface;
+}

tests/Collection/AbstractCollectionTestCase.php

@@ -304,6 +304,16 @@ public function testPartition(): void
         self::assertSame([2 => 'banana', 3 => 'cherry', 4 => 'date'], $collection->all()[1]->all(true));
     }
 
+    public function testReduction(): void
+    {
+        self::assertSame(
+            [1, 4, 9, 16, 25],
+            $this->collection()
+                ->reductions(static fn(int $acc, int $v, int $k): int => $acc + $v + $k, 0)
+                ->all(),
+        );
+    }
+
     public function testReject(): void
     {
         $predicate = static fn(int $item, string $key): bool => str_starts_with($key, 'd') && $item > 2;