Wip oneOf directive

src/Typesystem/Exception/OneOfDirectiveNotSatisfied.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Graphpinator\Typesystem\Exception;
+
+final class OneOfDirectiveNotSatisfied extends \Graphpinator\Typesystem\Exception\TypeError
+{
+    public const MESSAGE = 'Exactly one field must be specified and be not null.';
+
+    public function isOutputable() : bool
+    {
+        return true;
+    }
+}

src/Typesystem/Exception/OneOfInputInvalidFields.php

@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Graphpinator\Typesystem\Exception;
+
+final class OneOfInputInvalidFields extends \Graphpinator\Typesystem\Exception\TypeError
+{
+    public const MESSAGE = 'OneOf input type must have only nullable fields without default values.';
+}

src/Typesystem/Spec/OneOfDirective.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Graphpinator\Typesystem\Spec;
+
+use \Graphpinator\Value\ArgumentValueSet;
+use \Graphpinator\Value\InputValue;
+
+#[\Graphpinator\Typesystem\Attribute\Description('Built-in oneOf directive')]
+final class OneOfDirective extends \Graphpinator\Typesystem\Directive implements \Graphpinator\Typesystem\Location\InputObjectLocation
+{
+    protected const NAME = 'oneOf';
+
+    public function validateInputUsage(\Graphpinator\Typesystem\InputType $inputType, ArgumentValueSet $arguments) : bool
+    {
+        foreach ($inputType->getArguments() as $argument) {
+            if ($argument->getType() instanceof \Graphpinator\Typesystem\NotNullType ||
+                $argument->getDefaultValue() instanceof \Graphpinator\Value\ArgumentValue) {
+                throw new \Graphpinator\Typesystem\Exception\OneOfInputInvalidFields();
+            }
+        }
+    }
+
+    public function resolveInputObject(ArgumentValueSet $arguments, InputValue $inputValue) : void
+    {
+        $currentCount = 0;
+
+        foreach ($inputValue as $innerValue) {
+            \assert($innerValue instanceof \Graphpinator\Value\ArgumentValue);
+
+            if ($currentCount >= 1 || $innerValue->getValue() instanceof \Graphpinator\Value\NullValue) {
+                throw new \Graphpinator\Typesystem\Exception\OneOfDirectiveNotSatisfied();
+            }
+
+            ++$currentCount;
+        }
+    }
+
+    protected function getFieldDefinition() : \Graphpinator\Typesystem\Argument\ArgumentSet
+    {
+        return new \Graphpinator\Typesystem\Argument\ArgumentSet([]);
+    }
+}