<?php
/* --------------------------------------------------------------
   ColorHelper.php 2016-04-01
   Gambio GmbH
   http://www.gambio.de
   Copyright (c) 2016 Gambio GmbH
   Released under the GNU General Public License (Version 2)
   [http://www.gnu.org/licenses/gpl-2.0.html]
   --------------------------------------------------------------
*/

/**
 * Class ColorHelper
 *
 * @category   System
 * @package    Extensions
 * @subpackage Helpers
 */
class ColorHelper
{
    /**
     * Retrieves the percentage of the relative luminance (based on the photometric definition of luminance) of a
     * passed color.
     *
     * @param StringType $color Six-digit hexadecimal color definition (e.g: '002337').
     *
     * @return int Color luminance percentage.
     * @throws UnexpectedValueException On invalid color definition.
     */
    public static function getLuminance(StringType $color)
    {
        // Regex to extract the RGB color chunks of a hexadecimal color value.
        $regex = '/^#?([0-9a-fA-F]{2})([0-9a-fA-F]{2})([0-9a-fA-F]{2})$/';
        
        // Color values chunks (R, G, B) from input argument.
        $chunks = [];
        
        // Extract color values with regex.
        if (!preg_match($regex, $color->asString(), $chunks)) {
            throw new UnexpectedValueException('Expected six-digit hexadecimal color definition');
        }
        
        // Get numeric color values.
        $red   = intval($chunks[1], 16);
        $green = intval($chunks[2], 16);
        $blue  = intval($chunks[3], 16);
        
        // A reference for the formula can be seen here: http://alienryderflex.com/hsp.html
        $lightness = sqrt(pow($red, 2) * .299 + pow($green, 2) * .587 + pow($blue, 2) * .114);
        
        // Return lightness value.
        return $lightness;
    }
}