fix: check that the previous xref is not the just processed xref

src/Smalot/PdfParser/RawData/FilterHelper.php

@@ -282,7 +282,7 @@ protected function decodeFilterLZWDecode(string $data): string
         // convert string to binary string
         $bitstring = '';
         for ($i = 0; $i < $data_length; ++$i) {
-            $bitstring .= sprintf('%08b', \ord($data[$i]));
+            $bitstring .= \sprintf('%08b', \ord($data[$i]));
         }
         // get the number of bits
         $data_length = \strlen($bitstring);

src/Smalot/PdfParser/RawData/RawDataParser.php

@@ -214,8 +214,11 @@ protected function decodeXref(string $pdfData, int $startxref, array $xref = [])
                 }
             }
             if (preg_match('/Prev[\s]+([0-9]+)/i', $trailer_data, $matches) > 0) {
-                // get previous xref
-                $xref = $this->getXrefData($pdfData, (int) $matches[1], $xref);
+                $offset = (int) $matches[1];
+                if (0 != $offset) {
+                    // get previous xref
+                    $xref = $this->getXrefData($pdfData, $offset, $xref);
+                }
             }
         } else {
             throw new \Exception('Unable to find trailer');

tests/PHPUnit/Integration/RawData/RawDataParserTest.php

@@ -194,4 +194,24 @@ public function testGetXrefDataIssue673(): void
 
         self::assertStringContainsString('6 rue des Goutais', $text);
     }
+
+    /**
+     * Handle self referencing xref
+     *
+     * It seems that some pdf creators output `Prev 0` when there is no previous
+     * xref
+     *
+     * @see https://github.com/smalot/pdfparser/pull/727
+     */
+    public function testDecodeXrefIssue727(): void
+    {
+        $filename = $this->rootDir.'/samples/bugs/Issue727.pdf';
+
+        // Parsing this document would previously cause an infinite loop
+        $parser = $this->getParserInstance();
+        $document = $parser->parseFile($filename);
+        $text = $document->getText();
+
+        self::assertStringContainsString('', $text);
+    }
 }