define(['ModernizrProto', 'cssomPrefixes'], function(ModernizrProto, prefixes) {
  /**
   * atRule returns a given CSS property at-rule (eg @keyframes), possibly in
   * some prefixed form, or false, in the case of an unsupported rule
   *
   * @memberOf Modernizr
   * @name Modernizr.atRule
   * @optionName Modernizr.atRule()
   * @optionProp atRule
   * @access public
   * @function atRule
   * @param {string} prop - String name of the @-rule to test for
   * @returns {string|boolean} The string representing the (possibly prefixed)
   * valid version of the @-rule, or `false` when it is unsupported.
   * @example
   * ```js
   *  var keyframes = Modernizr.atRule('@keyframes');
   *
   *  if (keyframes) {
   *    // keyframes are supported
   *    // could be `@-webkit-keyframes` or `@keyframes`
   *  } else {
   *    // keyframes === `false`
   *  }
   * ```
   */
  var atRule = function(prop) {
    var length = prefixes.length;
    var cssrule = window.CSSRule;
    var rule;

    if (typeof cssrule === 'undefined') {
      return undefined;
    }

    if (!prop) {
      return false;
    }

    // remove literal @ from beginning of provided property
    prop = prop.replace(/^@/, '');

    // CSSRules use underscores instead of dashes
    rule = prop.replace(/-/g, '_').toUpperCase() + '_RULE';

    if (rule in cssrule) {
      return '@' + prop;
    }

    for (var i = 0; i < length; i++) {
      // prefixes gives us something like -o-, and we want O_
      var prefix = prefixes[i];
      var thisRule = prefix.toUpperCase() + '_' + rule;

      if (thisRule in cssrule) {
        return '@-' + prefix.toLowerCase() + '-' + prop;
      }
    }

    return false;
  };

  ModernizrProto.atRule = atRule;

  return atRule;
});
