edlp_d8/core/modules/color/color.module

<?php

/**
 * @file
 * Allows users to change the color scheme of themes.
 */

use Drupal\Component\Utility\Color;
use Drupal\Component\Utility\Unicode;
use Drupal\Core\Asset\CssOptimizer;
use Drupal\Component\Utility\Bytes;
use Drupal\Component\Utility\Environment;
use Drupal\Core\Block\BlockPluginInterface;
use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Language\LanguageInterface;
use Drupal\Core\Render\Element\Textfield;
use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function color_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.color':
      $output = '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Color module allows users with the <em>Administer site configuration</em> permission to change the color scheme (color of links, backgrounds, text, and other theme elements) of compatible themes. For more information, see the <a href=":color_do">online documentation for the Color module</a>.', [':color_do' => 'https://www.drupal.org/documentation/modules/color']) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Changing colors') . '</dt>';
      $output .= '<dd><p>' . t('To change the color settings, select the <em>Settings</em> link for your theme on the <a href=":appearance">Appearance</a> page. If the color picker does not appear then the theme is not compatible with the Color module.', [':appearance' => \Drupal::url('system.themes_page')]) . '</p>';
      $output .= '<p>' . t("The Color module saves a modified copy of the theme's specified stylesheets in the files directory. If you make any manual changes to your theme's stylesheet, <em>you must save your color settings again, even if you haven't changed the colors</em>. This step is required because the module stylesheets in the files directory need to be recreated to reflect your changes.") . '</p></dd>';
      $output .= '</dl>';
      return $output;
  }
}

/**
 * Implements hook_theme().
 */
function color_theme() {
  return [
    'color_scheme_form' => [
      'render element' => 'form',
    ],
  ];
}

/**
 * Implements hook_form_FORM_ID_alter().
 */
function color_form_system_theme_settings_alter(&$form, FormStateInterface $form_state) {
  $build_info = $form_state->getBuildInfo();
  if (isset($build_info['args'][0]) && ($theme = $build_info['args'][0]) && color_get_info($theme) && function_exists('gd_info')) {
    $form['color'] = [
      '#type' => 'details',
      '#title' => t('Color scheme'),
      '#open' => TRUE,
      '#weight' => -1,
      '#attributes' => ['id' => 'color_scheme_form'],
      '#theme' => 'color_scheme_form',
    ];
    $form['color'] += color_scheme_form($form, $form_state, $theme);
    $form['#validate'][] = 'color_scheme_form_validate';
    // Ensure color submission happens first so we can unset extra values.
    array_unshift($form['#submit'], 'color_scheme_form_submit');
  }
}

/**
 * Implements hook_library_info_alter().
 *
 * Replaces style sheets declared in libraries with color-altered style sheets.
 */
function color_library_info_alter(&$libraries, $extension) {
  $themes = array_keys(\Drupal::service('theme_handler')->listInfo());
  if (in_array($extension, $themes)) {
    $color_paths = \Drupal::config('color.theme.' . $extension)->get('stylesheets');
    if (!empty($color_paths)) {
      foreach (array_keys($libraries) as $name) {
        if (isset($libraries[$name]['css'])) {
          // Override stylesheets.
          foreach ($libraries[$name]['css'] as $category => $css_assets) {
            foreach ($css_assets as $path => $metadata) {
              // Loop over the path array with recolored CSS files to find matching
              // paths which could replace the non-recolored paths.
              foreach ($color_paths as $color_path) {
                // Color module currently requires unique file names to be used,
                // which allows us to compare different file paths.
                if (drupal_basename($path) == drupal_basename($color_path)) {
                  // Replace the path to the new css file.
                  // This keeps the order of the stylesheets intact.
                  $index = array_search($path, array_keys($libraries[$name]['css'][$category]));
                  $preceding_css_assets = array_slice($libraries[$name]['css'][$category], 0, $index);
                  $succeeding_css_assets = array_slice($libraries[$name]['css'][$category], $index + 1);
                  $libraries[$name]['css'][$category] = array_merge(
                    $preceding_css_assets,
                    [$color_path => $metadata],
                    $succeeding_css_assets
                  );
                }
              }
            }
          }
        }
      }
    }
  }
}

/**
 * Implements hook_block_view_BASE_BLOCK_ID_alter().
 */
function color_block_view_system_branding_block_alter(array &$build, BlockPluginInterface $block) {
  $build['#pre_render'][] = 'color_block_view_pre_render';
}

/**
 * #pre_render callback: Sets color preset logo.
 */
function color_block_view_pre_render(array $build) {
  $theme_key = \Drupal::theme()->getActiveTheme()->getName();
  $config = \Drupal::config('color.theme.' . $theme_key);
  CacheableMetadata::createFromRenderArray($build)
    ->addCacheableDependency($config)
    ->applyTo($build);

  // Override logo.
  $logo = $config->get('logo');
  if ($logo && $build['content']['site_logo'] && preg_match('!' . $theme_key . '/logo.svg$!', $build['content']['site_logo']['#uri'])) {
    $build['content']['site_logo']['#uri'] = file_url_transform_relative(file_create_url($logo));
  }

  return $build;
}

/**
 * Retrieves the Color module information for a particular theme.
 */
function color_get_info($theme) {
  static $theme_info = [];

  if (isset($theme_info[$theme])) {
    return $theme_info[$theme];
  }

  $path = drupal_get_path('theme', $theme);
  $file = \Drupal::root() . '/' . $path . '/color/color.inc';
  if ($path && file_exists($file)) {
    include $file;
    // Add in default values.
    $info += [
      // CSS files (excluding @import) to rewrite with new color scheme.
      'css' => [],
      // Files to copy.
      'copy' => [],
      // Gradient definitions.
      'gradients' => [],
      // Color areas to fill (x, y, width, height).
      'fill' => [],
      // Coordinates of all the theme slices (x, y, width, height) with their
      // filename as used in the stylesheet.
      'slices' => [],
      // Reference color used for blending.
      'blend_target' => '#ffffff',
    ];
    $theme_info[$theme] = $info;
    return $info;
  }
}

/**
 * Retrieves the color palette for a particular theme.
 */
function color_get_palette($theme, $default = FALSE) {
  // Fetch and expand default palette.
  $info = color_get_info($theme);
  $palette = $info['schemes']['default']['colors'];

  if ($default) {
    return $palette;
  }

  // Load variable.
  // @todo Default color config should be moved to yaml in the theme.
  // Getting a mutable override-free object because this function is only used
  // in forms. Color configuration is used to write CSS to the file system
  // making configuration overrides pointless.
  return \Drupal::configFactory()->getEditable('color.theme.' . $theme)->get('palette') ?: $palette;
}

/**
 * Form constructor for the color configuration form for a particular theme.
 *
 * @param $theme
 *   The machine name of the theme whose color settings are being configured.
 *
 * @see color_scheme_form_validate()
 * @see color_scheme_form_submit()
 */
function color_scheme_form($complete_form, FormStateInterface $form_state, $theme) {
  $info = color_get_info($theme);

  $info['schemes'][''] = ['title' => t('Custom'), 'colors' => []];
  $color_sets = [];
  $schemes = [];
  foreach ($info['schemes'] as $key => $scheme) {
    $color_sets[$key] = $scheme['title'];
    $schemes[$key] = $scheme['colors'];
    $schemes[$key] += $info['schemes']['default']['colors'];
  }

  // See if we're using a predefined scheme.
  // Note: we use the original theme when the default scheme is chosen.
  // Note: we use configuration without overrides since this information is used
  // in a form and therefore without doing this would bleed overrides into
  // active configuration. Furthermore, color configuration is used to write
  // CSS to the file system making configuration overrides pointless.
  $current_scheme = \Drupal::configFactory()->getEditable('color.theme.' . $theme)->get('palette');
  foreach ($schemes as $key => $scheme) {
    if ($current_scheme == $scheme) {
      $scheme_name = $key;
      break;
    }
  }
  if (empty($scheme_name)) {
    if (empty($current_scheme)) {
      $scheme_name = 'default';
    }
    else {
      $scheme_name = '';
    }
  }

  // Add scheme selector.
  $default_palette = color_get_palette($theme, TRUE);
  $form['scheme'] = [
    '#type' => 'select',
    '#title' => t('Color set'),
    '#options' => $color_sets,
    '#default_value' => $scheme_name,
    '#attached' => [
      'library' => [
        'color/drupal.color',
        'color/admin',
      ],
      // Add custom JavaScript.
      'drupalSettings' => [
        'color' => [
          'reference' => $default_palette,
          'schemes' => $schemes,
        ],
        'gradients' => $info['gradients'],
      ],
    ],
  ];

  // Add palette fields. Use the configuration if available.
  $palette = $current_scheme ?: $default_palette;
  $names = $info['fields'];
  $form['palette']['#tree'] = TRUE;
  foreach ($palette as $name => $value) {
    if (isset($names[$name])) {
      $form['palette'][$name] = [
        '#type' => 'textfield',
        '#title' => $names[$name],
        '#value_callback' => 'color_palette_color_value',
        '#default_value' => $value,
        '#size' => 8,
        '#attributes' => ['dir' => LanguageInterface::DIRECTION_LTR],
      ];
    }
  }
  $form['theme'] = ['#type' => 'value', '#value' => $theme];
  if (isset($info['#attached'])) {
    $form['#attached'] = $info['#attached'];
    unset($info['#attached']);
  }
  $form['info'] = ['#type' => 'value', '#value' => $info];

  return $form;
}

/**
 * Prepares variables for color scheme form templates.
 *
 * Default template: color-scheme-form.html.twig.
 *
 * @param array $variables
 *   An associative array containing:
 *   - form: A render element representing the form.
 */
function template_preprocess_color_scheme_form(&$variables) {
  $form = &$variables['form'];

  $theme = $form['theme']['#value'];
  $info = $form['info']['#value'];

  if (isset($info['preview_library'])) {
    $form['scheme']['#attached']['library'][] = $info['preview_library'];
  }

  // Attempt to load preview HTML if the theme provides it.
  $preview_html_path = \Drupal::root() . '/' . (isset($info['preview_html']) ? drupal_get_path('theme', $theme) . '/' . $info['preview_html'] : drupal_get_path('module', 'color') . '/preview.html');
  $variables['html_preview']['#markup'] = file_get_contents($preview_html_path);
}

/**
 * Determines the value for a palette color field.
 *
 * @param array $element
 *   The form element whose value is being populated.
 * @param string|bool $input
 *   The incoming input to populate the form element. If this is FALSE,
 *   the element's default value should be returned.
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 *   The current state of the form.
 *
 * @return string
 *   The data that will appear in the $form_state->getValues() collection for this
 *   element. Return nothing to use the default.
 */
function color_palette_color_value($element, $input, FormStateInterface $form_state) {
  // If we suspect a possible cross-site request forgery attack, only accept
  // hexadecimal CSS color strings from user input, to avoid problems when this
  // value is used in the JavaScript preview.
  if ($input !== FALSE) {
    // Start with the provided value for this textfield, and validate that if
    // necessary, falling back on the default value.
    $value = Textfield::valueCallback($element, $input, $form_state);
    $complete_form = $form_state->getCompleteForm();
    if (!$value || !isset($complete_form['#token']) || color_valid_hexadecimal_string($value) || \Drupal::csrfToken()->validate($form_state->getValue('form_token'), $complete_form['#token'])) {
      return $value;
    }
    else {
      return $element['#default_value'];
    }
  }
}

/**
 * Determines if a hexadecimal CSS color string is valid.
 *
 * @param string $color
 *   The string to check.
 *
 * @return bool
 *   TRUE if the string is a valid hexadecimal CSS color string, or FALSE if it
 *   isn't.
 */
function color_valid_hexadecimal_string($color) {
  return preg_match('/^#([a-f0-9]{3}){1,2}$/iD', $color);
}

/**
 * Form validation handler for color_scheme_form().
 *
 * @see color_scheme_form_submit()
 */
function color_scheme_form_validate($form, FormStateInterface $form_state) {
  // Only accept hexadecimal CSS color strings to avoid XSS upon use.
  foreach ($form_state->getValue('palette') as $key => $color) {
    if (!color_valid_hexadecimal_string($color)) {
      $form_state->setErrorByName('palette][' . $key, t('You must enter a valid hexadecimal color value for %name.', ['%name' => $form['color']['palette'][$key]['#title']]));
    }
  }
}

/**
 * Form submission handler for color_scheme_form().
 *
 * @see color_scheme_form_validate()
 */
function color_scheme_form_submit($form, FormStateInterface $form_state) {

  // Avoid color settings spilling over to theme settings.
  $color_settings = ['theme', 'palette', 'scheme'];
  if ($form_state->hasValue('info')) {
    $color_settings[] = 'info';
  }
  foreach ($color_settings as $setting_name) {
    ${$setting_name} = $form_state->getValue($setting_name);
    $form_state->unsetValue($setting_name);
  }
  if (!isset($info)) {
    return;
  }

  $config = \Drupal::configFactory()->getEditable('color.theme.' . $theme);

  // Resolve palette.
  if ($scheme != '') {
    foreach ($palette as $key => $color) {
      if (isset($info['schemes'][$scheme]['colors'][$key])) {
        $palette[$key] = $info['schemes'][$scheme]['colors'][$key];
      }
    }
    $palette += $info['schemes']['default']['colors'];
  }

  // Make sure enough memory is available.
  if (isset($info['base_image'])) {
    // Fetch source image dimensions.
    $source = drupal_get_path('theme', $theme) . '/' . $info['base_image'];
    list($width, $height) = getimagesize($source);

    // We need at least a copy of the source and a target buffer of the same
    // size (both at 32bpp).
    $required = $width * $height * 8;
    // We intend to prevent color scheme changes if there isn't enough memory
    // available.  memory_get_usage(TRUE) returns a more accurate number than
    // memory_get_usage(), therefore we won't inadvertently reject a color
    // scheme change based on a faulty memory calculation.
    $usage = memory_get_usage(TRUE);
    $memory_limit = ini_get('memory_limit');
    $size = Bytes::toInt($memory_limit);
    if (!Environment::checkMemoryLimit($usage + $required, $memory_limit)) {
      drupal_set_message(t('There is not enough memory available to PHP to change this theme\'s color scheme. You need at least %size more. Check the <a href="http://php.net/manual/ini.core.php#ini.sect.resource-limits">PHP documentation</a> for more information.', ['%size' => format_size($usage + $required - $size)]), 'error');
      return;
    }
  }

  // Delete old files.
  $files = $config->get('files');
  if (isset($files)) {
    foreach ($files as $file) {
      @drupal_unlink($file);
    }
  }
  if (isset($file) && $file = dirname($file)) {
    @drupal_rmdir($file);
  }

  // No change in color config, use the standard theme from color.inc.
  if (implode(',', color_get_palette($theme, TRUE)) == implode(',', $palette)) {
    $config->delete();
    return;
  }

  // Prepare target locations for generated files.
  $id = $theme . '-' . substr(hash('sha256', serialize($palette) . microtime()), 0, 8);
  $paths['color'] = 'public://color';
  $paths['target'] = $paths['color'] . '/' . $id;
  foreach ($paths as $path) {
    file_prepare_directory($path, FILE_CREATE_DIRECTORY);
  }
  $paths['target'] = $paths['target'] . '/';
  $paths['id'] = $id;
  $paths['source'] = drupal_get_path('theme', $theme) . '/';
  $paths['files'] = $paths['map'] = [];

  // Save palette and logo location.
  $config
    ->set('palette', $palette)
    ->set('logo', $paths['target'] . 'logo.svg')
    ->save();

  // Copy over neutral images.
  foreach ($info['copy'] as $file) {
    $base = drupal_basename($file);
    $source = $paths['source'] . $file;
    $filepath = file_unmanaged_copy($source, $paths['target'] . $base);
    $paths['map'][$file] = $base;
    $paths['files'][] = $filepath;
  }

  // Render new images, if image has been provided.
  if (isset($info['base_image'])) {
    _color_render_images($theme, $info, $paths, $palette);
  }

  // Rewrite theme stylesheets.
  $css = [];
  foreach ($info['css'] as $stylesheet) {
    // Build a temporary array with CSS files.
    $files = [];
    if (file_exists($paths['source'] . $stylesheet)) {
      $files[] = $stylesheet;
    }

    foreach ($files as $file) {
      $css_optimizer = new CssOptimizer();
      // Aggregate @imports recursively for each configured top level CSS file
      // without optimization. Aggregation and optimization will be
      // handled by drupal_build_css_cache() only.
      $style = $css_optimizer->loadFile($paths['source'] . $file, FALSE);

      // Return the path to where this CSS file originated from, stripping
      // off the name of the file at the end of the path.
      $css_optimizer->rewriteFileURIBasePath = base_path() . dirname($paths['source'] . $file) . '/';

      // Prefix all paths within this CSS file, ignoring absolute paths.
      $style = preg_replace_callback('/url\([\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\)/i', [$css_optimizer, 'rewriteFileURI'], $style);

      // Rewrite stylesheet with new colors.
      $style = _color_rewrite_stylesheet($theme, $info, $paths, $palette, $style);
      $base_file = drupal_basename($file);
      $css[] = $paths['target'] . $base_file;
      _color_save_stylesheet($paths['target'] . $base_file, $style, $paths);
    }
  }

  // Maintain list of files.
  $config
    ->set('stylesheets', $css)
    ->set('files', $paths['files'])
    ->save();
}

/**
 * Rewrites the stylesheet to match the colors in the palette.
 */
function _color_rewrite_stylesheet($theme, &$info, &$paths, $palette, $style) {
  // Prepare color conversion table.
  $conversion = $palette;
  foreach ($conversion as $k => $v) {
    $v = Unicode::strtolower($v);
    $conversion[$k] = Color::normalizeHexLength($v);
  }
  $default = color_get_palette($theme, TRUE);

  // Split off the "Don't touch" section of the stylesheet.
  $split = "Color Module: Don't touch";
  if (strpos($style, $split) !== FALSE) {
    list($style, $fixed) = explode($split, $style);
  }

  // Find all colors in the stylesheet and the chunks in between.
  $style = preg_split('/(#[0-9a-f]{6}|#[0-9a-f]{3})/i', $style, -1, PREG_SPLIT_DELIM_CAPTURE);
  $is_color = FALSE;
  $output = '';
  $base = 'base';

  // Iterate over all the parts.
  foreach ($style as $chunk) {
    if ($is_color) {
      $chunk = Unicode::strtolower($chunk);
      $chunk = Color::normalizeHexLength($chunk);
      // Check if this is one of the colors in the default palette.
      if ($key = array_search($chunk, $default)) {
        $chunk = $conversion[$key];
      }
      // Not a pre-set color. Extrapolate from the base.
      else {
        $chunk = _color_shift($palette[$base], $default[$base], $chunk, $info['blend_target']);
      }
    }
    else {
      // Determine the most suitable base color for the next color.

      // 'a' declarations. Use link.
      if (preg_match('@[^a-z0-9_-](a)[^a-z0-9_-][^/{]*{[^{]+$@i', $chunk)) {
        $base = 'link';
      }
      // 'color:' styles. Use text.
      elseif (preg_match('/(?<!-)color[^{:]*:[^{#]*$/i', $chunk)) {
        $base = 'text';
      }
      // Reset back to base.
      else {
        $base = 'base';
      }
    }
    $output .= $chunk;
    $is_color = !$is_color;
  }
  // Append fixed colors segment.
  if (isset($fixed)) {
    $output .= $fixed;
  }

  // Replace paths to images.
  foreach ($paths['map'] as $before => $after) {
    $before = base_path() . $paths['source'] . $before;
    $before = preg_replace('`(^|/)(?!../)([^/]+)/../`', '$1', $before);
    $output = str_replace($before, $after, $output);
  }

  return $output;
}

/**
 * Saves the rewritten stylesheet to disk.
 */
function _color_save_stylesheet($file, $style, &$paths) {
  $filepath = file_unmanaged_save_data($style, $file, FILE_EXISTS_REPLACE);
  $paths['files'][] = $filepath;

  // Set standard file permissions for webserver-generated files.
  drupal_chmod($file);
}

/**
 * Renders images that match a given palette.
 */
function _color_render_images($theme, &$info, &$paths, $palette) {
  // Prepare template image.
  $source = $paths['source'] . '/' . $info['base_image'];
  $source = imagecreatefrompng($source);
  $width = imagesx($source);
  $height = imagesy($source);

  // Prepare target buffer.
  $target = imagecreatetruecolor($width, $height);
  imagealphablending($target, TRUE);

  // Fill regions of solid color.
  foreach ($info['fill'] as $color => $fill) {
    imagefilledrectangle($target, $fill[0], $fill[1], $fill[0] + $fill[2], $fill[1] + $fill[3], _color_gd($target, $palette[$color]));
  }

  // Render gradients.
  foreach ($info['gradients'] as $gradient) {
    // Get direction of the gradient.
    if (isset($gradient['direction']) && $gradient['direction'] == 'horizontal') {
      // Horizontal gradient.
      for ($x = 0; $x < $gradient['dimension'][2]; $x++) {
        $color = _color_blend($target, $palette[$gradient['colors'][0]], $palette[$gradient['colors'][1]], $x / ($gradient['dimension'][2] - 1));
        imagefilledrectangle($target, ($gradient['dimension'][0] + $x), $gradient['dimension'][1], ($gradient['dimension'][0] + $x + 1), ($gradient['dimension'][1] + $gradient['dimension'][3]), $color);
      }
    }
    else {
      // Vertical gradient.
      for ($y = 0; $y < $gradient['dimension'][3]; $y++) {
        $color = _color_blend($target, $palette[$gradient['colors'][0]], $palette[$gradient['colors'][1]], $y / ($gradient['dimension'][3] - 1));
        imagefilledrectangle($target, $gradient['dimension'][0], $gradient['dimension'][1] + $y, $gradient['dimension'][0] + $gradient['dimension'][2], $gradient['dimension'][1] + $y + 1, $color);
      }
    }
  }

  // Blend over template.
  imagecopy($target, $source, 0, 0, 0, 0, $width, $height);

  // Clean up template image.
  imagedestroy($source);

  // Cut out slices.
  foreach ($info['slices'] as $file => $coord) {
    list($x, $y, $width, $height) = $coord;
    $base = drupal_basename($file);
    $image = \Drupal::service('file_system')->realpath($paths['target'] . $base);

    // Cut out slice.
    if ($file == 'screenshot.png') {
      $slice = imagecreatetruecolor(150, 90);
      imagecopyresampled($slice, $target, 0, 0, $x, $y, 150, 90, $width, $height);
      \Drupal::configFactory()->getEditable('color.theme.' . $theme)
        ->set('screenshot', $image)
        ->save();
    }
    else {
      $slice = imagecreatetruecolor($width, $height);
      imagecopy($slice, $target, 0, 0, $x, $y, $width, $height);
    }

    // Save image.
    imagepng($slice, $image);
    imagedestroy($slice);
    $paths['files'][] = $image;

    // Set standard file permissions for webserver-generated files
    drupal_chmod($image);

    // Build before/after map of image paths.
    $paths['map'][$file] = $base;
  }

  // Clean up target buffer.
  imagedestroy($target);
}

/**
 * Shifts a given color, using a reference pair and a target blend color.
 *
 * Note: this function is significantly different from the JS version, as it
 * is written to match the blended images perfectly.
 *
 * Constraint: if (ref2 == target + (ref1 - target) * delta) for some fraction
 * delta then (return == target + (given - target) * delta).
 *
 * Loose constraint: Preserve relative positions in saturation and luminance
 * space.
 */
function _color_shift($given, $ref1, $ref2, $target) {
  // We assume that ref2 is a blend of ref1 and target and find
  // delta based on the length of the difference vectors.

  // delta = 1 - |ref2 - ref1| / |white - ref1|
  $target = _color_unpack($target, TRUE);
  $ref1 = _color_unpack($ref1, TRUE);
  $ref2 = _color_unpack($ref2, TRUE);
  $numerator = 0;
  $denominator = 0;
  for ($i = 0; $i < 3; ++$i) {
    $numerator += ($ref2[$i] - $ref1[$i]) * ($ref2[$i] - $ref1[$i]);
    $denominator += ($target[$i] - $ref1[$i]) * ($target[$i] - $ref1[$i]);
  }
  $delta = ($denominator > 0) ? (1 - sqrt($numerator / $denominator)) : 0;

  // Calculate the color that ref2 would be if the assumption was true.
  for ($i = 0; $i < 3; ++$i) {
    $ref3[$i] = $target[$i] + ($ref1[$i] - $target[$i]) * $delta;
  }

  // If the assumption is not true, there is a difference between ref2 and ref3.
  // We measure this in HSL space. Notation: x' = hsl(x).
  $ref2 = _color_rgb2hsl($ref2);
  $ref3 = _color_rgb2hsl($ref3);
  for ($i = 0; $i < 3; ++$i) {
    $shift[$i] = $ref2[$i] - $ref3[$i];
  }

  // Take the given color, and blend it towards the target.
  $given = _color_unpack($given, TRUE);
  for ($i = 0; $i < 3; ++$i) {
    $result[$i] = $target[$i] + ($given[$i] - $target[$i]) * $delta;
  }

  // Finally, we apply the extra shift in HSL space.
  // Note: if ref2 is a pure blend of ref1 and target, then |shift| = 0.
  $result = _color_rgb2hsl($result);
  for ($i = 0; $i < 3; ++$i) {
    $result[$i] = min(1, max(0, $result[$i] + $shift[$i]));
  }
  $result = _color_hsl2rgb($result);

  // Return hex color.
  return _color_pack($result, TRUE);
}

/**
 * Converts a hex triplet into a GD color.
 */
function _color_gd($img, $hex) {
  $c = array_merge([$img], _color_unpack($hex));
  return call_user_func_array('imagecolorallocate', $c);
}

/**
 * Blends two hex colors and returns the GD color.
 */
function _color_blend($img, $hex1, $hex2, $alpha) {
  $in1 = _color_unpack($hex1);
  $in2 = _color_unpack($hex2);
  $out = [$img];
  for ($i = 0; $i < 3; ++$i) {
    $out[] = $in1[$i] + ($in2[$i] - $in1[$i]) * $alpha;
  }

  return call_user_func_array('imagecolorallocate', $out);
}

/**
 * Converts a hex color into an RGB triplet.
 */
function _color_unpack($hex, $normalize = FALSE) {
  if (strlen($hex) == 4) {
    $hex = $hex[1] . $hex[1] . $hex[2] . $hex[2] . $hex[3] . $hex[3];
  }
  $c = hexdec($hex);
  for ($i = 16; $i >= 0; $i -= 8) {
    $out[] = (($c >> $i) & 0xFF) / ($normalize ? 255 : 1);
  }

  return $out;
}

/**
 * Converts an RGB triplet to a hex color.
 */
function _color_pack($rgb, $normalize = FALSE) {
  $out = 0;
  foreach ($rgb as $k => $v) {
    $out |= (($v * ($normalize ? 255 : 1)) << (16 - $k * 8));
  }

  return '#' . str_pad(dechex($out), 6, 0, STR_PAD_LEFT);
}

/**
 * Converts an HSL triplet into RGB.
 */
function _color_hsl2rgb($hsl) {
  $h = $hsl[0];
  $s = $hsl[1];
  $l = $hsl[2];
  $m2 = ($l <= 0.5) ? $l * ($s + 1) : $l + $s - $l * $s;
  $m1 = $l * 2 - $m2;

  return [
    _color_hue2rgb($m1, $m2, $h + 0.33333),
    _color_hue2rgb($m1, $m2, $h),
    _color_hue2rgb($m1, $m2, $h - 0.33333),
  ];
}

/**
 * Helper function for _color_hsl2rgb().
 */
function _color_hue2rgb($m1, $m2, $h) {
  $h = ($h < 0) ? $h + 1 : (($h > 1) ? $h - 1 : $h);
  if ($h * 6 < 1) {
    return $m1 + ($m2 - $m1) * $h * 6;
  }
  if ($h * 2 < 1) {
    return $m2;
  }
  if ($h * 3 < 2) {
    return $m1 + ($m2 - $m1) * (0.66666 - $h) * 6;
  }

  return $m1;
}

/**
 * Converts an RGB triplet to HSL.
 */
function _color_rgb2hsl($rgb) {
  $r = $rgb[0];
  $g = $rgb[1];
  $b = $rgb[2];
  $min = min($r, min($g, $b));
  $max = max($r, max($g, $b));
  $delta = $max - $min;
  $l = ($min + $max) / 2;
  $s = 0;

  if ($l > 0 && $l < 1) {
    $s = $delta / ($l < 0.5 ? (2 * $l) : (2 - 2 * $l));
  }

  $h = 0;
  if ($delta > 0) {
    if ($max == $r && $max != $g) {
      $h += ($g - $b) / $delta;
    }
    if ($max == $g && $max != $b) {
      $h += (2 + ($b - $r) / $delta);
    }
    if ($max == $b && $max != $r) {
      $h += (4 + ($r - $g) / $delta);
    }
    $h /= 6;
  }

  return [$h, $s, $l];
}