using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace DCIT.Core.Utilities
{
    public static class HashUtility
    {
        private static readonly byte[] EncryptionKey = ComputeSha256Bytes("DCIT::DiffEncryptionKey::20260424");
        private static readonly byte[] TamperKey = ComputeSha256Bytes("DCIT::TamperProtectionKey::20260424");

        public static string ComputeSha256Base64(byte[] bytes)
        {
            using (var sha = SHA256.Create())
            {
                return Convert.ToBase64String(sha.ComputeHash(bytes ?? new byte[0]));
            }
        }

        public static string ComputeSha256Base64(string value)
        {
            return ComputeSha256Base64(Encoding.UTF8.GetBytes(value ?? string.Empty));
        }

        public static string ComputeFileSha256Base64(string path)
        {
            using (var stream = File.OpenRead(path))
            using (var sha = SHA256.Create())
            {
                return Convert.ToBase64String(sha.ComputeHash(stream));
            }
        }

        public static string ComputeHmacBase64(byte[] bytes)
        {
            using (var hmac = new HMACSHA256(TamperKey))
            {
                return Convert.ToBase64String(hmac.ComputeHash(bytes ?? new byte[0]));
            }
        }

        public static string ComputeFileHmacBase64(string path)
        {
            using (var stream = File.OpenRead(path))
            using (var hmac = new HMACSHA256(TamperKey))
            {
                return Convert.ToBase64String(hmac.ComputeHash(stream));
            }
        }

        public static EncryptedPayload Encrypt(byte[] plaintextBytes)
        {
            var iv = new byte[16];
            using (var rng = RandomNumberGenerator.Create())
            {
                rng.GetBytes(iv);
            }

            byte[] cipherBytes;
            using (var aes = Aes.Create())
            {
                aes.Key = EncryptionKey;
                aes.IV = iv;
                aes.Mode = CipherMode.CBC;
                aes.Padding = PaddingMode.PKCS7;

                using (var encryptor = aes.CreateEncryptor())
                {
                    cipherBytes = encryptor.TransformFinalBlock(plaintextBytes ?? new byte[0], 0, (plaintextBytes ?? new byte[0]).Length);
                }
            }

            var tagInput = Combine(iv, cipherBytes);
            using (var hmac = new HMACSHA256(TamperKey))
            {
                return new EncryptedPayload
                {
                    CiphertextBase64 = Convert.ToBase64String(cipherBytes),
                    NonceBase64 = Convert.ToBase64String(iv),
                    TagBase64 = Convert.ToBase64String(hmac.ComputeHash(tagInput)),
                };
            }
        }

        public static EncryptedPayloadHeader EncryptStream(Stream plaintextStream, Stream ciphertextStream)
        {
            if (plaintextStream == null)
            {
                throw new ArgumentNullException("plaintextStream");
            }

            if (ciphertextStream == null)
            {
                throw new ArgumentNullException("ciphertextStream");
            }

            if (!ciphertextStream.CanSeek)
            {
                throw new ArgumentException("密文流必须支持 Seek 以便计算防篡改标签。", "ciphertextStream");
            }

            var iv = new byte[16];
            using (var rng = RandomNumberGenerator.Create())
            {
                rng.GetBytes(iv);
            }

            using (var aes = Aes.Create())
            {
                aes.Key = EncryptionKey;
                aes.IV = iv;
                aes.Mode = CipherMode.CBC;
                aes.Padding = PaddingMode.PKCS7;

                using (var encryptor = aes.CreateEncryptor())
                using (var cryptoStream = new CryptoStream(ciphertextStream, encryptor, CryptoStreamMode.Write, leaveOpen: true))
                {
                    plaintextStream.CopyTo(cryptoStream);
                    cryptoStream.FlushFinalBlock();
                }
            }

            ciphertextStream.Position = 0;
            using (var hmac = new HMACSHA256(TamperKey))
            {
                hmac.TransformBlock(iv, 0, iv.Length, iv, 0);
                var buffer = new byte[65536];
                int read;
                while ((read = ciphertextStream.Read(buffer, 0, buffer.Length)) > 0)
                {
                    hmac.TransformBlock(buffer, 0, read, buffer, 0);
                }

                hmac.TransformFinalBlock(new byte[0], 0, 0);
                return new EncryptedPayloadHeader
                {
                    NonceBase64 = Convert.ToBase64String(iv),
                    TagBase64 = Convert.ToBase64String(hmac.Hash),
                };
            }
        }

        public static byte[] Decrypt(string ciphertextBase64, string nonceBase64, string tagBase64)
        {
            var cipherBytes = Convert.FromBase64String(ciphertextBase64 ?? string.Empty);
            var iv = Convert.FromBase64String(nonceBase64 ?? string.Empty);
            var providedTag = Convert.FromBase64String(tagBase64 ?? string.Empty);
            var expectedTag = Convert.FromBase64String(ComputeHmacBase64(Combine(iv, cipherBytes)));
            if (!FixedTimeEquals(providedTag, expectedTag))
            {
                throw new CryptographicException("差异载荷验证失败。");
            }

            using (var aes = Aes.Create())
            {
                aes.Key = EncryptionKey;
                aes.IV = iv;
                aes.Mode = CipherMode.CBC;
                aes.Padding = PaddingMode.PKCS7;

                using (var decryptor = aes.CreateDecryptor())
                {
                    return decryptor.TransformFinalBlock(cipherBytes, 0, cipherBytes.Length);
                }
            }
        }

        public static void DecryptStream(Stream cipherStream, Stream plaintextStream, byte[] iv, byte[] expectedTag)
        {
            if (cipherStream == null)
            {
                throw new ArgumentNullException("cipherStream");
            }

            if (plaintextStream == null)
            {
                throw new ArgumentNullException("plaintextStream");
            }

            if (!cipherStream.CanSeek)
            {
                throw new ArgumentException("密文流必须支持 Seek 以便校验防篡改标签。", "cipherStream");
            }

            using (var hmac = new HMACSHA256(TamperKey))
            {
                hmac.TransformBlock(iv ?? new byte[0], 0, (iv ?? new byte[0]).Length, iv ?? new byte[0], 0);
                cipherStream.Position = 0;
                var buffer = new byte[65536];
                int read;
                while ((read = cipherStream.Read(buffer, 0, buffer.Length)) > 0)
                {
                    hmac.TransformBlock(buffer, 0, read, buffer, 0);
                }

                hmac.TransformFinalBlock(new byte[0], 0, 0);
                if (!FixedTimeEquals(hmac.Hash, expectedTag ?? new byte[0]))
                {
                    throw new CryptographicException("差异载荷防篡改校验失败。");
                }
            }

            cipherStream.Position = 0;
            using (var aes = Aes.Create())
            {
                aes.Key = EncryptionKey;
                aes.IV = iv;
                aes.Mode = CipherMode.CBC;
                aes.Padding = PaddingMode.PKCS7;

                using (var decryptor = aes.CreateDecryptor())
                using (var cryptoStream = new CryptoStream(cipherStream, decryptor, CryptoStreamMode.Read, leaveOpen: true))
                {
                    cryptoStream.CopyTo(plaintextStream);
                }
            }
        }

        private static byte[] ComputeSha256Bytes(string value)
        {
            using (var sha = SHA256.Create())
            {
                return sha.ComputeHash(Encoding.UTF8.GetBytes(value ?? string.Empty));
            }
        }

        private static bool FixedTimeEquals(byte[] left, byte[] right)
        {
            if (left == null || right == null || left.Length != right.Length)
            {
                return false;
            }

            var diff = 0;
            for (var index = 0; index < left.Length; index++)
            {
                diff |= left[index] ^ right[index];
            }

            return diff == 0;
        }

        private static byte[] Combine(byte[] left, byte[] right)
        {
            left = left ?? new byte[0];
            right = right ?? new byte[0];
            var bytes = new byte[left.Length + right.Length];
            Buffer.BlockCopy(left, 0, bytes, 0, left.Length);
            Buffer.BlockCopy(right, 0, bytes, left.Length, right.Length);
            return bytes;
        }
    }

    public class EncryptedPayload
    {
        public string CiphertextBase64 { get; set; }

        public string NonceBase64 { get; set; }

        public string TagBase64 { get; set; }
    }

    public class EncryptedPayloadHeader
    {
        public string NonceBase64 { get; set; }

        public string TagBase64 { get; set; }
    }
}