Add blocks preprocessor

Currently, we need to wrap top-level non-column blocks into a single
column. This is done in the preprocessor.

[MAILPOET-5540]

mailpoet/lib/EmailEditor/Engine/Renderer/Preprocessor.php

@@ -0,0 +1,52 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\EmailEditor\Engine\Renderer;
+
+class Preprocessor {
+
+  const SINGLE_COLUMN_TEMPLATE = [
+    'blockName' => 'core/columns',
+    'attrs' => [],
+    'innerBlocks' => [[
+      'blockName' => 'core/column',
+      'attrs' => [],
+      'innerBlocks' => [],
+    ]],
+  ];
+
+  public function preprocess(array $parsedBlocks): array {
+    return $this->addTopLevelColumns($parsedBlocks);
+  }
+
+  /**
+   * In the editor we allow putting content blocks directly into the root level of the email.
+   * But for rendering purposes it is more convenient to have them wrapped in a single column.
+   * This method walks through the first level of blocks and wraps non column blocks into a single column.
+   */
+  private function addTopLevelColumns(array $parsedBlocks): array {
+    $wrappedParsedBlocks = [];
+    $nonColumnsBlocksBuffer = [];
+    foreach ($parsedBlocks as $block) {
+      // The next block is columns so we can flush the buffer and add the columns block
+      if ($block['blockName'] === 'core/columns') {
+        if ($nonColumnsBlocksBuffer) {
+          $columnsBlock = self::SINGLE_COLUMN_TEMPLATE;
+          $columnsBlock['innerBlocks'][0]['innerBlocks'] = $nonColumnsBlocksBuffer;
+          $nonColumnsBlocksBuffer = [];
+          $wrappedParsedBlocks[] = $columnsBlock;
+        }
+        $wrappedParsedBlocks[] = $block;
+        continue;
+      }
+      // Non columns block so we add it to the buffer
+      $nonColumnsBlocksBuffer[] = $block;
+    }
+    // Flush the buffer if there are any blocks left
+    if ($nonColumnsBlocksBuffer) {
+      $columnsBlock = self::SINGLE_COLUMN_TEMPLATE;
+      $columnsBlock['innerBlocks'][0]['innerBlocks'] = $nonColumnsBlocksBuffer;
+      $wrappedParsedBlocks[] = $columnsBlock;
+    }
+    return $wrappedParsedBlocks;
+  }
+}