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function compress(&$chars) {
    $n = count($chars);
    if ($n <= 1) {
        return $n;
    }
    
    $anchor = 0;
    $write = 0;
    
    for ($read = 0; $read <= $n; $read++) {
        // When reaching the end or encountering a different character
        if ($read == $n || $chars[$read] !== $chars[$anchor]) {
            // Write the character
            $chars[$write++] = $chars[$anchor];
            
            // Calculate count and write if greater than 1
            $count = $read - $anchor;
            if ($count > 1) {
                foreach (str_split((string) $count) as $digit) {
                    $chars[$write++] = $digit;
                }
            }
            
            // Update anchor for the next group
            if ($read < $n) {
                $anchor = $read;
            }
        }
    }
    
    return $write;
}