SocialPub/PrivaPub/Federation/Objects/InstanceDocuments.cs

using System.Globalization;
using System.Text.Json;
using System.Text.RegularExpressions;

namespace PrivaPub.Federation.Objects
{
	public sealed record NodeInfoFacts(string Version, string Software, string SoftwareVersion, string NodeName, string NodeDescription,
		List<string> Protocols, bool? OpenRegistrations, long? UsersTotal, long? UsersActiveMonth, long? UsersActiveHalfyear, long? LocalPosts,
		long? LocalComments, List<string> Languages);

	public sealed record InstanceApiFacts(string Title, List<string> Languages, string RegistrationMode, int? MaxPostChars, int? ApiVersion, string SourceUrl);

	public static partial class InstanceDocuments
	{
		public static readonly string[] Schemas =
		{
			"http://nodeinfo.diaspora.software/ns/schema/2.2",
			"http://nodeinfo.diaspora.software/ns/schema/2.1",
			"http://nodeinfo.diaspora.software/ns/schema/2.0"
		};

		static readonly HashSet<string> MastodonLike = new(StringComparer.OrdinalIgnoreCase)
		{
			"mastodon", "hometown", "glitch-soc", "gotosocial", "pleroma", "akkoma", "sharkey", "iceshrimp", "iceshrimp.net", "firefish", "catodon",
			"friendica", "pixelfed", "takahe", "snac", "honk", "mitra", "pachli"
		};

		const long MaxCount = 1_000_000_000;

		public static bool SpeaksMastodonApi(string software) => software != default && MastodonLike.Contains(software);

		public static (string Href, string Version) NodeInfoLink(JsonElement links)
		{
			if (links.ValueKind != JsonValueKind.Object || !links.TryGetProperty("links", out var list) || list.ValueKind != JsonValueKind.Array)
				return default;
			foreach (var schema in Schemas)
				foreach (var link in list.EnumerateArray())
					if (link.ValueKind == JsonValueKind.Object && Text(link, "rel") == schema && Text(link, "href") is { } href
						&& href.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
						return (href, schema[(schema.LastIndexOf('/') + 1)..]);
			return default;
		}

		public static NodeInfoFacts NodeInfo(JsonElement root, string version)
		{
			var software = Section(root, "software");
			var usage = Section(root, "usage");
			var users = Section(usage, "users");
			var metadata = Section(root, "metadata");
			// (NodeInfo wants the name in lower case; Mobilizon says "Mobilizon")
			return new NodeInfoFacts(
				version,
				Text(software, "name", 200)?.ToLowerInvariant(),
				Text(software, "version", 200),
				Text(metadata, "nodeName", 200),
				Plain(Text(metadata, "nodeDescription", 2000), 500),
				Strings(root, "protocols", 16, value => value.Length <= 40),
				root.ValueKind == JsonValueKind.Object && root.TryGetProperty("openRegistrations", out var open) && open.ValueKind is JsonValueKind.True or JsonValueKind.False
					? open.GetBoolean()
					: (bool?)null,
				Count(users, "total"),
				Count(users, "activeMonth"),
				Count(users, "activeHalfyear"),
				Count(usage, "localPosts"),
				Count(usage, "localComments"),
				Languages(metadata, "languages") ?? Languages(metadata, "langs"));
		}

		public static InstanceApiFacts InstanceApi(JsonElement root)
		{
			var registrations = root.ValueKind == JsonValueKind.Object && root.TryGetProperty("registrations", out var r) ? r : default;
			string mode;
			if (registrations.ValueKind == JsonValueKind.Object)
				mode = Flag(registrations, "enabled") switch
				{
					true => Flag(registrations, "approval_required") == true ? "approval" : "open",
					false => "closed",
					_ => default
				};
			else if (registrations.ValueKind is JsonValueKind.True or JsonValueKind.False)
				mode = registrations.GetBoolean() ? Flag(root, "approval_required") == true ? "approval" : "open" : "closed";
			else
				mode = default;
			var statuses = Section(Section(root, "configuration"), "statuses");
			var maxChars = Count(statuses, "max_characters") ?? Count(root, "max_toot_chars");
			var apiVersion = Count(Section(root, "api_versions"), "mastodon");
			var source = Text(root, "source_url", 300);
			return new InstanceApiFacts(
				Plain(Text(root, "title", 300), 200),
				Languages(root, "languages"),
				mode,
				maxChars is { } chars ? (int)Math.Min(chars, int.MaxValue) : (int?)null,
				apiVersion is { } api ? (int)Math.Min(api, int.MaxValue) : (int?)null,
				source != default && source.StartsWith("https://", StringComparison.OrdinalIgnoreCase) ? source : default);
		}

		static JsonElement Section(JsonElement parent, string name) =>
			parent.ValueKind == JsonValueKind.Object && parent.TryGetProperty(name, out var value) && value.ValueKind == JsonValueKind.Object ? value : default;

		static string Text(JsonElement parent, string name, int max = 2048)
		{
			if (parent.ValueKind != JsonValueKind.Object || !parent.TryGetProperty(name, out var value) || value.ValueKind != JsonValueKind.String)
				return default;
			var text = value.GetString();
			return text.Length <= max ? text : text[..max];
		}

		static bool? Flag(JsonElement parent, string name) =>
			parent.ValueKind == JsonValueKind.Object && parent.TryGetProperty(name, out var value) && value.ValueKind is JsonValueKind.True or JsonValueKind.False
				? value.GetBoolean()
				: (bool?)null;

		static long? Count(JsonElement parent, string name)
		{
			if (parent.ValueKind != JsonValueKind.Object || !parent.TryGetProperty(name, out var value))
				return default;
			long? number = value.ValueKind switch
			{
				JsonValueKind.Number when value.TryGetInt64(out var whole) => whole,
				JsonValueKind.Number when value.TryGetDouble(out var real) && real is >= 0 and < MaxCount => (long)real,
				JsonValueKind.String when long.TryParse(value.GetString(), NumberStyles.Integer, CultureInfo.InvariantCulture, out var parsed) => parsed,
				_ => (long?)null
			};
			return number is { } n ? Math.Clamp(n, 0, MaxCount) : (long?)null;
		}

		static List<string> Strings(JsonElement parent, string name, int max, Func<string, bool> keep) =>
			parent.ValueKind == JsonValueKind.Object && parent.TryGetProperty(name, out var value) && value.ValueKind == JsonValueKind.Array
				? value.EnumerateArray().Where(v => v.ValueKind == JsonValueKind.String).Select(v => v.GetString()).Where(keep).Distinct().Take(max).ToList()
				: new List<string>();

		static List<string> Languages(JsonElement parent, string name)
		{
			var languages = Strings(parent, name, 16, language => LanguageCode().IsMatch(language.ToLowerInvariant()))
				.Select(language => language.ToLowerInvariant()).Distinct().ToList();
			return languages.Count == 0 ? default : languages;
		}

		static string Plain(string text, int max)
		{
			if (string.IsNullOrWhiteSpace(text))
				return default;
			var plain = Tags().Replace(text, " ").Trim();
			plain = Spaces().Replace(plain, " ");
			return plain.Length <= max ? plain : plain[..max];
		}

		[GeneratedRegex("^[a-z]{2,3}(-[a-z0-9]{2,8})?$")]
		private static partial Regex LanguageCode();

		[GeneratedRegex("<[^>]*>")]
		private static partial Regex Tags();

		[GeneratedRegex("\\s+")]
		private static partial Regex Spaces();
	}
}