public function addFont(\PhpOffice\PhpSpreadsheet\Style\Font $font)
    {
        $fontHashCode = $font->getHashCode();
        if (isset($this->addedFonts[$fontHashCode])) {
            $fontIndex = $this->addedFonts[$fontHashCode];
        } else {
            $countFonts = count($this->fontWriters);
            $fontIndex = ($countFonts < 4) ? $countFonts : $countFonts + 1;

            $fontWriter = new Font($font);
            $fontWriter->setColorIndex($this->addColor($font->getColor()->getRGB()));
            $this->fontWriters[] = $fontWriter;

            $this->addedFonts[$fontHashCode] = $fontIndex;
        }

        return $fontIndex;
    }