using System.Net;
using System.Net.Mail;
using System.Security.Cryptography;
using System.Text;
using Lagerbestand.Models;
using QRCoder;

namespace Lagerbestand.Services;

public interface ITwoFactorService
{
    /// <summary>
    /// Generates a new random Base32-encoded TOTP secret key.
    /// </summary>
    string GenerateSecretKey();

    /// <summary>
    /// Generates the otpauth:// URI for authenticator app setup.
    /// </summary>
    string GenerateOtpAuthUri(string secret, string username, string issuer);

    /// <summary>
    /// Validates a 6-digit TOTP code against a secret (±1 time step tolerance).
    /// </summary>
    bool ValidateTotpCode(string secret, string code);

    /// <summary>
    /// Generates a random 6-digit email verification code.
    /// </summary>
    string GenerateEmailCode();

    /// <summary>
    /// Sends a 2FA code via email using configured SMTP settings.
    /// </summary>
    Task<bool> SendEmailCodeAsync(string toEmail, string code, string username);

    /// <summary>
    /// Generates a QR code as a base64 PNG data URL from an otpauth URI.
    /// </summary>
    string GenerateQrCodeDataUrl(string otpAuthUri);
}

public class TwoFactorService : ITwoFactorService
{
    private const int SecretKeyLength = 20; // 160 bits
    private const int TotpCodeLength = 6;
    private const int TotpTimeStepSeconds = 30;
    private const int TotpTimeTolerance = 1; // ±1 step (30 seconds each side)

    private static readonly char[] Base32Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567".ToCharArray();

    private readonly ISiteSettingsService _siteSettingsService;
    private readonly ILogger<TwoFactorService> _logger;

    public TwoFactorService(ISiteSettingsService siteSettingsService, ILogger<TwoFactorService> logger)
    {
        _siteSettingsService = siteSettingsService;
        _logger = logger;
    }

    public string GenerateSecretKey()
    {
        var secretBytes = RandomNumberGenerator.GetBytes(SecretKeyLength);
        return Base32Encode(secretBytes);
    }

    public string GenerateOtpAuthUri(string secret, string username, string issuer)
    {
        var encodedIssuer = Uri.EscapeDataString(issuer);
        var encodedUsername = Uri.EscapeDataString(username);
        return $"otpauth://totp/{encodedIssuer}:{encodedUsername}?secret={secret}&issuer={encodedIssuer}&digits={TotpCodeLength}&period={TotpTimeStepSeconds}";
    }

    public bool ValidateTotpCode(string secret, string code)
    {
        if (string.IsNullOrWhiteSpace(code) || code.Length != TotpCodeLength)
            return false;

        var secretBytes = Base32Decode(secret);
        var unixTimestamp = DateTimeOffset.UtcNow.ToUnixTimeSeconds();

        // Check current time step and ±tolerance steps
        for (int i = -TotpTimeTolerance; i <= TotpTimeTolerance; i++)
        {
            var timeStep = (unixTimestamp / TotpTimeStepSeconds) + i;
            var generatedCode = ComputeTotp(secretBytes, timeStep);
            if (CryptographicOperations.FixedTimeEquals(
                    Encoding.UTF8.GetBytes(generatedCode),
                    Encoding.UTF8.GetBytes(code)))
            {
                return true;
            }
        }

        return false;
    }

    public string GenerateEmailCode()
    {
        return RandomNumberGenerator.GetInt32(0, 1_000_000).ToString("D6");
    }

    public async Task<bool> SendEmailCodeAsync(string toEmail, string code, string username)
    {
        try
        {
            var smtpSettings = await _siteSettingsService.GetSmtpSettingsAsync();
            if (!smtpSettings.IsEnabled || !smtpSettings.IsConfigured)
            {
                _logger.LogWarning("SMTP is not configured. Cannot send 2FA email to {Email}", toEmail);
                return false;
            }

            using var client = new SmtpClient(smtpSettings.Host, smtpSettings.Port)
            {
                Credentials = new NetworkCredential(smtpSettings.Username, smtpSettings.Password),
                EnableSsl = smtpSettings.UseSsl,
                Timeout = 15_000
            };

            using var message = new MailMessage
            {
                From = new MailAddress(smtpSettings.FromEmail, smtpSettings.FromName),
                Subject = $"Ihr Anmeldecode – {code}",
                IsBodyHtml = true,
                Body = $"""
                    <div style="font-family: Arial, sans-serif; max-width: 480px; margin: 0 auto; padding: 24px;">
                        <h2 style="color: #dc2626;">Reifen Späth – Anmeldecode</h2>
                        <p>Hallo <strong>{username}</strong>,</p>
                        <p>Ihr Einmal-Code für die Anmeldung lautet:</p>
                        <div style="background: #f3f4f6; border-radius: 8px; padding: 16px; text-align: center; margin: 16px 0;">
                            <span style="font-size: 32px; font-weight: bold; letter-spacing: 6px; color: #111;">{code}</span>
                        </div>
                        <p style="color: #666; font-size: 14px;">Dieser Code ist 5 Minuten gültig. Falls Sie diese Anmeldung nicht angefordert haben, ignorieren Sie diese E-Mail.</p>
                    </div>
                    """
            };
            message.To.Add(toEmail);

            await client.SendMailAsync(message);
            _logger.LogInformation("2FA email code sent to {Email} for user {Username}", toEmail, username);
            return true;
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Failed to send 2FA email to {Email}", toEmail);
            return false;
        }
    }

    public string GenerateQrCodeDataUrl(string otpAuthUri)
    {
        using var qrGenerator = new QRCodeGenerator();
        using var qrCodeData = qrGenerator.CreateQrCode(otpAuthUri, QRCodeGenerator.ECCLevel.M);
        using var qrCode = new PngByteQRCode(qrCodeData);
        var pngBytes = qrCode.GetGraphic(8, new byte[] { 30, 30, 32 }, new byte[] { 255, 255, 255 });
        return $"data:image/png;base64,{Convert.ToBase64String(pngBytes)}";
    }

    #region TOTP Internals

    private static string ComputeTotp(byte[] secret, long timeStep)
    {
        var counterBytes = BitConverter.GetBytes(timeStep);
        if (BitConverter.IsLittleEndian)
            Array.Reverse(counterBytes);

        var hash = HMACSHA1.HashData(secret, counterBytes);

        var offset = hash[^1] & 0x0F;
        var code = ((hash[offset] & 0x7F) << 24) |
                   ((hash[offset + 1] & 0xFF) << 16) |
                   ((hash[offset + 2] & 0xFF) << 8) |
                   (hash[offset + 3] & 0xFF);

        return (code % 1_000_000).ToString("D6");
    }

    #endregion

    #region Base32

    private static string Base32Encode(byte[] data)
    {
        var sb = new StringBuilder((data.Length * 8 + 4) / 5);
        int buffer = data[0];
        int next = 1;
        int bitsLeft = 8;

        while (bitsLeft > 0 || next < data.Length)
        {
            if (bitsLeft < 5)
            {
                if (next < data.Length)
                {
                    buffer <<= 8;
                    buffer |= data[next++] & 0xFF;
                    bitsLeft += 8;
                }
                else
                {
                    int pad = 5 - bitsLeft;
                    buffer <<= pad;
                    bitsLeft += pad;
                }
            }
            int index = 0x1F & (buffer >> (bitsLeft - 5));
            bitsLeft -= 5;
            sb.Append(Base32Chars[index]);
        }

        return sb.ToString();
    }

    private static byte[] Base32Decode(string base32)
    {
        var cleanInput = base32.TrimEnd('=').ToUpperInvariant();
        var output = new byte[cleanInput.Length * 5 / 8];
        int buffer = 0;
        int bitsLeft = 0;
        int outputIndex = 0;

        foreach (var c in cleanInput)
        {
            var val = Array.IndexOf(Base32Chars, c);
            if (val < 0) continue;

            buffer <<= 5;
            buffer |= val & 0x1F;
            bitsLeft += 5;

            if (bitsLeft >= 8)
            {
                output[outputIndex++] = (byte)(buffer >> (bitsLeft - 8));
                bitsLeft -= 8;
            }
        }

        return output;
    }

    #endregion
}