using System.Security.Cryptography;
using System.Text;
using Konscious.Security.Cryptography;
using KronaStaffTerminal.Models;

namespace KronaStaffTerminal.Services;

public static class AdminPinService
{
    public const string Argon2Prefix = "$argon2id$";
    public const int DefaultArgon2Iterations = 3;
    public const int DefaultMemorySizeKb = 65536; // 64 MB
    public const int DefaultParallelism = 2;
    public const int HashLength = 32;

    public const int LegacyPbkdf2Iterations = 600_000;

    public static (string Salt, string Hash, int Iterations) Hash(string pin)
    {
        var salt = RandomNumberGenerator.GetBytes(16);
        using var argon2 = new Argon2id(Encoding.UTF8.GetBytes(pin))
        {
            Salt = salt,
            DegreeOfParallelism = DefaultParallelism,
            Iterations = DefaultArgon2Iterations,
            MemorySize = DefaultMemorySizeKb
        };
        var hash = argon2.GetBytes(HashLength);
        return (Convert.ToBase64String(salt), Argon2Prefix + Convert.ToBase64String(hash), DefaultArgon2Iterations);
    }

    public static bool Verify(string pin, string saltBase64, string hashBase64, int iterations)
    {
        if (string.IsNullOrWhiteSpace(pin) || string.IsNullOrWhiteSpace(saltBase64) || string.IsNullOrWhiteSpace(hashBase64))
        {
            return false;
        }

        try
        {
            var salt = Convert.FromBase64String(saltBase64);

            if (hashBase64.StartsWith(Argon2Prefix, StringComparison.Ordinal))
            {
                var rawHash = hashBase64[Argon2Prefix.Length..];
                var expected = Convert.FromBase64String(rawHash);
                using var argon2 = new Argon2id(Encoding.UTF8.GetBytes(pin))
                {
                    Salt = salt,
                    DegreeOfParallelism = DefaultParallelism,
                    Iterations = Math.Max(1, iterations),
                    MemorySize = DefaultMemorySizeKb
                };
                var actual = argon2.GetBytes(expected.Length);
                return CryptographicOperations.FixedTimeEquals(actual, expected);
            }

            // Backwards compatibility with legacy PBKDF2 hashes
            var legacyExpected = Convert.FromBase64String(hashBase64);
            var legacyActual = Rfc2898DeriveBytes.Pbkdf2(
                pin,
                salt,
                Math.Clamp(iterations, 120_000, LegacyPbkdf2Iterations),
                HashAlgorithmName.SHA256,
                legacyExpected.Length);
            return CryptographicOperations.FixedTimeEquals(legacyActual, legacyExpected);
        }
        catch (FormatException)
        {
            return false;
        }
    }

    public static TimeSpan CalculateLockout(int failedAttempts)
    {
        if (failedAttempts < 3) return TimeSpan.Zero;
        if (failedAttempts < 5) return TimeSpan.FromSeconds(30 * (failedAttempts - 2)); // 30s, 60s
        if (failedAttempts < 8) return TimeSpan.FromMinutes(5); // 5 minutes
        return TimeSpan.FromMinutes(15); // 15 minutes max
    }

    public static (bool Allowed, string? ErrorMessage) CheckLockout(TerminalSettings settings, DateTimeOffset nowUtc)
    {
        if (settings.AdminPinBlockedUntilUtc > nowUtc)
        {
            var remaining = settings.AdminPinBlockedUntilUtc - nowUtc;
            var seconds = Math.Max(1, (int)Math.Ceiling(remaining.TotalSeconds));
            return (false, $"Ввод PIN временно заблокирован из-за неверных попыток. Попробуйте через {seconds} сек.");
        }
        return (true, null);
    }

    public static bool VerifyAndRecord(
        string pin,
        TerminalSettings settings,
        DateTimeOffset nowUtc,
        out string? errorMessage,
        out bool rehashNeeded)
    {
        var (allowed, lockoutMessage) = CheckLockout(settings, nowUtc);
        if (!allowed)
        {
            errorMessage = lockoutMessage;
            rehashNeeded = false;
            return false;
        }

        var isMatch = Verify(pin, settings.AdminPinSalt, settings.AdminPinHash, settings.AdminPinIterations);
        if (isMatch)
        {
            settings.FailedAdminPinAttempts = 0;
            settings.AdminPinBlockedUntilUtc = DateTimeOffset.MinValue;
            errorMessage = null;

            // Automatically upgrade legacy PBKDF2 hash to Argon2id
            if (!settings.AdminPinHash.StartsWith(Argon2Prefix, StringComparison.Ordinal))
            {
                var (newSalt, newHash, newIterations) = Hash(pin);
                settings.AdminPinSalt = newSalt;
                settings.AdminPinHash = newHash;
                settings.AdminPinIterations = newIterations;
                rehashNeeded = true;
            }
            else
            {
                rehashNeeded = false;
            }

            return true;
        }

        settings.FailedAdminPinAttempts++;
        var lockout = CalculateLockout(settings.FailedAdminPinAttempts);
        if (lockout > TimeSpan.Zero)
        {
            settings.AdminPinBlockedUntilUtc = nowUtc.Add(lockout);
            var seconds = (int)lockout.TotalSeconds;
            errorMessage = $"Неверный PIN. Ввод заблокирован на {seconds} сек.";
        }
        else
        {
            var remainingTries = 3 - settings.FailedAdminPinAttempts;
            errorMessage = $"Неверный PIN. Осталось попыток до блокировки: {remainingTries}.";
        }

        rehashNeeded = false;
        return false;
    }
}