using System;
using System.Text;

namespace SKIT.FlurlHttpClient.Wechat.TenpayV3
{
    /// <summary>
    /// 为 <see cref="WechatTenpayClient"/> 提供响应签名验证的扩展方法。
    /// </summary>
    public static class WechatTenpayClientResponseVerificationExtensions
    {
        /// <summary>
        /// <para>验证响应签名。</para>
        /// <para>REF: https://pay.weixin.qq.com/wiki/doc/apiv3/wechatpay/wechatpay4_1.shtml </para>
        /// <para>REF: https://pay.weixin.qq.com/wiki/doc/apiv3_partner/wechatpay/wechatpay4_1.shtml </para>
        /// </summary>
        /// <typeparam name="TResponse"></typeparam>
        /// <param name="client"></param>
        /// <param name="response"></param>
        /// <returns></returns>
        public static bool VerifyResponseSignature<TResponse>(this WechatTenpayClient client, TResponse response)
            where TResponse : WechatTenpayResponse
        {
            return VerifyResponseSignature(client, response, out _);
        }

        /// <summary>
        /// <para>验证响应签名。</para>
        /// <para>REF: https://pay.weixin.qq.com/wiki/doc/apiv3/wechatpay/wechatpay4_1.shtml </para>
        /// <para>REF: https://pay.weixin.qq.com/wiki/doc/apiv3_partner/wechatpay/wechatpay4_1.shtml </para>
        /// </summary>
        /// <typeparam name="TResponse"></typeparam>
        /// <param name="client"></param>
        /// <param name="response"></param>
        /// <param name="error"></param>
        /// <returns></returns>
        /// <exception cref="ArgumentNullException"></exception>
        public static bool VerifyResponseSignature<TResponse>(this WechatTenpayClient client, TResponse response, out Exception? error)
            where TResponse : WechatTenpayResponse
        {
            if (client == null) throw new ArgumentNullException(nameof(client));
            if (response == null) throw new ArgumentNullException(nameof(response));

            if (client.CertificateManager != null)
            {
                try
                {
                    var cert = client.CertificateManager.GetEntry(response.WechatpayCertSerialNumber)!;
                    if (!cert.HasValue)
                    {
                        error = new Exceptions.WechatTenpayResponseVerificationException("Verify signature of response failed, because there is no platform certificate matched the serial number.");
                        return false;
                    }

                    error = null;
                    return Utilities.RSAUtility.VerifyWithSHA256ByCertificate(
                        certificate: cert.Value.Certificate,
                        plainText: GetPlainTextForSignature(response),
                        signature: response.WechatpaySignature
                    );
                }
                catch (Exception ex) 
                {
                    error = new Exceptions.WechatTenpayResponseVerificationException("Verify signature of response failed. Please see the `InnerException` for more details.", ex);
                    return false;
                }
            }

            error = new Exceptions.WechatTenpayResponseVerificationException("Verify signature of response failed, because there is no platform certificate in the manager.");
            return false;
        }

        private static string GetPlainTextForSignature(WechatTenpayResponse response)
        {
            string timestamp = response.WechatpayTimestamp;
            string nonce = response.WechatpayNonce;
            string body = Encoding.UTF8.GetString(response.RawBytes);
            return $"{timestamp}\n{nonce}\n{body}\n";
        }
    }
}