Flesh out OPDS generation backend to be more robust and to support generic Atom feeds

lib/AtomFeed.php

@@ -0,0 +1,54 @@
+<?
+use function Safe\file_get_contents;
+use function Safe\file_put_contents;
+use function Safe\preg_replace;
+use function Safe\rename;
+use function Safe\tempnam;
+use function Safe\unlink;
+
+class AtomFeed extends Feed{
+	public $Id;
+
+	public function __construct(string $url, string $title, string $path, array $entries){
+		parent::__construct($url, $title, $path, $entries);
+		$this->Id = 'https://standardebooks.org' . $url;
+	}
+
+	private function Sha1Entries(string $xmlString): string{
+		try{
+			$xml = new SimpleXMLElement(str_replace('xmlns=', 'ns=', $xmlString));
+			$xml->registerXPathNamespace('dc', 'http://purl.org/dc/elements/1.1/');
+			$xml->registerXPathNamespace('schema', 'http://schema.org/');
+
+			// Remove any <updated> elements, we don't want to compare against those.
+			foreach($xml->xpath('/feed/updated') ?: [] as $element){
+				unset($element[0]);
+			}
+
+			$output = '';
+			foreach($xml->xpath('/feed/entry') ?: [] as $entry){
+				$output .= $entry->asXml();
+			}
+
+			return sha1(preg_replace('/\s/ius', '', $output));
+		}
+		catch(Exception $ex){
+			// Invalid XML
+			return '';
+		}
+	}
+
+	protected function GetXmlString(): string{
+		if($this->XmlString === null){
+			$feed = Template::AtomFeed(['id' => $this->Id, 'url' => $this->Url, 'title' => $this->Title, 'entries' => $this->Entries]);
+
+			$this->XmlString = $this->CleanXmlString($feed);
+		}
+
+		return $this->XmlString;
+	}
+
+	protected function HasChanged(string $path): bool{
+		return !is_file($path) || ($this->Sha1Entries($this->GetXmlString()) != $this->Sha1Entries(file_get_contents($path)));
+	}
+}