[CLEANUP] Don't store array length as a property

Since PHP 5, if not earlier, the length of an array is internally stored by PHP
and can be accessed in O(1) time.
Maintaining both the array and its length in class properties is error-prone
(particularly regarding possible future code changes).
When used in a loop, `\count($array)` is more repetitively expensive than
`$arrayLength`,
but the latter can be set up as a local variable as and when needed.

Reference:
https://stackoverflow.com/questions/5835241/is-phps-count-function-o1-or-on-for-arrays

This change also ensures that the characters are always a (dazzling) array.
And adds a type parameter to `ParserState::setCharset()`,
which was unavoidably required.

Resolves #953.

Author: JakeQZ

config/phpstan-baseline.neon

@@ -174,12 +174,6 @@ parameters:
 			count: 2
 			path: ../src/Parsing/ParserState.php
 
-		-
-			message: '#^Parameters should have "string" types as the only types passed to this method$#'
-			identifier: typePerfect.narrowPublicClassMethodParamType
-			count: 1
-			path: ../src/Parsing/ParserState.php
-
 		-
 			message: '#^Parameters should have "string\|int\|null" types as the only types passed to this method$#'
 			identifier: typePerfect.narrowPublicClassMethodParamType

src/Parsing/ParserState.php

@@ -30,7 +30,7 @@ class ParserState
     /**
      * @var array<int, string>
      */
-    private $characters;
+    private $characters = [];
 
     /**
      * @var int
@@ -44,11 +44,6 @@ class ParserState
      */
     private $charset;
 
-    /**
-     * @var int
-     */
-    private $length;
-
     /**
      * @var int
      */
@@ -69,15 +64,21 @@ public function __construct($text, Settings $parserSettings, $lineNumber = 1)
     /**
      * Sets the charset to be used if the CSS does not contain an `@charset` declaration.
      *
-     * @param string $charset
+     * @throws SourceException
      */
-    public function setCharset($charset): void
+    public function setCharset(string $charset): void
     {
+        // The charset property must be set for `strsplit()`, but may need to be reverted if it fails.
+        $oldCharset = $this->charset;
         $this->charset = $charset;
-        $this->characters = $this->strsplit($this->text);
-        if (\is_array($this->characters)) {
-            $this->length = \count($this->characters);
+
+        $newCharacters = $this->strsplit($this->text);
+        if (!\is_array($newCharacters)) {
+            $this->charset = $oldCharset;
+            throw new SourceException('Charset `' . $charset . '` cannot be reconciled with the CSS provided.');
         }
+
+        $this->characters = $newCharacters;
     }
 
     /**
@@ -227,7 +228,7 @@ public function consumeWhiteSpace(): array
                 try {
                     $oComment = $this->consumeComment();
                 } catch (UnexpectedEOFException $e) {
-                    $this->currentPosition = $this->length;
+                    $this->currentPosition = \count($this->characters);
                     return $comments;
                 }
             } else {
@@ -259,7 +260,7 @@ public function comes($sString, $bCaseInsensitive = false): bool
     public function peek($length = 1, $iOffset = 0): string
     {
         $iOffset += $this->currentPosition;
-        if ($iOffset >= $this->length) {
+        if ($iOffset >= \count($this->characters)) {
             return '';
         }
         return $this->substr($iOffset, $length);
@@ -288,7 +289,7 @@ public function consume($mValue = 1): string
             $this->currentPosition += $this->strlen($mValue);
             return $mValue;
         } else {
-            if ($this->currentPosition + $mValue > $this->length) {
+            if ($this->currentPosition + $mValue > \count($this->characters)) {
                 throw new UnexpectedEOFException((string) $mValue, $this->peek(5), 'count', $this->lineNumber);
             }
             $result = $this->substr($this->currentPosition, $mValue);
@@ -345,7 +346,7 @@ public function consumeComment()
 
     public function isEnd(): bool
     {
-        return $this->currentPosition >= $this->length;
+        return $this->currentPosition >= \count($this->characters);
     }
 
     /**
@@ -438,10 +439,10 @@ public function strlen($sString): int
     private function substr($iStart, $length): string
     {
         if ($length < 0) {
-            $length = $this->length - $iStart + $length;
+            $length = \count($this->characters) - $iStart + $length;
         }
-        if ($iStart + $length > $this->length) {
-            $length = $this->length - $iStart;
+        if ($iStart + $length > \count($this->characters)) {
+            $length = \count($this->characters) - $iStart;
         }
         $result = '';
         while ($length > 0) {