using System.Security.Cryptography;
using System.Text.Json;

namespace PrivaPub.Federation.Actors
{
	public static class ActorFeatures
	{
		public static List<string> Detect(JsonElement root)
		{
			var found = new List<string>();
			if (root.ValueKind != JsonValueKind.Object)
				return found;
			void When(bool condition, string feature)
			{
				if (condition)
					found.Add(feature);
			}
			bool Has(string name) => root.TryGetProperty(name, out var value) && value.ValueKind is not (JsonValueKind.Null or JsonValueKind.Undefined);
			bool True(string name) => root.TryGetProperty(name, out var value) && value.ValueKind == JsonValueKind.True;
			IEnumerable<JsonElement> Items(string name) => !root.TryGetProperty(name, out var value)
				? Enumerable.Empty<JsonElement>()
				: value.ValueKind == JsonValueKind.Array ? value.EnumerateArray() : new[] { value };
			bool Typed(JsonElement item, string type) =>
				item.ValueKind == JsonValueKind.Object && item.TryGetProperty("type", out var t) && t.ValueKind == JsonValueKind.String && t.GetString() == type;

			When(Has("featured"), "featured");
			When(Has("featuredTags"), "featured-tags");
			When(Items("alsoKnownAs").Any(), "also-known-as");
			When(Has("movedTo"), "moved-to");
			When(root.TryGetProperty("endpoints", out var endpoints) && endpoints.ValueKind == JsonValueKind.Object && endpoints.TryGetProperty("sharedInbox", out _), "shared-inbox");
			When(Has("assertionMethod"), "fep-521a-assertion-method");
			When(Has("proof"), "fep-8b32-proof");
			When(Items("attachment").Any(a => Typed(a, "PropertyValue")), "property-value");
			When(Items("attachment").Any(a => Typed(a, "VerifiableIdentityStatement") || Typed(a, "IdentityProof")), "fep-c390-identity-proof");
			When(Items("tag").Any(t => Typed(t, "Emoji")), "custom-emoji");
			When(root.EnumerateObject().Any(p => p.Name.StartsWith("_misskey", StringComparison.Ordinal)), "misskey");
			When(True("isCat"), "is-cat");
			When(Has("webfinger"), "fep-2c59-webfinger");
			When(Has("attributionDomains"), "attribution-domains");
			When(Has("searchableBy"), "fep-268d-searchable-by");
			When(True("indexable"), "indexable");
			When(root.TryGetProperty("discoverable", out var discoverable) && discoverable.ValueKind == JsonValueKind.False, "undiscoverable");
			When(True("manuallyApprovesFollowers"), "locked");
			When(True("memorial"), "memorial");
			When(Has("generator"), "generator");
			When(root.TryGetProperty("publicKey", out var keys) && keys.ValueKind == JsonValueKind.Array, "public-key-array");
			if (KeyBits(root) is { } bits)
				found.Add($"key:rsa-{bits}");
			return found;
		}

		static int? KeyBits(JsonElement root)
		{
			if (!root.TryGetProperty("publicKey", out var key))
				return default;
			if (key.ValueKind == JsonValueKind.Array)
				key = key.EnumerateArray().FirstOrDefault(k => k.ValueKind == JsonValueKind.Object);
			if (key.ValueKind != JsonValueKind.Object || !key.TryGetProperty("publicKeyPem", out var pem) || pem.ValueKind != JsonValueKind.String)
				return default;
			try
			{
				using var rsa = RSA.Create();
				rsa.ImportFromPem(pem.GetString());
				return rsa.KeySize;
			}
			catch (Exception ex) when (ex is ArgumentException or CryptographicException)
			{
				return default;
			}
		}
	}
}