// Data structures associated with RSS.

using System.Xml;
using System.Xml.Linq;


class Feed
{
	public readonly string? Error;
	public readonly Channel? Channel;
	public readonly DateTime DateRetrieved;

	private Feed(string? error, Channel? channel)
	{
		Error = error;
		Channel = channel;
		DateRetrieved = DateTime.Now;
	}

	public Feed(string content)
	{
		try
		{
			var xml = XDocument.Parse(content.TrimStart());  // NOTE: loper-os site returns RSS with beginning whitespace, which the XDocument parser does not tolerate.

			var xmlChannel = xml.Get("rss").Get("channel");
			var xmlItems = xmlChannel?.Elements("item") ?? Enumerable.Empty<XElement>();

			var channel = new Channel();
			var items = xmlItems.Select(item => 
			{
				var pubDate = item.Get("pubDate")?.Value;
				return new Item(
					Channel: channel,
					Title: item.Get("title")?.Value?.Trim(),
					Description: item.Get("description")?.Value?.Trim(),
					Author: item.Get("author")?.Value?.Trim(),
					Link: item.Get("link")?.Value?.Trim(),
					PubDate: pubDate != null ? DateTime.Parse(pubDate) : null,
					Guid: item.Get("guid")?.Value?.Trim()
				);
			}).Where(item => !item.Empty()).ToArray();

			channel.Title = xmlChannel.Get("title")?.Value?.Trim();
			channel.Link = xmlChannel.Get("link")?.Value?.Trim();
			channel.Description = xmlChannel.Get("description")?.Value?.Trim();
			channel.Items = items;
			
			this.DateRetrieved = DateTime.Now;
			this.Error = null;
			this.Channel = channel;
		}
		catch (XmlException e)
		{
			this.Error = e.ToString();
			this.Channel = null;
		}
	}

	// when it fails.
	public static Feed Failed(string error) => new(error, null);

	public override string ToString()
	{
		if (Channel == null)
		{
			return $"FEED ERROR: {Error}";
		}

		var description = Channel.Description != null ? "(" + Channel.Description + ")" : "";
		var items = string.Concat(Channel.Items.Select(item => $"\t{item.Title ?? item.Description ?? "<uh oh>"}\n"));
		return $"Feed {Channel.Title ?? "<no chan title>"} {description}:\n{items}";
	}

	public Feed Compose(Feed feed)
	{
		// creates a new feed from two feeds.
		// literally just copies entries from the feed from the argument

		// if any of them have an error, return the other one!
		if (feed.Error != null) return this;
		if (this.Error != null) return feed;

		var newChannel = this.Channel.Compose(feed.Channel);
		var newFeed = new Feed(null, newChannel);

		return newFeed;
	}
}

static class FindElement
{
	public static XElement? Get(this XContainer? elem, string name) => elem?.Elements(name)?.SingleOrDefault();
}

// not a record so we can recursively define Channel in Item.
public class Channel
{
	public string? Title;
	public string? Link;
	public string? Description;
	public Item[] Items;

	public Channel Compose(Channel other)
	{
		var newChannel = (Channel)this.MemberwiseClone();
		newChannel.Items = Items.Concat(other.Items).Select(item => item with { Channel = newChannel }).ToArray();
		return newChannel;
	}
}

public record Item(Channel Channel, string? Title, string? Description, string? Author, string? Link, DateTime? PubDate, string? Guid)
{
	public bool Empty() => Title == null && Description == null && Author == null && Link == null && PubDate == null && Guid == null;
}