diff --git a/lib/avro/datum.php b/lib/avro/datum.php
index b328853..334a757 100644
--- a/lib/avro/datum.php
+++ b/lib/avro/datum.php
@@ -76,19 +76,24 @@ public static function encode_long($n): string
   {
     $n = (int) $n;
     $n = ($n << 1) ^ ($n >> 63);
-    $str = '';
+
+    if ($n >= 0 && $n < 0x80) {
+      return chr($n);
+    }
+
+    $buf = [];
     if (($n & ~0x7F) != 0) {
-      $str .= chr(($n | 0x80) & 0xFF);
-      $n = self::unsigned_right_shift($n, 7);
+      $buf []= ($n | 0x80) & 0xFF;
+      $n = ($n >> 7) ^ (($n >> 63) << 57); // unsigned shift right ($n >>> 7)
 
       while ($n > 0x7F) {
-        $str .= chr(($n | 0x80) & 0xFF);
-        $n = self::unsigned_right_shift($n, 7);
+        $buf []= ($n | 0x80) & 0xFF;
+        $n >>= 7; // $n is always positive here
       }
     }
 
-    $str .= chr($n);
-    return $str;
+    $buf []= $n;
+    return pack("C*", ...$buf);
   }
 
   public static function decode_long(array $bytes): int {
diff --git a/test/DatumIOTest.php b/test/DatumIOTest.php
index a2086c1..dc9bd06 100644
--- a/test/DatumIOTest.php
+++ b/test/DatumIOTest.php
@@ -100,7 +100,13 @@ function data_provider()
 
                  array('"long"', (int) -9223372036854775808, "\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\x01"),
                  array('"long"', -(1<<62), "\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\x7F"),
+                 array('"long"', -(1<<61), "\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\x3F"),
                  array('"long"', -4294967295, "\xFD\xFF\xFF\xFF\x1F"),
+                 array('"long"', -1<<24, "\xFF\xFF\xFF\x0F"),
+                 array('"long"', -1<<16, "\xFF\xFF\x07"),
+                 array('"long"', -255, "\xFD\x03"),
+                 array('"long"', -128, "\xFF\x01"),
+                 array('"long"', -127, "\xFD\x01"),
                  array('"long"', -10, "\x13"),
                  array('"long"', -3, "\005"),
                  array('"long"', -2, "\003"),
@@ -110,7 +116,13 @@ function data_provider()
                  array('"long"',  2, "\004"),
                  array('"long"',  3, "\006"),
                  array('"long"', 10, "\x14"),
+                 array('"long"', 127, "\xFE\x01"),
+                 array('"long"', 128, "\x80\x02"),
+                 array('"long"', 255, "\xFE\x03"),
+                 array('"long"', 1<<16, "\x80\x80\x08"),
+                 array('"long"', 1<<24, "\x80\x80\x80\x10"),
                  array('"long"', 4294967295, "\xFE\xFF\xFF\xFF\x1F"),
+                 array('"long"', 1<<61, "\x80\x80\x80\x80\x80\x80\x80\x80\x40"),
                  array('"long"', 1<<62, "\x80\x80\x80\x80\x80\x80\x80\x80\x80\x01"),
                  array('"long"', 9223372036854775807, "\xFE\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\x01"),