function forecastHoltWinters($data) {
        $alpha = 0.2;
        $beta = 0.1;
        $forecast_length = 1;
        $data_length = count($data);
        $level = $data[0];
        $trend = ($data[1] - $data[0]) / 2;
        for ($i = 2; $i < $data_length; $i++) {
            $last_level = $level;
            $last_trend = $trend;
            $level = $alpha * $data[$i] + (1 - $alpha) * ($last_level + $last_trend);
            $trend = $beta * ($level - $last_level) + (1 - $beta) * $last_trend;
        }
        $last_level = $level;
        $last_trend = $trend;
        for ($i = 0; $i < $forecast_length; $i++) {
            $forecast = $last_level + $last_trend * ($i + 1);
        }
        return ($forecast);
    }