lib: Add FileExtensionFilterIterator

Add iterator for iterating over files having a specific file extension.

library/Icinga/File/FileExtensionFilterIterator.php

@@ -0,0 +1,71 @@
+<?php
+// {{{ICINGA_LICENSE_HEADER}}}
+// {{{ICINGA_LICENSE_HEADER}}}
+
+namespace Icinga\File;
+
+use FilterIterator;
+use Iterator;
+
+/**
+ * Iterator over files having a specific file extension
+ *
+ * Usage example:
+ * <code>
+ * <?php
+ *
+ * namespace Icinga\Example;
+ *
+ * use RecursiveDirectoryIterator;
+ * use RecursiveIteratorIterator;
+ * use Icinga\File\FileExtensionFilterIterator;
+ *
+ * $markdownFiles = new FileExtensionFilterIterator(
+ *     new RecursiveIteratorIterator(
+ *         new RecursiveDirectoryIterator(__DIR__),
+ *         RecursiveIteratorIterator::SELF_FIRST
+ *     ),
+ *     'md'
+ * );
+ * </code>
+ */
+class FileExtensionFilterIterator extends FilterIterator
+{
+    /**
+     * The extension to filter for
+     *
+     * @type string
+     */
+    protected $extension;
+
+    /**
+     * Create a new FileExtensionFilterIterator
+     *
+     * @param Iterator  $iterator   Apply filter to this iterator
+     * @param string    $extension  The file extension to filter for. The file extension may not contain the leading dot
+     */
+    public function __construct(Iterator $iterator, $extension)
+    {
+        $this->extension = '.' . ltrim(strtolower((string) $extension), '.');
+        parent::__construct($iterator);
+    }
+
+    /**
+     * Accept files which match the file extension to filter for
+     *
+     * @return bool Whether the current element of the iterator is acceptable
+     *              through this filter
+     */
+    public function accept()
+    {
+        $current = $this->current();
+        /* @var $current \SplFileInfo */
+        if (! $current->isFile()) {
+            return false;
+        }
+        // SplFileInfo::getExtension() is only available since PHP 5 >= 5.3.6
+        $filename = $current->getFilename();
+        $sfx = substr($filename, -strlen($this->extension));
+        return $sfx === false ? false : strtolower($sfx) === $this->extension;
+    }
+}