using System.Collections.Concurrent;
using System.Security.Cryptography;
using System.Text;

namespace PostgresService.WebApi;

public sealed class SecretStore
{
    private const int Iterations = 210_000;
    private readonly string _dataRoot;
    private readonly object _sync = new();

    public SecretStore(IConfiguration configuration)
    {
        _dataRoot = configuration["POSTGRES_SERVICE_DATA_ROOT"]
            ?? Environment.GetEnvironmentVariable("POSTGRES_SERVICE_DATA_ROOT")
            ?? Path.Combine(AppContext.BaseDirectory, "data");
        Directory.CreateDirectory(_dataRoot);
    }

    public bool VerifyAdminPassword(string value) => Verify("admin-password", value);
    public bool VerifyEnrollmentToken(string value) => Verify("enrollment-token", value);

    public string RotateEnrollmentToken()
    {
        var token = Convert.ToHexString(RandomNumberGenerator.GetBytes(32)).ToLowerInvariant();
        WriteHash("enrollment-token", token);
        return token;
    }

    public void EnsureInitialized()
    {
        lock (_sync)
        {
            PromoteSeed("admin-password");
            PromoteSeed("enrollment-token");
        }
    }

    private void PromoteSeed(string name)
    {
        var hashPath = Path.Combine(_dataRoot, $"{name}.hash");
        if (File.Exists(hashPath))
        {
            return;
        }

        var seedPath = Path.Combine(_dataRoot, $"{name}.seed");
        if (!File.Exists(seedPath))
        {
            throw new InvalidOperationException($"缺少 {name} 初始化文件。");
        }

        var seed = File.ReadAllText(seedPath).TrimEnd('\r', '\n');
        if (string.IsNullOrWhiteSpace(seed))
        {
            throw new InvalidOperationException($"{name} 不能为空。");
        }

        WriteHash(name, seed);
        File.Delete(seedPath);
    }

    private bool Verify(string name, string value)
    {
        if (string.IsNullOrEmpty(value))
        {
            return false;
        }

        var path = Path.Combine(_dataRoot, $"{name}.hash");
        if (!File.Exists(path))
        {
            return false;
        }

        var parts = File.ReadAllText(path).Trim().Split('$');
        if (parts.Length != 4 || parts[0] != "pbkdf2-sha256" || !int.TryParse(parts[1], out var iterations))
        {
            return false;
        }

        try
        {
            var salt = Convert.FromBase64String(parts[2]);
            var expected = Convert.FromBase64String(parts[3]);
            var actual = Rfc2898DeriveBytes.Pbkdf2(
                Encoding.UTF8.GetBytes(value), salt, iterations, HashAlgorithmName.SHA256, expected.Length);
            return CryptographicOperations.FixedTimeEquals(actual, expected);
        }
        catch (FormatException)
        {
            return false;
        }
    }

    private void WriteHash(string name, string value)
    {
        var salt = RandomNumberGenerator.GetBytes(16);
        var hash = Rfc2898DeriveBytes.Pbkdf2(
            Encoding.UTF8.GetBytes(value), salt, Iterations, HashAlgorithmName.SHA256, 32);
        var content = $"pbkdf2-sha256${Iterations}${Convert.ToBase64String(salt)}${Convert.ToBase64String(hash)}\n";
        var destination = Path.Combine(_dataRoot, $"{name}.hash");
        var temporary = destination + ".tmp";
        File.WriteAllText(temporary, content, new UTF8Encoding(false));
        File.Move(temporary, destination, true);
        if (!OperatingSystem.IsWindows())
        {
            File.SetUnixFileMode(destination, UnixFileMode.UserRead | UnixFileMode.UserWrite);
        }
    }
}

public sealed class AdminSessionStore
{
    private static readonly TimeSpan Lifetime = TimeSpan.FromHours(12);
    private readonly ConcurrentDictionary<string, DateTimeOffset> _sessions = new();

    public string Create()
    {
        RemoveExpired();
        var token = Convert.ToHexString(RandomNumberGenerator.GetBytes(32)).ToLowerInvariant();
        _sessions[token] = DateTimeOffset.UtcNow.Add(Lifetime);
        return token;
    }

    public bool Validate(string? token)
    {
        if (string.IsNullOrWhiteSpace(token) || !_sessions.TryGetValue(token, out var expiresAt))
        {
            return false;
        }

        if (expiresAt <= DateTimeOffset.UtcNow)
        {
            _sessions.TryRemove(token, out _);
            return false;
        }

        _sessions[token] = DateTimeOffset.UtcNow.Add(Lifetime);
        return true;
    }

    public void Revoke(string? token)
    {
        if (!string.IsNullOrWhiteSpace(token))
        {
            _sessions.TryRemove(token, out _);
        }
    }

    private void RemoveExpired()
    {
        var now = DateTimeOffset.UtcNow;
        foreach (var entry in _sessions.Where(item => item.Value <= now))
        {
            _sessions.TryRemove(entry.Key, out _);
        }
    }
}