diff --git a/.travis.yml b/.travis.yml
index 8855bac..e8e9767 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,8 +2,9 @@ language: php
 dist: trusty
 
 php:
-  - '7.2'
-  - '7.3'
+  - '7.4'
+  - '8.0'
+  - '8.1'
 
 before_script:
   - travis_retry composer self-update
diff --git a/composer.json b/composer.json
index ae10ee3..fcca784 100644
--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,7 @@
   "minimum-stability": "dev",
   "prefer-stable": true,
   "require": {
-    "php": "7.4 - 8.0",
+    "php": "^7.4 || ^8.0",
     "ext-mbstring": "*"
   },
   "require-dev": {
diff --git a/src/Atn/ATNDeserializer.php b/src/Atn/ATNDeserializer.php
index 1f56f62..bd68745 100644
--- a/src/Atn/ATNDeserializer.php
+++ b/src/Atn/ATNDeserializer.php
@@ -68,9 +68,14 @@ public function __construct(?ATNDeserializationOptions $options = null)
         $this->deserializationOptions = $options ?? ATNDeserializationOptions::defaultOptions();
     }
 
-    public function deserialize(string $data) : ATN
+    /**
+     * @param array<int> $data
+     */
+    public function deserialize(array $data) : ATN
     {
-        $this->reset($data);
+        $this->data = $data;
+        $this->pos = 0;
+
         $this->checkVersion();
         $atn = $this->readATN();
         $this->readStates($atn);
@@ -104,22 +109,6 @@ public function deserialize(string $data) : ATN
         return $atn;
     }
 
-    private function reset(string $data) : void
-    {
-        $characters = \preg_split('//u', $data, -1, \PREG_SPLIT_NO_EMPTY);
-
-        if ($characters === false) {
-            return;
-        }
-
-        for ($i = 0, $length = \count($characters); $i < $length; $i++) {
-            $this->data[] = StringUtils::codePoint($characters[$i]);
-        }
-
-        $this->pos = 0;
-    }
-
-
     private function checkVersion() : void
     {
         $version = $this->readInt();
diff --git a/src/Parser.php b/src/Parser.php
index 94e19c0..cc9393a 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -32,10 +32,8 @@ abstract class Parser extends Recognizer
      * {@see ATN} with bypass alternatives.
      *
      * @see ATNDeserializationOptions::isGenerateRuleBypassTransitions()
-     *
-     * @var array<string, ATN>
      */
-    private static $bypassAltsAtnCache = [];
+    private static ?ATN $bypassAltsAtnCache = null;
 
     /**
      * The error handling strategy for the parser. The default value is a new
@@ -394,17 +392,14 @@ public function setTokenFactory(TokenFactory $factory) : void
      */
     public function getATNWithBypassAlts() : ATN
     {
-        $serializedAtn = $this->getSerializedATN();
-        $result = self::$bypassAltsAtnCache[$serializedAtn] ?? null;
-
-        if ($result === null) {
+        if (self::$bypassAltsAtnCache === null) {
             $deserializationOptions = new ATNDeserializationOptions();
             $deserializationOptions->setGenerateRuleBypassTransitions(true);
-            $result = (new ATNDeserializer($deserializationOptions))->deserialize($serializedAtn);
-            self::$bypassAltsAtnCache[$serializedAtn] = $result;
+            self::$bypassAltsAtnCache = (new ATNDeserializer($deserializationOptions))
+                ->deserialize($this->getSerializedATN());
         }
 
-        return $result;
+        return self::$bypassAltsAtnCache;
     }
 
     public function getErrorHandler() : ANTLRErrorStrategy
diff --git a/src/Recognizer.php b/src/Recognizer.php
index e54a98b..95901e9 100644
--- a/src/Recognizer.php
+++ b/src/Recognizer.php
@@ -113,8 +113,10 @@ public function getTokenType(string $tokenName) : int
      * If this recognizer was generated, it will have a serialized ATN
      * representation of the grammar. For interpreters, we don't know
      * their serialized ATN despite having created the interpreter from it.
+     *
+     * @return array<int>
      */
-    public function getSerializedATN() : string
+    public function getSerializedATN() : array
     {
         throw new \InvalidArgumentException('there is no serialized ATN');
     }