fix: improve Unigram Tokenizer handling of multibyte strings

Author: CodeWithKyrian

Diff for: src/DataStructures/CharTrie.php

@@ -40,9 +40,10 @@ public function extend(array $texts): void
     public function push(string $text): void
     {
         $node = $this->root;
+        $length = mb_strlen($text);
 
-        for ($i = 0, $length = strlen($text); $i < $length; $i++) {
-            $ch = $text[$i];
+        for ($i = 0; $i < $length; $i++) {
+            $ch = mb_substr($text, $i, 1);
             $node = $node->getChild($ch);
         }
 
@@ -59,10 +60,13 @@ public function commonPrefixSearch(string $text): Generator
     {
         $node = $this->root;
         $prefix = "";
-        for ($i = 0; $i < strlen($text) && $node != null; $i++) {
-            $ch = $text[$i];
+        $length = mb_strlen($text);
+        
+        for ($i = 0; $i < $length && $node != null; $i++) {
+            $ch = mb_substr($text, $i, 1);
             $prefix .= $ch;
             $node = $node->getChild($ch);
+
             if ($node?->isLeaf) {
                 yield $prefix;
             }

Diff for: src/DataStructures/TokenLattice.php

@@ -36,7 +36,7 @@ public function __construct(
         public ?int    $bosTokenId,
         public ?int    $eosTokenId)
     {
-        $this->len = strlen($sentence);
+        $this->len = mb_strlen($sentence);
         $this->beginNodes = array_fill(0, $this->len + 1, []);
         $this->endNodes = array_fill(0, $this->len + 1, []);
 
@@ -124,7 +124,7 @@ public function viterbi(): array
      */
     public function piece(TokenLatticeNode $node): string
     {
-        return substr($this->sentence, $node->pos, $node->length);
+        return mb_substr($this->sentence, $node->pos, $node->length);
     }
 
     /**