Add no duplicate edges rule for workflow validation

[MAILPOET-4629]

mailpoet/lib/Automation/Engine/Validation/WorkflowRules/NoDuplicateEdgesRule.php

@@ -0,0 +1,26 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Validation\WorkflowRules;
+
+use MailPoet\Automation\Engine\Data\Workflow;
+use MailPoet\Automation\Engine\Exceptions;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNode;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNodeVisitor;
+
+class NoDuplicateEdgesRule implements WorkflowNodeVisitor {
+  public function initialize(Workflow $workflow): void {
+  }
+
+  public function visitNode(Workflow $workflow, WorkflowNode $node): void {
+    $visitedNextStepIdsMap = [];
+    foreach ($node->getStep()->getNextSteps() as $nextStep) {
+      if (isset($visitedNextStepIdsMap[$nextStep->getId()])) {
+        throw Exceptions::workflowStructureNotValid(__('Duplicate next step definition found', 'mailpoet'));
+      }
+      $visitedNextStepIdsMap[$nextStep->getId()] = true;
+    }
+  }
+
+  public function complete(Workflow $workflow): void {
+  }
+}

mailpoet/tests/unit/Automation/Engine/Validation/WorkflowRules/NoDuplicateEdgesTest.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Validation\WorkflowRules;
+
+require_once __DIR__ . '/WorkflowRuleTest.php';
+
+use MailPoet\Automation\Engine\Exceptions\UnexpectedValueException;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowWalker;
+
+class NoDuplicateEdgesTest extends WorkflowRuleTest {
+  public function testItDetectsDuplicateEdges(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['a', 'a'],
+      'a' => [],
+    ]);
+
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('Invalid workflow structure: Duplicate next step definition found');
+    (new WorkflowWalker())->walk($workflow, [new NoDuplicateEdgesRule()]);
+  }
+
+  public function testItPassesWithSingleEdges(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['a'],
+      'a' => ['b'],
+      'b' => ['c'],
+      'c' => [],
+    ]);
+
+    (new WorkflowWalker())->walk($workflow, [new NoDuplicateEdgesRule()]);
+    // no exception thrown
+  }
+}