vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/Trend/ExponentialBestFit.php

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<?php

namespace PhpOffice\PhpSpreadsheet\Shared\Trend;

class ExponentialBestFit extends BestFit
{
    /**
     * Algorithm type to use for best-fit
     * (Name of this Trend class).
     *
     * @var string
     */
    protected $bestFitType = 'exponential';

    /**
     * Return the Y-Value for a specified value of X.
     *
     * @param float $xValue X-Value
     *
     * @return float Y-Value
     */
    public function getValueOfYForX($xValue)
    {
        return $this->getIntersect() * $this->getSlope() ** ($xValue - $this->xOffset);
    }

    /**
     * Return the X-Value for a specified value of Y.
     *
     * @param float $yValue Y-Value
     *
     * @return float X-Value
     */
    public function getValueOfXForY($yValue)
    {
        return log(($yValue + $this->yOffset) / $this->getIntersect()) / log($this->getSlope());
    }

    /**
     * Return the Equation of the best-fit line.
     *
     * @param int $dp Number of places of decimal precision to display
     *
     * @return string
     */
    public function getEquation($dp = 0)
    {
        $slope = $this->getSlope($dp);
        $intersect = $this->getIntersect($dp);

        return 'Y = ' . $intersect . ' * ' . $slope . '^X';
    }

    /**
     * Return the Slope of the line.
     *
     * @param int $dp Number of places of decimal precision to display
     *
     * @return float
     */