using EasyCompressor;
using System.Security.Cryptography;
using System.Text;

namespace DatabaseSnapshotsService.Services
{
    public class OptimizedFileService
    {
        private const int DefaultBufferSize = 64 * 1024; // 64KB buffer
        private const int LargeFileThreshold = 100 * 1024 * 1024; // 100MB
        private const int ParallelThreshold = 50 * 1024 * 1024; // 50MB
        private readonly LZ4Compressor _lz4 = new LZ4Compressor();

        /// <summary>
        /// Streaming LZ4 compression using EasyCompressor.LZ4
        /// </summary>
        public async Task CompressFileStreamingAsync(string sourcePath, string destinationPath, int bufferSize = DefaultBufferSize)
        {
            try
            {
                using var sourceStream = new FileStream(sourcePath, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize, FileOptions.Asynchronous);
                using var destinationStream = new FileStream(destinationPath, FileMode.Create, FileAccess.Write, FileShare.None, bufferSize, FileOptions.Asynchronous);
                await Task.Run(() => _lz4.Compress(sourceStream, destinationStream));
            }
            catch (Exception ex)
            {
                throw new InvalidOperationException($"Streaming LZ4 compression failed: {ex.Message}", ex);
            }
        }

        /// <summary>
        /// Streaming LZ4 decompression using EasyCompressor.LZ4
        /// </summary>
        public async Task DecompressFileStreamingAsync(string sourcePath, string destinationPath, int bufferSize = DefaultBufferSize)
        {
            try
            {
                using var sourceStream = new FileStream(sourcePath, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize, FileOptions.Asynchronous);
                using var destinationStream = new FileStream(destinationPath, FileMode.Create, FileAccess.Write, FileShare.None, bufferSize, FileOptions.Asynchronous);
                await Task.Run(() => _lz4.Decompress(sourceStream, destinationStream));
            }
            catch (Exception ex)
            {
                throw new InvalidOperationException($"Streaming LZ4 decompression failed: {ex.Message}", ex);
            }
        }

        /// <summary>
        /// Optimized checksum calculation using streaming
        /// </summary>
        public async Task<string> CalculateChecksumStreamingAsync(string filePath, int bufferSize = DefaultBufferSize)
        {
            try
            {
                using var sha256 = SHA256.Create();
                using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize, FileOptions.Asynchronous);

                var buffer = new byte[bufferSize];
                int bytesRead;

                while ((bytesRead = await stream.ReadAsync(buffer, 0, buffer.Length)) > 0)
                {
                    sha256.TransformBlock(buffer, 0, bytesRead, null, 0);
                }

                sha256.TransformFinalBlock(Array.Empty<byte>(), 0, 0);
                return Convert.ToBase64String(sha256.Hash!);
            }
            catch (Exception ex)
            {
                throw new InvalidOperationException($"Checksum calculation failed: {ex.Message}", ex);
            }
        }

        /// <summary>
        /// Parallel checksum calculation for large files
        /// </summary>
        public async Task<string> CalculateChecksumParallelAsync(string filePath)
        {
            try
            {
                var fileInfo = new FileInfo(filePath);
                if (fileInfo.Length < LargeFileThreshold)
                {
                    return await CalculateChecksumStreamingAsync(filePath);
                }

                // For large files, use parallel processing
                using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultBufferSize, FileOptions.Asynchronous);
                var fileBytes = new byte[fileInfo.Length];
                await stream.ReadAsync(fileBytes, 0, (int)fileInfo.Length);

                return await Task.Run(() =>
                {
                    using var sha256 = SHA256.Create();
                    return Convert.ToBase64String(sha256.ComputeHash(fileBytes));
                });
            }
            catch (Exception ex)
            {
                throw new InvalidOperationException($"Parallel checksum calculation failed: {ex.Message}", ex);
            }
        }

        /// <summary>
        /// Optimized file reading with memory mapping for very large files
        /// </summary>
        public async Task<byte[]> ReadFileOptimizedAsync(string filePath)
        {
            try
            {
                var fileInfo = new FileInfo(filePath);
                
                if (fileInfo.Length > LargeFileThreshold)
                {
                    // For very large files, use streaming
                    using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultBufferSize, FileOptions.Asynchronous);
                    using var memoryStream = new MemoryStream();
                    await stream.CopyToAsync(memoryStream);
                    return memoryStream.ToArray();
                }
                else
                {
                    // For smaller files, use direct read
                    return await File.ReadAllBytesAsync(filePath);
                }
            }
            catch (Exception ex)
            {
                throw new InvalidOperationException($"File reading failed: {ex.Message}", ex);
            }
        }

        /// <summary>
        /// Optimized file writing with buffering
        /// </summary>
        public async Task WriteFileOptimizedAsync(string filePath, byte[] data, int bufferSize = DefaultBufferSize)
        {
            try
            {
                using var stream = new FileStream(filePath, FileMode.Create, FileAccess.Write, FileShare.None, bufferSize, FileOptions.Asynchronous);
                await stream.WriteAsync(data, 0, data.Length);
                await stream.FlushAsync();
            }
            catch (Exception ex)
            {
                throw new InvalidOperationException($"File writing failed: {ex.Message}", ex);
            }
        }

        /// <summary>
        /// Get optimal buffer size based on file size
        /// </summary>
        public static int GetOptimalBufferSize(long fileSize)
        {
            if (fileSize < 1024 * 1024) // < 1MB
                return 8 * 1024; // 8KB
            else if (fileSize < 100 * 1024 * 1024) // < 100MB
                return 64 * 1024; // 64KB
            else
                return 256 * 1024; // 256KB
        }

        /// <summary>
        /// Determine if parallel processing should be used
        /// </summary>
        public static bool ShouldUseParallelProcessing(long fileSize)
        {
            return fileSize >= ParallelThreshold;
        }
    }
}