using System;
using System.Web;
using System.Text;
using System.Net;
using System.Threading.Tasks;
using System.IO;
using System.Linq;
using Newtonsoft.Json;
using System.Security.Cryptography;

namespace Sample.ReceiveCallback
{
    public class ReceiveCallbackHandler : HttpTaskAsyncHandler
    {
        // callback data as received from PayWiser
        private class Callback
        {
            public string Type { get; set; }
            public string Data { get; set; }
            public string HMAC { get; set; }
        }

        // inner callback data for pos payment as received from PayWiser
        private class POSPaymentCallback
        {
            public string PGReferenceID { get; set; }
            public string CustomerReferenceID { get; set; }
            public int TransactionType { get; set; }
            public int PaymentStatus { get; set; }
            public string PaymentDescription { get; set; }
            public string CardScheme { get; set; }
            public string CardType { get; set; }
            public string TerminalID { get; set; }
            public string TerminalIP { get; set; }
            public string TerminalPort { get; set; }
        }

        // inner callback data for pos refund as received from PayWiser (same structure as payment)
        private class POSRefundCallback: POSPaymentCallback
        {
        }

        public override bool IsReusable
        {
            get { return false; }
        }

        // main processing code
        public override async Task ProcessRequestAsync(HttpContext context)
        {
            // reject Get etc calls
            if (HttpContext.Current.Request.HttpMethod != "POST")
            {
                context.Response.StatusCode = (int)HttpStatusCode.MethodNotAllowed;     //405
                context.Response.SuppressContent = true;
                context.Response.TrySkipIisCustomErrors = true;
                return;
            }

            // reject if running on HTTP instead of HTTPS
            if (!HttpContext.Current.Request.IsSecureConnection)
            {
                context.Response.StatusCode = (int)HttpStatusCode.HttpVersionNotSupported; //505
                context.Response.SuppressContent = true;
                context.Response.TrySkipIisCustomErrors = true;
                return;
            }

            try
            {
                // your own confirmation string, as entered into PayWiser PaymentGateway Dashboard for your account
                string confirmationString = "1234qwertz";
                
                // allow only calls from this IP
                string allowedIP = "192.168.1.1";

                // get POST data
                string postData = ReadParameters(context.Request.InputStream);

                // get sender (caller) IP
                string senderIP = GetClientIP(HttpContext.Current.Request);

                // reject calls from wrong IPs
                if (!senderIP.Equals(allowedIP))
                {
                    context.Response.StatusCode = (int)HttpStatusCode.NotFound; //404
                    context.Response.SuppressContent = true;
                    context.Response.TrySkipIisCustomErrors = true;
                    return;
                }

                // deserialize data
                Callback callback = JsonConvert.DeserializeObject<Callback>(postData);

                // calculate HMAC
                string calculatedHMAC = CalculateHMAC(callback.Data, confirmationString);

                // reject calls with wrong HMAC
                if (!calculatedHMAC.Equals(callback.HMAC))
                {
                    context.Response.StatusCode = (int)HttpStatusCode.NotFound; //404
                    context.Response.SuppressContent = true;
                    context.Response.TrySkipIisCustomErrors = true;
                    return;
                }

                // deserialize inner callback data
                if (callback.Type.Equals("pos_payment"))
                {
                    POSPaymentCallback posPayment = JsonConvert.DeserializeObject<POSPaymentCallback>(callback.Data);
                    // your own logic goes here
                }
                else if (callback.Type.Equals("pos_refund"))
                {
                    POSRefundCallback posRefund = JsonConvert.DeserializeObject<POSRefundCallback>(callback.Data);
                    // your own logic goes here
                }

                // successfully received
                context.Response.StatusCode = (int)HttpStatusCode.OK; //200
                return;
            }
            catch
            {
                // PayWiser will retry sending
                context.Response.StatusCode = (int)HttpStatusCode.BadRequest; //400
                return;
            }
        }

        private string ReadParameters(Stream streamParameters)
        {
            string ret = "";
            try
            {
                using (StreamReader reader = new StreamReader(streamParameters))
                    ret = reader.ReadToEnd();
            }
            catch { }

            return ret;
        }

        private string GetClientIP(HttpRequest httpRequest)
        {
            // in case of load balancers etc
            string ip = httpRequest.ServerVariables["HTTP_X_FORWARDED_FOR"];

            if (String.IsNullOrWhiteSpace(ip))
                ip = httpRequest.ServerVariables["REMOTE_ADDR"];
            else ip = ip.Split(',').Last().Trim();

            return ip;
        }

        static private string CalculateHMAC(string payload, string confirmationSequence)
        {
            if (String.IsNullOrWhiteSpace(payload) || String.IsNullOrWhiteSpace(confirmationSequence))
                return "";

            var data = Encoding.UTF8.GetBytes(payload);
            var key = Encoding.UTF8.GetBytes(confirmationSequence);

            // Create HMAC-MD5 Algorithm;
            var hmac = new HMACMD5(key);

            // Compute hash.
            var hashBytes = hmac.ComputeHash(data);

            // Convert to HEX string.
            return System.BitConverter.ToString(hashBytes).Replace("-", "").ToLower();
        }
    }
}