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 * 
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 * with or without modification, are permitted provided 
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 * 
 * *   Redistributions of source code must retain the above 
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 *     following disclaimer. 
 * *   Redistributions in binary form must reproduce the above 
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 *     following disclaimer in the documentation and/or other 
 *     materials provided with the distribution. 
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND 
 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, 
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR 
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 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 
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 */

using System;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

namespace XenCenterLib
{
    public static class StreamUtilities
    {
        private enum HashMethod
        {
            Sha1,
            Sha256
        }

        private static string StringOf(this HashMethod method)
        {
            switch (method)
            {
                case HashMethod.Sha1:
                    return "SHA1";
                case HashMethod.Sha256:
                    return "SHA256";
                default:
                    throw new ArgumentOutOfRangeException(nameof(method), method, null);
            }
        }

        /// <summary>
        /// Perform a copy of the contents of one stream class to another in a buffered fashion
        /// 
        /// Buffer size is a hard-coded 2Mb
        /// </summary>
        /// <param name="inputData">Source data</param>
        /// <param name="outputData">Target stream</param>
        public static void BufferedStreamCopy(Stream inputData, Stream outputData)
        {
            if( inputData == null)
                throw new ArgumentNullException("inputData", "BufferedStreamCopy argument cannot be null");

            if (outputData == null)
                throw new ArgumentNullException("outputData", "BufferedStreamCopy argument cannot be null");

            const long bufferSize = 2*1024*1024;

            byte[] buffer = new byte[bufferSize];
            int n;
            while ((n = inputData.Read(buffer, 0, buffer.Length)) > 0)
            {
                outputData.Write(buffer, 0, n);
            }

            outputData.Flush();
        }

        public static byte[] ComputeHash(Stream stream, out string hashAlgorithm)
        {
            hashAlgorithm = HashMethod.Sha256.StringOf();

            using (var hasher = HashAlgorithm.Create(hashAlgorithm))
                return hasher?.ComputeHash(stream);
        }

        public static byte[] ComputeSignedHash(Stream stream, X509Certificate2 certificate, out string hashAlgorithm)
        {
            hashAlgorithm = HashMethod.Sha256.StringOf();

            if (!certificate.SignatureAlgorithm.FriendlyName.ToUpper().Contains(hashAlgorithm))
            {
                hashAlgorithm = HashMethod.Sha1.StringOf();
                if (!certificate.SignatureAlgorithm.FriendlyName.ToUpper().Contains(hashAlgorithm))
                    throw new NotSupportedException("Unsupported hash algorithm");
            }

            byte[] hash;
            using (var hasher = HashAlgorithm.Create(hashAlgorithm))
                hash = hasher?.ComputeHash(stream);

            if (hash == null || !(certificate.PrivateKey is RSACryptoServiceProvider csp))
                return null;

            if (hashAlgorithm == HashMethod.Sha256.StringOf())
                return csp.SignData(hash, CryptoConfig.MapNameToOID(hashAlgorithm));

            return csp.SignHash(hash, CryptoConfig.MapNameToOID(hashAlgorithm));
        }

        public static bool VerifyAgainstDigest(Stream stream, long limit, string algorithmName, byte[] digest, X509Certificate2 certificate = null)
        {
            int bytesRead = 0;
            long offset = 0;
            byte[] buffer = new byte[2 * 1024 * 1024];

            using (var hashAlgorithm = HashAlgorithm.Create(algorithmName))
            {
                // Validate the algorithm.
                if (hashAlgorithm == null)
                    throw new NotSupportedException($"{algorithmName} is not a valid hash algorithm");

                while (offset < limit)
                {
                    bytesRead = stream.Read(buffer, 0, buffer.Length);

                    if (bytesRead <= 0)
                        break;

                    if (offset + bytesRead < limit)
                    {
                        // This is not the last block. Compute the partial hash.
                        hashAlgorithm.TransformBlock(buffer, 0, bytesRead, buffer, 0);
                    }

                    offset += bytesRead;
                }

                // It is necessary to call TransformBlock at least once and TransformFinalBlock only once before getting the hash.
                // If only the last buffer had content, then TransformBlock would not have been called at least once.
                // So, split the last buffer and hash it even if it is empty.
                // Note: TransformBlock will accept an "inputCount" that is zero.
                hashAlgorithm.TransformBlock(buffer, 0, bytesRead / 2, buffer, 0);

                // Compute the final hash.
                hashAlgorithm.TransformFinalBlock(buffer, bytesRead / 2, bytesRead / 2 + bytesRead % 2);

                if (certificate == null)
                    return digest.SequenceEqual(hashAlgorithm.Hash);

                if (!(certificate.PublicKey.Key is RSACryptoServiceProvider csp))
                    return false;

                if (algorithmName == HashMethod.Sha256.StringOf())
                    return csp.VerifyData(hashAlgorithm.Hash, CryptoConfig.MapNameToOID(algorithmName), digest);

                return csp.VerifyHash(hashAlgorithm.Hash, CryptoConfig.MapNameToOID(algorithmName), digest);
            }
        }
    }
}