From b5895c517393cc767d77383b239d3d8627b86518 Mon Sep 17 00:00:00 2001
From: Ruslan <mavlitov98@icloud.com>
Date: Wed, 15 Jun 2022 15:04:56 +0300
Subject: [PATCH 1/2] Add groupMapReduce

---
 composer.json                                 |  1 +
 .../Functions/Collection/GroupMapReduce.php   | 55 +++++++++++++++++++
 .../Collection/GroupMapReduceTest.php         | 42 ++++++++++++++
 3 files changed, 98 insertions(+)
 create mode 100644 src/Fp/Functions/Collection/GroupMapReduce.php
 create mode 100644 tests/Runtime/Functions/Collection/GroupMapReduceTest.php

diff --git a/composer.json b/composer.json
index 46f10bfb..9942193e 100644
--- a/composer.json
+++ b/composer.json
@@ -60,6 +60,7 @@
             "src/Fp/Functions/Collection/Fold.php",
             "src/Fp/Functions/Collection/ForAll.php",
             "src/Fp/Functions/Collection/GroupBy.php",
+            "src/Fp/Functions/Collection/GroupMapReduce.php",
             "src/Fp/Functions/Collection/Head.php",
             "src/Fp/Functions/Collection/Keys.php",
             "src/Fp/Functions/Collection/Last.php",
diff --git a/src/Fp/Functions/Collection/GroupMapReduce.php b/src/Fp/Functions/Collection/GroupMapReduce.php
new file mode 100644
index 00000000..80a82f2d
--- /dev/null
+++ b/src/Fp/Functions/Collection/GroupMapReduce.php
@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Fp\Collection;
+
+/**
+ * Partitions this iterable collection into a map according to a discriminator function key.
+ * All the values that have the same discriminator are then transformed by the value function and
+ * then reduced into a single value with the reduce function.
+ *
+ * ```php
+ * >>> groupMapReduce(
+ *         collection: [
+ *             ['id' => 10, 'val' => 10],
+ *             ['id' => 10, 'val' => 15],
+ *             ['id' => 10, 'val' => 20],
+ *             ['id' => 20, 'val' => 10],
+ *             ['id' => 20, 'val' => 15],
+ *             ['id' => 30, 'val' => 20],
+ *         ],
+ *         group: fn(array $a) => $a['id'],
+ *         map: fn(array $a) => [$a['val']],
+ *         reduce: fn(array $old, array $new) => array_merge($old, $new),
+ *     );
+ * => [10 => [10, 15, 20], 20 => [10, 15], 30 => [20]]
+ * ```
+ *
+ * @template K of array-key
+ * @template A
+ * @template KOut of array-key
+ * @template B
+ *
+ * @param iterable<K, A> $collection
+ * @param callable(A): KOut $group
+ * @param callable(A): B $map
+ * @param callable(B, B): B $reduce
+ * @return array<KOut, B>
+ */
+function groupMapReduce(iterable $collection, callable $group, callable $map, callable $reduce): array
+{
+    $grouped = [];
+
+    foreach ($collection as $item) {
+        $key = $group($item);
+
+        if (array_key_exists($key, $grouped)) {
+            $grouped[$key] = $reduce($grouped[$key], $map($item));
+        } else {
+            $grouped[$key] = $map($item);
+        }
+    }
+
+    return $grouped;
+}
diff --git a/tests/Runtime/Functions/Collection/GroupMapReduceTest.php b/tests/Runtime/Functions/Collection/GroupMapReduceTest.php
new file mode 100644
index 00000000..edab2568
--- /dev/null
+++ b/tests/Runtime/Functions/Collection/GroupMapReduceTest.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Runtime\Functions\Collection;
+
+use PHPUnit\Framework\TestCase;
+use function Fp\Collection\groupBy;
+use function Fp\Collection\groupMapReduce;
+
+final class GroupMapReduceTest extends TestCase
+{
+    public function testGroup(): void
+    {
+        $this->assertEquals(
+            [
+                10 => [10, 15, 20],
+                20 => [10, 15],
+                30 => [20],
+            ],
+            groupMapReduce(
+                [
+                    ['id' => 10, 'sum' => 10],
+                    ['id' => 10, 'sum' => 15],
+                    ['id' => 10, 'sum' => 20],
+                    ['id' => 20, 'sum' => 10],
+                    ['id' => 20, 'sum' => 15],
+                    ['id' => 30, 'sum' => 20],
+                ],
+                /** @param array{id: int, sum: int} $a */
+                fn(array $a) => $a['id'],
+                /** @param array{id: int, sum: int} $a */
+                fn(array $a) => [$a['sum']],
+                /**
+                 * @param list<int> $old
+                 * @param list<int> $new
+                 */
+                fn(array $old, array $new) => array_merge($old, $new),
+            )
+        );
+    }
+}

From 967946c3d977f394a40bfc4c1d339882fa4f324d Mon Sep 17 00:00:00 2001
From: Ruslan <mavlitov98@icloud.com>
Date: Wed, 15 Jun 2022 15:17:07 +0300
Subject: [PATCH 2/2] More specific return type for groupMapReduce

---
 src/Fp/Functions/Collection/GroupMapReduce.php | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/Fp/Functions/Collection/GroupMapReduce.php b/src/Fp/Functions/Collection/GroupMapReduce.php
index 80a82f2d..1cc1d292 100644
--- a/src/Fp/Functions/Collection/GroupMapReduce.php
+++ b/src/Fp/Functions/Collection/GroupMapReduce.php
@@ -36,6 +36,10 @@
  * @param callable(A): B $map
  * @param callable(B, B): B $reduce
  * @return array<KOut, B>
+ *
+ * @psalm-return ($collection is non-empty-array
+ *     ? non-empty-array<KOut, B>
+ *     : array<KOut, B>)
  */
 function groupMapReduce(iterable $collection, callable $group, callable $map, callable $reduce): array
 {