Extract subscribers to a separate table in the Bounce worker [MAILPOET-940]

lib/Tasks/Subscribers/BatchIterator.php

@@ -0,0 +1,59 @@
+<?php
+namespace MailPoet\Tasks\Subscribers;
+
+use MailPoet\Models\ScheduledTaskSubscriber;
+use MailPoet\Util\Helpers;
+
+if(!defined('ABSPATH')) exit;
+
+class BatchIterator implements \Iterator, \Countable {
+  private $task_id;
+  private $batch_size;
+  private $offset = 0;
+
+  function __construct($task_id, $batch_size) {
+    if($task_id <= 0) {
+      throw new \Exception('Task ID must be greater than zero');
+    } elseif($batch_size <= 0) {
+      throw new \Exception('Batch size must be greater than zero');
+    }
+    $this->task_id = (int)$task_id;
+    $this->batch_size = (int)$batch_size;
+  }
+
+  function rewind() {
+    $this->offset = 0;
+  }
+
+  function current() {
+    $subscribers = $this->getSubscribers()
+      ->orderByAsc('subscriber_id')
+      ->limit($this->batch_size)
+      ->offset($this->offset)
+      ->findArray();
+    $subscribers = Helpers::arrayColumn($subscribers, 'subscriber_id');
+    return $subscribers;
+  }
+
+  function key() {
+    return $this->offset;
+  }
+
+  function next() {
+    $this->offset += $this->batch_size;
+  }
+
+  function valid() {
+    return $this->offset < $this->count();
+  }
+
+  function count() {
+    return $this->getSubscribers()->count();
+  }
+
+  private function getSubscribers() {
+    return ScheduledTaskSubscriber::select('subscriber_id')
+      ->where('task_id', $this->task_id)
+      ->where('processed', ScheduledTaskSubscriber::STATUS_TO_PROCESS);
+  }
+}