namespace Yes.Infrastructure.Rss
{
    public class RssGenerator
    {    

        private readonly XNamespace _contentNs = XNamespace.Get("http://purl.org/rss/1.0/modules/content/");
        private readonly XNamespace _dcNs = XNamespace.Get("http://purl.org/dc/elements/1.1/");
        private readonly XNamespace _slashNs = XNamespace.Get("http://purl.org/rss/1.0/modules/slash/");
        private readonly XNamespace _wfwNs = XNamespace.Get("http://wellformedweb.org/CommentAPI/");

        public string GenerateRss(Channel channel, IEnumerable<Item> items)
        {
            var doc = new XDocument(
                new XDeclaration("1.0", "UTF-8", null),
                new XElement("rss",
                    new XAttribute("version", "2.0"),
                    GetNamespaceAttributes(),
                    new XElement("channel",
                        RequiredChannelElements(channel),
                        GenerateItems(items)
                    )
                )
            );

            return doc.ToStringWithDeclaration();
        }


        private IEnumerable<XAttribute> GetNamespaceAttributes()
        {
            yield return new XAttribute(XNamespace.Xmlns + "content", "http://purl.org/rss/1.0/modules/content/");
            yield return new XAttribute(XNamespace.Xmlns + "dc", "http://purl.org/dc/elements/1.1/");
            yield return new XAttribute(XNamespace.Xmlns + "atom", "http://www.w3.org/2005/Atom");
            yield return new XAttribute(XNamespace.Xmlns + "wfw", "http://wellformedweb.org/CommentAPI/");
            yield return new XAttribute(XNamespace.Xmlns + "slash", "http://purl.org/rss/1.0/modules/slash/");
        }

        private IEnumerable<XElement> RequiredChannelElements(Channel channel)
        {
            yield return new XElement("title", channel.Title);
            yield return new XElement("link", channel.Link);
            yield return new XElement("description", channel.Description);
            yield return new XElement("language", channel.Language);
            yield return new XElement("lastBuildDate", FormatRfc1123(channel.LastBuildDate));
            yield return new XElement("pubDate", FormatRfc1123(channel.PubDate));
            yield return new XElement(XNamespace.Get("http://www.w3.org/2005/Atom") + "link",
                new XAttribute("href", channel.FeedUrl),
                new XAttribute("rel", "self"),
                new XAttribute("type", "application/rss+xml"));
        }

        private static string FormatRfc1123(DateTimeOffset date)
        {
            return date.ToString("r", System.Globalization.CultureInfo.InvariantCulture);
        }

        private IEnumerable<XElement> GenerateItems(IEnumerable<Item> items)
        {
            foreach (var item in items)
            {
                yield return new XElement("item",
                    new XElement("title", item.Title),
                    new XElement("link", item.Url),
                    new XElement("guid", new XAttribute("isPermaLink", "true"), item.Url),
                    new XElement("pubDate", item.PublishDate.ToString("ddd, dd MMM yyyy HH:mm:ss zzz")),
                    new XElement(_dcNs + "creator", item.Creator),
                    new XElement("description",
                        new XCData(item.Description)),
                    new XElement(_contentNs + "encoded",
                        new XAttribute(XNamespace.Xml + "lang", "zh-CN"),
                        new XCData(item.Content)),
                    new XElement(_slashNs + "comments", item.CommentsCount),
                    new XElement("comments", item.CommentsUrl),
                    new XElement(_wfwNs + "commentRss", item.CommentRssUrl)
                );
            }
        }
    }
}