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

using LetsEncrypt.Entities;

namespace LetsEncrypt.Services
{
    public interface IJwsService {
        void Init(RSA rsa, string keyId);
        JwsMessage Encode<TPayload>(TPayload payload, JwsHeader protectedHeader);
        string GetKeyAuthorization(string token);
        string Base64UrlEncoded(byte[] arg);

        void SetKeyId(Account account);
    }


    public class JwsService : IJwsService {

        public Jwk _jwk;
        private RSA _rsa;

        public JwsService() {
            
        }

        public void Init(RSA rsa, string keyId) {
            _rsa = rsa ?? throw new ArgumentNullException(nameof(rsa));

            var publicParameters = rsa.ExportParameters(false);

            _jwk = new Jwk
            {
                KeyType = "RSA",
                Exponent = Base64UrlEncoded(publicParameters.Exponent),
                Modulus = Base64UrlEncoded(publicParameters.Modulus),
                KeyId = keyId
            };
        }



        public JwsMessage Encode<TPayload>(TPayload payload, JwsHeader protectedHeader)
        {

            protectedHeader.Algorithm = "RS256";
            if (_jwk.KeyId != null)
            {
                protectedHeader.KeyId = _jwk.KeyId;
            }
            else
            {
                protectedHeader.Key = _jwk;
            }

            var message = new JwsMessage
            {
                Payload = Base64UrlEncoded(JsonConvert.SerializeObject(payload)),
                Protected = Base64UrlEncoded(JsonConvert.SerializeObject(protectedHeader))
            };

            message.Signature = Base64UrlEncoded(
                _rsa.SignData(Encoding.ASCII.GetBytes(message.Protected + "." + message.Payload),
                    HashAlgorithmName.SHA256,
                    RSASignaturePadding.Pkcs1));

            return message;
        }

        private string GetSha256Thumbprint()
        {
            var json = "{\"e\":\"" + _jwk.Exponent + "\",\"kty\":\"RSA\",\"n\":\"" + _jwk.Modulus + "\"}";

            using (var sha256 = SHA256.Create())
            {
                return Base64UrlEncoded(sha256.ComputeHash(Encoding.UTF8.GetBytes(json)));
            }
        }

        public string GetKeyAuthorization(string token)
        {
            return token + "." + GetSha256Thumbprint();
        }



        public string Base64UrlEncoded(string s)
        {
            return Base64UrlEncoded(Encoding.UTF8.GetBytes(s));
        }

        public string Base64UrlEncoded(byte[] arg)
        {
            var s = Convert.ToBase64String(arg); // Regular base64 encoder
            s = s.Split('=')[0]; // Remove any trailing '='s
            s = s.Replace('+', '-'); // 62nd char of encoding
            s = s.Replace('/', '_'); // 63rd char of encoding
            return s;
        }

        public void SetKeyId(Account account)
        {
            _jwk.KeyId = account.Id;
        }


    }
}