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

namespace CampusErrand.Services;

/// <summary>
/// 腾讯 IM 服务，负责生成 UserSig
/// </summary>
public class TencentIMService
{
    private readonly long _sdkAppId;
    private readonly string _secretKey;

    public TencentIMService(IConfiguration configuration)
    {
        var config = configuration.GetSection("TencentIM");
        _sdkAppId = config.GetValue<long>("SDKAppId");
        _secretKey = config["SecretKey"]!;
    }

    public long SDKAppId => _sdkAppId;

    /// <summary>
    /// 生成 UserSig
    /// </summary>
    /// <param name="userId">用户标识</param>
    /// <param name="expireSeconds">有效期（秒），默认7天</param>
    public string GenerateUserSig(string userId, int expireSeconds = 604800)
    {
        var now = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
        var obj = new Dictionary<string, object>
        {
            ["TLS.ver"] = "2.0",
            ["TLS.identifier"] = userId,
            ["TLS.sdkappid"] = _sdkAppId,
            ["TLS.expire"] = expireSeconds,
            ["TLS.time"] = now
        };

        var contentToSign = $"TLS.identifier:{userId}\nTLS.sdkappid:{_sdkAppId}\nTLS.time:{now}\nTLS.expire:{expireSeconds}\n";
        using var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(_secretKey));
        var sig = hmac.ComputeHash(Encoding.UTF8.GetBytes(contentToSign));
        obj["TLS.sig"] = Convert.ToBase64String(sig);

        var jsonBytes = Encoding.UTF8.GetBytes(JsonSerializer.Serialize(obj));

        // zlib 压缩
        using var output = new MemoryStream();
        using (var zlib = new System.IO.Compression.ZLibStream(output, System.IO.Compression.CompressionLevel.Optimal))
        {
            zlib.Write(jsonBytes, 0, jsonBytes.Length);
        }

        // Base64 URL 安全编码
        return Convert.ToBase64String(output.ToArray())
            .Replace('+', '*')
            .Replace('/', '-')
            .Replace('=', '_');
    }
}