feat: create Folded Scalar function with Test (#1449)

* feat: create Folded Scalar function with Test

* Add Folded ScalarFunction

* Remove from DSL and fix to ScalarFunctionChain

src/core/etl/src/Flow/ETL/Function/ScalarFunctionChain.php

@@ -471,6 +471,11 @@ public function startsWith(ScalarFunction|string $needle) : self
         return new StartsWith($this, $needle);
     }
 
+    public function stringFold() : self
+    {
+        return new StringFold($this);
+    }
+
     public function strPad(int $length, string $pad_string = ' ', int $type = STR_PAD_RIGHT) : self
     {
         return new StrPad($this, $length, $pad_string, $type);

src/core/etl/src/Flow/ETL/Function/StringFold.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Flow\ETL\Function;
+
+use function Flow\ETL\DSL\type_string;
+use function Symfony\Component\String\u;
+use Flow\ETL\Function\ScalarFunction\TypedScalarFunction;
+use Flow\ETL\PHP\Type\Type;
+use Flow\ETL\Row;
+
+final class StringFold extends ScalarFunctionChain implements TypedScalarFunction
+{
+    public function __construct(private readonly ScalarFunction|string $string)
+    {
+    }
+
+    public function eval(Row $row) : mixed
+    {
+        $string = (new Parameter($this->string))->asString($row);
+
+        if ($string === null) {
+            return null;
+        }
+
+        return u($string)->folded()->toString();
+    }
+
+    public function returns() : Type
+    {
+        return type_string();
+    }
+}

src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringFoldTest.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Flow\ETL\Tests\Unit\Function;
+
+use function Flow\ETL\DSL\row;
+use function Flow\ETL\DSL\{ref, str_entry};
+use Flow\ETL\Tests\FlowTestCase;
+
+final class StringFoldTest extends FlowTestCase
+{
+    public function test_string_folded() : void
+    {
+        self::assertSame(
+            "die o'brian strasse",
+            ref('str')->stringFold()->eval(
+                row(str_entry('str', "Die O'Brian Straße"))
+            )
+        );
+    }
+}