Add FoldLeft and FoldRight transformations.

spec/loophp/collection/CollectionSpec.php

@@ -95,6 +95,21 @@ static function ($item) {
             ->shouldIterateAs([1, 2, 3, 4, 5, 6]);
     }
 
+    public function it_can_be_constructed_from_a_stream(): void
+    {
+        $string = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.';
+
+        $stream = fopen('data://text/plain,' . $string, 'rb');
+        $this::with($stream)
+            ->count()
+            ->shouldReturn(56);
+
+        $stream = fopen('data://text/plain,' . $string, 'rb');
+        $this::with($stream)
+            ->implode('')
+            ->shouldReturn($string);
+    }
+
     public function it_can_be_constructed_from_array(): void
     {
         $this
@@ -520,6 +535,40 @@ public function it_can_flip(): void
             ->shouldIterateAs(['a', 'b', 'c', 'd', 'a']);
     }
 
+    public function it_can_fold_from_the_left(): void
+    {
+        $this
+            ->beConstructedThrough('with', [range('A', 'C')]);
+
+        $this
+            ->foldLeft(
+                static function (string $carry, string $item): string {
+                    $carry .= $item;
+
+                    return $carry;
+                },
+                ''
+            )
+            ->shouldReturn('ABC');
+    }
+
+    public function it_can_fold_from_the_right(): void
+    {
+        $this
+            ->beConstructedThrough('with', [range('A', 'C')]);
+
+        $this
+            ->foldRight(
+                static function (string $carry, string $item): string {
+                    $carry .= $item;
+
+                    return $carry;
+                },
+                ''
+            )
+            ->shouldReturn('CBA');
+    }
+
     public function it_can_forget(): void
     {
         $this
@@ -1291,19 +1340,4 @@ public function it_is_initializable(): void
     {
         $this->shouldHaveType(Collection::class);
     }
-
-    public function it_can_be_constructed_from_a_stream(): void
-    {
-        $string = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.';
-
-        $stream = fopen('data://text/plain,' . $string, 'rb');
-        $this::with($stream)
-            ->count()
-            ->shouldReturn(56);
-
-        $stream = fopen('data://text/plain,' . $string, 'rb');
-        $this::with($stream)
-            ->implode('')
-            ->shouldReturn($string);
-    }
 }

src/Collection.php

@@ -18,6 +18,8 @@
 use loophp\collection\Operation\Filter;
 use loophp\collection\Operation\Flatten;
 use loophp\collection\Operation\Flip;
+use loophp\collection\Operation\FoldLeft;
+use loophp\collection\Operation\FoldRight;
 use loophp\collection\Operation\Forget;
 use loophp\collection\Operation\Intersperse;
 use loophp\collection\Operation\Iterate;
@@ -199,6 +201,22 @@ public function flip(): BaseInterface
         return $this->run(new Flip());
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function foldLeft(callable $callback, $initial = null)
+    {
+        return $this->transform(new FoldLeft($callback, $initial));
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function foldRight(callable $callback, $initial = null)
+    {
+        return $this->transform(new FoldRight($callback, $initial));
+    }
+
     /**
      * {@inheritdoc}
      */

src/Contract/Collection.php

@@ -26,6 +26,8 @@ interface Collection extends
     Firstable,
     Flattenable,
     Flipable,
+    FoldLeftable,
+    FoldRightable,
     Forgetable,
     Getable,
     Implodeable,

src/Contract/FoldLeftable.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace loophp\collection\Contract;
+
+/**
+ * Interface FoldLeftable.
+ */
+interface FoldLeftable
+{
+    /**
+     * Fold the collection from the left to the right.
+     *
+     * @param callable $callback
+     * @param mixed $initial
+     *
+     * @return mixed
+     */
+    public function foldLeft(callable $callback, $initial = null);
+}

src/Contract/FoldRightable.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace loophp\collection\Contract;
+
+/**
+ * Interface FoldRightable.
+ */
+interface FoldRightable
+{
+    /**
+     * Fold the collection from the right to the left.
+     *
+     * @param callable $callback
+     * @param mixed $initial
+     *
+     * @return mixed
+     */
+    public function foldRight(callable $callback, $initial = null);
+}

src/Operation/FoldLeft.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace loophp\collection\Operation;
+
+use loophp\collection\Contract\Transformation;
+
+/**
+ * Class FoldLeft.
+ */
+final class FoldLeft implements Transformation
+{
+    /**
+     * @var callable
+     */
+    private $callback;
+
+    /**
+     * @var mixed
+     */
+    private $initial;
+
+    /**
+     * FoldLeft constructor.
+     *
+     * @param callable $callback
+     * @param mixed|null $initial
+     */
+    public function __construct(callable $callback, $initial = null)
+    {
+        $this->callback = $callback;
+        $this->initial = $initial;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function on(iterable $collection)
+    {
+        $callback = $this->callback;
+        $initial = $this->initial;
+
+        foreach ($collection as $key => $value) {
+            $initial = $callback($initial, $value, $key);
+        }
+
+        return $initial;
+    }
+}

src/Operation/FoldRight.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace loophp\collection\Operation;
+
+use loophp\collection\Contract\Transformation;
+
+/**
+ * Class FoldRight.
+ */
+final class FoldRight implements Transformation
+{
+    /**
+     * @var callable
+     */
+    private $callback;
+
+    /**
+     * @var mixed
+     */
+    private $initial;
+
+    /**
+     * FoldRight constructor.
+     *
+     * @param callable $callback
+     * @param mixed|null $initial
+     */
+    public function __construct(callable $callback, $initial = null)
+    {
+        $this->callback = $callback;
+        $this->initial = $initial;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function on(iterable $collection)
+    {
+        $callback = $this->callback;
+        $initial = $this->initial;
+
+        foreach ((new Reverse())->on($collection)() as $key => $value) {
+            $initial = $callback($initial, $value, $key);
+        }
+
+        return $initial;
+    }
+}