class ExtensionList

abstract class ExtensionList {
    
    /**
     * The type of the extension: "module", "theme" or "profile".
     *
     * @var string
     */
    protected $type;
    
    /**
     * The app root.
     *
     * @var string
     */
    protected $root;
    
    /**
     * The cache.
     *
     * @var \Drupal\Core\Cache\CacheBackendInterface
     */
    protected $cache;
    
    /**
     * Default values to be merged into *.info.yml file arrays.
     *
     * @var mixed[]
     */
    protected $defaults = [];
    
    /**
     * The info parser.
     *
     * @var \Drupal\Core\Extension\InfoParserInterface
     */
    protected $infoParser;
    
    /**
     * The module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * The cached extensions.
     *
     * @var \Drupal\Core\Extension\Extension[]|null
     */
    protected $extensions;
    
    /**
     * Static caching for extension info.
     *
     * Access this property's value through static::getAllInfo().
     *
     * @var array[]|null
     *   Keys are extension names, and values their info arrays (mixed[]).
     *
     * @see \Drupal\Core\Extension\ExtensionList::getAllAvailableInfo
     */
    protected $extensionInfo;
    
    /**
     * A list of extension folder names keyed by extension name.
     *
     * @var string[]|null
     */
    protected $pathNames;
    
    /**
     * A list of extension folder names directly added in code (not discovered).
     *
     * It is important to keep a separate list to ensure that it takes priority
     * over the discovered extension folders.
     *
     * @var string[]
     *
     * @internal
     */
    protected $addedPathNames = [];
    
    /**
     * The state store.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * The install profile used by the site.
     *
     * @var string
     */
    protected $installProfile;
    
    /**
     * Constructs a new instance.
     *
     * @param string $root
     *   The app root.
     * @param string $type
     *   The extension type.
     * @param \Drupal\Core\Cache\CacheBackendInterface $cache
     *   The cache.
     * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
     *   The info parser.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     * @param \Drupal\Core\State\StateInterface $state
     *   The state.
     * @param string $install_profile
     *   The install profile used by the site.
     */
    public function __construct($root, $type, CacheBackendInterface $cache, InfoParserInterface $info_parser, ModuleHandlerInterface $module_handler, StateInterface $state, $install_profile) {
        $this->root = $root;
        $this->type = $type;
        $this->cache = $cache;
        $this->infoParser = $info_parser;
        $this->moduleHandler = $module_handler;
        $this->state = $state;
        $this->installProfile = $install_profile;
    }
    
    /**
     * Returns the extension discovery.
     *
     * @return \Drupal\Core\Extension\ExtensionDiscovery
     */
    protected function getExtensionDiscovery() {
        return new ExtensionDiscovery($this->root);
    }
    
    /**
     * Resets the stored extension list.
     *
     * We don't reset statically added filenames, as it is a static cache which
     * logically can't change. This is done for performance reasons of the
     * installer.
     *
     * @return $this
     */
    public function reset() {
        $this->extensions = NULL;
        $this->cache
            ->delete($this->getListCacheId());
        $this->extensionInfo = NULL;
        $this->cache
            ->delete($this->getInfoCacheId());
        $this->pathNames = NULL;
        try {
            $this->state
                ->delete($this->getPathnamesCacheId());
        } catch (DatabaseExceptionWrapper $e) {
            // Ignore exceptions caused by a non existing {key_value} table in the
            // early installer.
        }
        $this->cache
            ->delete($this->getPathnamesCacheId());
        // @todo In the long run it would be great to add the reset, but the early
        //   installer fails due to that. https://www.drupal.org/node/2719315 could
        //   help to resolve with that.
        return $this;
    }
    
    /**
     * Returns the extension list cache ID.
     *
     * @return string
     *   The list cache ID.
     */
    protected function getListCacheId() {
        return 'core.extension.list.' . $this->type;
    }
    
    /**
     * Returns the extension info cache ID.
     *
     * @return string
     *   The info cache ID.
     */
    protected function getInfoCacheId() {
        return "system.{$this->type}.info";
    }
    
    /**
     * Returns the extension filenames cache ID.
     *
     * @return string
     *   The filename cache ID.
     */
    protected function getPathnamesCacheId() {
        return "system.{$this->type}.files";
    }
    
    /**
     * Determines if an extension exists in the filesystem.
     *
     * @param string $extension_name
     *   The machine name of the extension.
     *
     * @return bool
     *   TRUE if the extension exists (regardless installed or not) and FALSE if
     *   not.
     */
    public function exists($extension_name) {
        $extensions = $this->getList();
        return isset($extensions[$extension_name]);
    }
    
    /**
     * Returns the human-readable name of the extension.
     *
     * @param string $extension_name
     *   The machine name of the extension.
     *
     * @return string
     *   The human-readable name of the extension.
     *
     * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
     *   If there is no extension with the supplied machine name.
     */
    public function getName($extension_name) {
        return $this->get($extension_name)->info['name'];
    }
    
    /**
     * Returns a single extension.
     *
     * @param string $extension_name
     *   The machine name of the extension.
     *
     * @return \Drupal\Core\Extension\Extension
     *   A processed extension object for the extension with the specified machine
     *   name.
     *
     * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
     *   If there is no extension with the supplied name.
     */
    public function get($extension_name) {
        $extensions = $this->getList();
        if (isset($extensions[$extension_name])) {
            return $extensions[$extension_name];
        }
        throw new UnknownExtensionException("The {$this->type} {$extension_name} does not exist.");
    }
    
    /**
     * Returns all available extensions.
     *
     * @return \Drupal\Core\Extension\Extension[]
     *   Processed extension objects, keyed by machine name.
     */
    public function getList() {
        if ($this->extensions !== NULL) {
            return $this->extensions;
        }
        if ($cache = $this->cache
            ->get($this->getListCacheId())) {
            $this->extensions = $cache->data;
            return $this->extensions;
        }
        $extensions = $this->doList();
        $this->cache
            ->set($this->getListCacheId(), $extensions);
        $this->extensions = $extensions;
        return $this->extensions;
    }
    
    /**
     * Scans the available extensions.
     *
     * Overriding this method gives other code the chance to add additional
     * extensions to this raw listing.
     *
     * @return \Drupal\Core\Extension\Extension[]
     *   Unprocessed extension objects, keyed by machine name.
     */
    protected function doScanExtensions() {
        return $this->getExtensionDiscovery()
            ->scan($this->type);
    }
    
    /**
     * Builds the list of extensions.
     *
     * @return \Drupal\Core\Extension\Extension[]
     *   Processed extension objects, keyed by machine name.
     *
     * @throws \Drupal\Core\Extension\InfoParserException
     *   If one of the .info.yml files is incomplete, or causes a parsing error.
     */
    protected function doList() {
        // Find extensions.
        $extensions = $this->doScanExtensions();
        // Read info files for each extension.
        foreach ($extensions as $extension) {
            $extension->info = $this->createExtensionInfo($extension);
            // Invoke hook_system_info_alter() to give installed modules a chance to
            // modify the data in the .info.yml files if necessary.
            $this->moduleHandler
                ->alter('system_info', $extension->info, $extension, $this->type);
        }
        return $extensions;
    }
    
    /**
     * Returns information about a specified extension.
     *
     * This function returns the contents of the .info.yml file for the specified
     * extension.
     *
     * @param string $extension_name
     *   The name of an extension whose information shall be returned.
     *
     * @return mixed[]
     *   An associative array of extension information.
     *
     * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
     *   If there is no extension with the supplied name.
     */
    public function getExtensionInfo($extension_name) {
        $all_info = $this->getAllInstalledInfo();
        if (isset($all_info[$extension_name])) {
            return $all_info[$extension_name];
        }
        throw new UnknownExtensionException("The {$this->type} {$extension_name} does not exist or is not installed.");
    }
    
    /**
     * Returns an array of info files information of available extensions.
     *
     * This function returns the processed contents (with added defaults) of the
     * .info.yml files.
     *
     * @return array[]
     *   An associative array of extension information arrays, keyed by extension
     *   name.
     */
    public function getAllAvailableInfo() {
        if ($this->extensionInfo === NULL) {
            $cache_id = $this->getInfoCacheId();
            if ($cache = $this->cache
                ->get($cache_id)) {
                $info = $cache->data;
            }
            else {
                $info = $this->recalculateInfo();
                $this->cache
                    ->set($cache_id, $info);
            }
            $this->extensionInfo = $info;
        }
        return $this->extensionInfo;
    }
    
    /**
     * Returns a list of machine names of installed extensions.
     *
     * @return string[]
     *   The machine names of all installed extensions of this type.
     */
    protected abstract function getInstalledExtensionNames();
    
    /**
     * Returns an array of info files information of installed extensions.
     *
     * This function returns the processed contents (with added defaults) of the
     * .info.yml files.
     *
     * @return array[]
     *   An associative array of extension information arrays, keyed by extension
     *   name.
     */
    public function getAllInstalledInfo() {
        return array_intersect_key($this->getAllAvailableInfo(), array_flip($this->getInstalledExtensionNames()));
    }
    
    /**
     * Generates the information from .info.yml files for extensions of this type.
     *
     * @return array[]
     *   An array of arrays of .info.yml entries keyed by the machine name.
     */
    protected function recalculateInfo() {
        return array_map(function (Extension $extension) {
            return $extension->info;
        }, $this->getList());
    }
    
    /**
     * Returns a list of extension file paths keyed by machine name.
     *
     * @return string[]
     */
    public function getPathnames() {
        if ($this->pathNames === NULL) {
            $cache_id = $this->getPathnamesCacheId();
            if ($cache = $this->cache
                ->get($cache_id)) {
                $path_names = $cache->data;
            }
            elseif (!($path_names = $this->state
                ->get($cache_id))) {
                $path_names = $this->recalculatePathnames();
                // Store filenames to allow static::getPathname() to retrieve them
                // without having to rebuild or scan the filesystem.
                $this->state
                    ->set($cache_id, $path_names);
                $this->cache
                    ->set($cache_id, $path_names);
            }
            $this->pathNames = $path_names;
        }
        return $this->pathNames;
    }
    
    /**
     * Generates a sorted list of .info.yml file locations for all extensions.
     *
     * @return string[]
     *   An array of .info.yml file locations keyed by the extension machine name.
     */
    protected function recalculatePathnames() {
        $extensions = $this->getList();
        ksort($extensions);
        return array_map(function (Extension $extension) {
            return $extension->getPathname();
        }, $extensions);
    }
    
    /**
     * Sets the pathname for an extension.
     *
     * This method is used in the Drupal bootstrapping phase, when the extension
     * system is not fully initialized, to manually set locations of modules and
     * profiles needed to complete bootstrapping.
     *
     * It is not recommended to call this method except in those rare cases.
     *
     * @param string $extension_name
     *   The machine name of the extension.
     * @param string $pathname
     *   The pathname of the extension which is to be set explicitly rather
     *   than by consulting the dynamic extension listing.
     *
     * @internal
     *
     * @see ::getPathname
     */
    public function setPathname($extension_name, $pathname) {
        $this->addedPathNames[$extension_name] = $pathname;
    }
    
    /**
     * Gets the info file path for an extension.
     *
     * The info path, whether provided, cached, or retrieved from the database, is
     * only returned if the file exists.
     *
     * This function plays a key role in allowing Drupal's extensions (modules,
     * themes, profiles, theme_engines, etc.) to be located in different places
     * depending on a site's configuration. For example, a module 'foo' may
     * legally be located in any of these four places:
     *
     * - core/modules/foo/foo.info.yml
     * - modules/foo/foo.info.yml
     * - sites/all/modules/foo/foo.info.yml
     * - sites/example.com/modules/foo/foo.info.yml
     *
     * while a theme 'bar' may be located in any of the following four places:
     *
     * - core/themes/bar/bar.info.yml
     * - themes/bar/bar.info.yml
     * - sites/all/themes/bar/bar.info.yml
     * - sites/example.com/themes/bar/bar.info.yml
     *
     * An installation profile maybe be located in any of the following places:
     *
     * - core/profiles/baz/baz.info.yml
     * - profiles/baz/baz.info.yml
     *
     * Calling ExtensionList::getPathname('foo') will give you one of the above,
     * depending on where the extension is located and what type it is.
     *
     * @param string $extension_name
     *   The machine name of the extension for which the pathname is requested.
     *
     * @return string
     *   The drupal-root relative filename and path of the requested extension's
     *   .info.yml file.
     *
     * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
     *   If there is no extension with the supplied machine name.
     */
    public function getPathname($extension_name) {
        if (isset($this->addedPathNames[$extension_name])) {
            return $this->addedPathNames[$extension_name];
        }
        elseif (isset($this->pathNames[$extension_name])) {
            return $this->pathNames[$extension_name];
        }
        elseif (($path_names = $this->getPathnames()) && isset($path_names[$extension_name])) {
            return $path_names[$extension_name];
        }
        throw new UnknownExtensionException("The {$this->type} {$extension_name} does not exist.");
    }
    
    /**
     * Gets the path to an extension of a specific type (module, theme, etc.).
     *
     * The path is the directory in which the .info file is located. This name is
     * coming from \SplFileInfo.
     *
     * @param string $extension_name
     *   The machine name of the extension for which the path is requested.
     *
     * @return string
     *   The Drupal-root-relative path to the specified extension.
     *
     * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
     *   If there is no extension with the supplied name.
     */
    public function getPath($extension_name) {
        return dirname($this->getPathname($extension_name));
    }
    
    /**
     * Creates the info value for an extension object.
     *
     * @param \Drupal\Core\Extension\Extension $extension
     *   The extension whose info is to be altered.
     *
     * @return array
     *   The extension info array.
     */
    protected function createExtensionInfo(Extension $extension) {
        $info = $this->infoParser
            ->parse($extension->getPathname());
        // Add the info file modification time, so it becomes available for
        // contributed extensions to use for ordering extension lists.
        $info['mtime'] = $extension->getMTime();
        // Merge extension type-specific defaults.
        $info += $this->defaults;
        return $info;
    }
    
    /**
     * Tests the compatibility of an extension.
     *
     * @param string $name
     *   The extension name to check.
     *
     * @return bool
     *   TRUE if the extension is incompatible and FALSE if not.
     *
     * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
     *   If there is no extension with the supplied name.
     */
    public function checkIncompatibility($name) {
        $extension = $this->get($name);
        return $extension->info['core_incompatible'] || isset($extension->info['php']) && version_compare(phpversion(), $extension->info['php']) < 0;
    }
    
    /**
     * Array sorting callback; sorts extensions by their name.
     *
     * @param \Drupal\Core\Extension\Extension $a
     *   The first extension to compare.
     * @param \Drupal\Core\Extension\Extension $b
     *   The second extension to compare.
     *
     * @return int
     *   Less than 0 if $a is less than $b, more than 0 if $a is greater than $b,
     *   and 0 if they are equal.
     */
    public static function sortByName(Extension $a, Extension $b) : int {
        return strcasecmp($a->info['name'], $b->info['name']);
    }

}