class ConfigCache

CachedFormulaLoader.php in drupal/core/vendor/kriswallsmith/assetic/src/Assetic/Factory/Loader/CachedFormulaLoader.php

ConfigCacheTest.php in drupal/core/vendor/kriswallsmith/assetic/tests/Assetic/Test/Cache/ConfigCacheTest.php

class ConfigCache {
  private $dir;

  /**
   * Construct.
   *
   * @param string $dir The cache directory
   */
  public function __construct($dir) {
    $this->dir = $dir;
  }

  /**
   * Checks of the cache has a file.
   *
   * @param string $resource A cache key
   *
   * @return Boolean True if a file exists
   */
  public function has($resource) {
    return file_exists($this
      ->getSourcePath($resource));
  }

  /**
   * Writes a value to a file.
   *
   * @param string $resource A cache key
   * @param mixed  $value    A value to cache
   */
  public function set($resource, $value) {
    $path = $this
      ->getSourcePath($resource);
    if (!is_dir($dir = dirname($path)) && false === @mkdir($dir, 0777, true)) {

      // @codeCoverageIgnoreStart
      throw new \RuntimeException('Unable to create directory ' . $dir);

      // @codeCoverageIgnoreEnd
    }
    if (false === @file_put_contents($path, sprintf("<?php\n\n// {$resource}\nreturn %s;\n", var_export($value, true)))) {

      // @codeCoverageIgnoreStart
      throw new \RuntimeException('Unable to write file ' . $path);

      // @codeCoverageIgnoreEnd
    }
  }

  /**
   * Loads and returns the value for the supplied cache key.
   *
   * @param string $resource A cache key
   *
   * @return mixed The cached value
   */
  public function get($resource) {
    $path = $this
      ->getSourcePath($resource);
    if (!file_exists($path)) {
      throw new \RuntimeException('There is no cached value for ' . $resource);
    }
    return include $path;
  }

  /**
   * Returns a timestamp for when the cache was created.
   *
   * @param string $resource A cache key
   *
   * @return integer A UNIX timestamp
   */
  public function getTimestamp($resource) {
    $path = $this
      ->getSourcePath($resource);
    if (!file_exists($path)) {
      throw new \RuntimeException('There is no cached value for ' . $resource);
    }
    if (false === ($mtime = @filemtime($path))) {

      // @codeCoverageIgnoreStart
      throw new \RuntimeException('Unable to determine file mtime for ' . $path);

      // @codeCoverageIgnoreEnd
    }
    return $mtime;
  }

  /**
   * Returns the path where the file corresponding to the supplied cache key can be included from.
   *
   * @param string $resource A cache key
   *
   * @return string A file path
   */
  private function getSourcePath($resource) {
    $key = md5($resource);
    return $this->dir . '/' . $key[0] . '/' . $key . '.php';
  }

}