using System.Xml.Linq;

namespace GroupVerify;

// Reads a Steam group's public member list (memberslistxml), runs off the game thread.
internal static class SteamGroup
{
    // 1000 members a page, so this allows groups of up to 100,000.
    private const int MaxPages = 100;
    // A pause between pages, to stay clear of Steam's rate limit.
    private static readonly TimeSpan PageDelay = TimeSpan.FromMilliseconds(500);

    private static readonly HttpClient Http = CreateClient();

    private static HttpClient CreateClient()
    {
        var client = new HttpClient { Timeout = TimeSpan.FromSeconds(30) };
        client.DefaultRequestHeaders.UserAgent.ParseAdd("GroupVerify/1.0 (CounterStrikeSharp)");
        return client;
    }

    // Every member's SteamID64, following nextPageLink. Throws if any page fails, so a partial list
    // never replaces a whole one.
    public static async Task<HashSet<ulong>> FetchMembersAsync(ulong groupId64, CancellationToken cancel)
    {
        var members = new HashSet<ulong>();
        Uri? url = new($"https://steamcommunity.com/gid/{groupId64}/memberslistxml/?xml=1");

        for (var page = 1; url != null; page++)
        {
            if (page > MaxPages)
                throw new InvalidDataException($"the member list has more than {MaxPages} pages");
            if (page > 1)
                await Task.Delay(PageDelay, cancel);

            var root = XDocument.Parse(await Http.GetStringAsync(url, cancel)).Root;
            if (root?.Name.LocalName != "memberList")
                throw new InvalidDataException($"page {page} isn't a member list (<{root?.Name.LocalName}>)");

            foreach (var id in root.Element("members")?.Elements("steamID64") ?? [])
            {
                if (ulong.TryParse(id.Value.Trim(), out var steamId))
                    members.Add(steamId);
            }

            url = NextPage(root.Element("nextPageLink")?.Value);
        }
        return members;
    }

    // Only ever follows links back to steamcommunity.com, over https.
    private static Uri? NextPage(string? link)
    {
        if (string.IsNullOrWhiteSpace(link) || !Uri.TryCreate(link.Trim(), UriKind.Absolute, out var uri))
            return null;
        if (uri.Host != "steamcommunity.com" || (uri.Scheme != Uri.UriSchemeHttps && uri.Scheme != Uri.UriSchemeHttp))
            throw new InvalidDataException($"unexpected nextPageLink {uri}");
        return new UriBuilder(uri) { Scheme = Uri.UriSchemeHttps, Port = -1 }.Uri;
    }
}