// simple version of $X with XHTML
// $X(exp);
// $X(exp, context);
function $X (exp, context) {
	context || (context = document);
	var _document  = context.ownerDocument || document,
	documentElement = _document.documentElement;
	var isXHTML = documentElement.tagName !== 'HTML' && _document.createElement('p').tagName === 'p';
	var defaultPrefix = null;
	if (isXHTML) {
		defaultPrefix = '__default__';
		exp = addDefaultPrefix(exp, defaultPrefix);
	}
	function resolver (prefix) {
		return context.lookupNamespaceURI(prefix === defaultPrefix ? null : prefix) ||
			   documentElement.namespaceURI || "";
	}

	var result = _document.evaluate(exp, context, resolver, XPathResult.ANY_TYPE, null);
		switch (result.resultType) {
			case XPathResult.STRING_TYPE : return result.stringValue;
			case XPathResult.NUMBER_TYPE : return result.numberValue;
			case XPathResult.BOOLEAN_TYPE: return result.booleanValue;
			case XPathResult.UNORDERED_NODE_ITERATOR_TYPE:
				// not ensure the order.
				var ret = [], i = null;
				while (i = result.iterateNext()) ret.push(i);
				return ret;
		}
	return null;
}

// XPath 式中の接頭辞のない名前テストに接頭辞 prefix を追加する
// e.g. '//body[@class = "foo"]/p' -> '//prefix:body[@class = "foo"]/prefix:p'
// http://nanto.asablo.jp/blog/2008/12/11/4003371
function addDefaultPrefix(xpath, prefix) {
	var tokenPattern = /([A-Za-z_\u00c0-\ufffd][\w\-.\u00b7-\ufffd]*|\*)\s*(::?|\()?|(".*?"|'.*?'|\d+(?:\.\d*)?|\.(?:\.|\d+)?|[\)\]])|(\/\/?|!=|[<>]=?|[\(\[|,=+-])|([@$])/g;
	var TERM = 1, OPERATOR = 2, MODIFIER = 3;
	var tokenType = OPERATOR;
	prefix += ':';
	function replacer(token, identifier, suffix, term, operator, modifier) {
		if (suffix) {
			tokenType = 
				(suffix == ':' || (suffix == '::' && (identifier == 'attribute' || identifier == 'namespace')))
				? MODIFIER : OPERATOR;
		} else if (identifier) {
			if (tokenType == OPERATOR && identifier != '*')
				token = prefix + token;
			tokenType = (tokenType == TERM) ? OPERATOR : TERM;
		} else {
			tokenType = term ? TERM : operator ? OPERATOR : MODIFIER;
		}
		return token;
	}
	return xpath.replace(tokenPattern, replacer);
}