Simplify offsetGet even more

Signed-off-by: Kamil Tekiela <tekiela246@gmail.com>

Author: kamil-tekiela

src/UtfString.php

@@ -116,10 +116,8 @@ public function offsetGet($offset): string|null
 
         if ($delta > 0) {
             // Fast forwarding.
-            while ($delta-- > 0) {
-                $this->byteIdx += strlen(mb_substr(substr($this->str, $this->byteIdx, 4), 0, 1));
-                ++$this->charIdx;
-            }
+            $this->byteIdx += strlen(mb_substr(substr($this->str, $this->byteIdx, 4 * $delta), 0, $delta));
+            $this->charIdx += $delta;
         } elseif ($delta < 0) {
             // Rewinding.
             while ($delta++ < 0) {
@@ -132,7 +130,7 @@ public function offsetGet($offset): string|null
                 --$this->charIdx;
             }
         }
-        
+
         // Fetch the first Unicode character within the next 4 bytes in the string.
         return mb_substr(substr($this->str, $this->byteIdx, 4), 0, 1);
     }

tests/Misc/UtfStringTest.php

@@ -89,7 +89,7 @@ public static function utf8StringsProvider(): array
                 'č',
             ],
             'emoji' => [
-                '😂😄😃😀😊😉😍😘😚😗😂👿😮😨😱😠😡😤😖😆😋👯',
+                '🦋😄😃😀😊😉😍😘😚😗😂👿😮😨😱😠😡😤😖😆😋👯',
                 '😂',
                 '😋',
             ],
@@ -98,6 +98,11 @@ public static function utf8StringsProvider(): array
                 null,
                 null,
             ],
+            'random' => [
+                'xℤⅿↈⅬ⅀ↆℜℝ⅗ℾ℧ⅰℓⅯⅵⅣ⅒21⅞',
+                'ℾ',
+                '⅞',
+            ],
         ];
     }
 }