using MongoDB.Entities;

using OtpNet;

using PrivaPub.Models.User;

using QRCoder;

using System.Security.Cryptography;

namespace PrivaPub.Services
{
	// Two-factor sign-in by TOTP (completion program R3): the authenticator app's six digits, a step of 30 seconds and one
	// step of slack either way, each code taken once (RootTotp.LastStep, moved forward only by a conditional update, so two
	// sign-ins with one code can't both pass); or one of ten recovery codes, each kept as its SHA-256 and pulled when used.
	public static class TwoFactor
	{
		public const string CodeNeeded = "Enter the code from your authenticator app, or a recovery code.";
		public const string CodeInvalid = "That code is not valid.";
		public const string Issuer = "PrivaPub";
		const string RecoveryAlphabet = "abcdefghjkmnpqrstuvwxyz23456789";//no 0/o, 1/l/i

		public static string NewSecret() => Base32Encoding.ToString(KeyGeneration.GenerateRandomKey(20)).TrimEnd('=');

		public static string OtpAuthUri(string secret, string userName) =>
			$"otpauth://totp/{Uri.EscapeDataString($"{Issuer}:{userName}")}?secret={secret}&issuer={Issuer}&algorithm=SHA1&digits=6&period=30";

		public static string QrDataUri(string text)
		{
			using var generator = new QRCodeGenerator();
			using var data = generator.CreateQrCode(text, QRCodeGenerator.ECCLevel.M);
			var svg = new SvgQRCode(data).GetGraphic(6);
			return "data:image/svg+xml;base64," + Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(svg));
		}

		public static List<string> NewRecoveryCodes(int count = 10) =>
			Enumerable.Range(0, count).Select(_ =>
			{
				var chars = new char[10];
				for (var i = 0; i < chars.Length; i++)
					chars[i] = RecoveryAlphabet[RandomNumberGenerator.GetInt32(RecoveryAlphabet.Length)];
				return $"{new string(chars, 0, 5)}-{new string(chars, 5, 5)}";
			}).ToList();

		public static string Normalize(string code) => (code ?? string.Empty).Trim().Replace(" ", "").Replace("-", "").ToLowerInvariant();

		public static string HashRecovery(string code) => RecoveryJob.Hash(Normalize(code));

		/// <summary>The step a six-digit code matches in the secret's window, or null.</summary>
		public static long? Step(string secret, string code)
		{
			var digits = Normalize(code);
			if (digits.Length != 6 || !digits.All(char.IsAsciiDigit) || string.IsNullOrEmpty(secret))
				return null;
			var totp = new Totp(Base32Encoding.ToBytes(secret));
			return totp.VerifyTotp(digits, out var step, VerificationWindow.RfcSpecifiedNetworkDelay) ? step : null;
		}

		/// <summary>Takes a code for a login with two-factor on: a fresh authenticator code, or an unused recovery code.</summary>
		public static async Task<bool> Consume(RootUser user, string code, CancellationToken token)
		{
			if (user?.Totp is not { EnabledAt: not null } totp)
				return false;
			if (Step(totp.Secret, code) is { } step)
			{
				if (step <= totp.LastStep)
					return false;//used already
				var moved = await DB.Default.Update<RootUser>()
					.Match(u => u.ID == user.ID && u.Totp.LastStep < step)
					.Modify(u => u.Totp.LastStep, step)
					.ExecuteAsync(token);
				return moved.ModifiedCount == 1;
			}
			var hash = HashRecovery(code);
			if (!totp.RecoveryCodeHashes.Contains(hash))
				return false;
			var pulled = await DB.Default.Update<RootUser>()
				.Match(u => u.ID == user.ID && u.Totp.RecoveryCodeHashes.Contains(hash))
				.Modify(b => b.Pull(u => u.Totp.RecoveryCodeHashes, hash))
				.ExecuteAsync(token);
			return pulled.ModifiedCount == 1;
		}
	}
}