using System;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using System.IO.Compression;
using System.Text;

namespace StardewModdingAPI.Web.Framework.Compression
{
    /// <summary>Handles GZip compression logic.</summary>
    internal class GzipHelper : IGzipHelper
    {
        /*********
        ** Fields
        *********/
        /// <summary>The first bytes in a valid zip file.</summary>
        /// <remarks>See <a href="https://en.wikipedia.org/wiki/Zip_(file_format)#File_headers"/>.</remarks>
        private const uint GzipLeadBytes = 0x8b1f;


        /*********
        ** Public methods
        *********/
        /// <summary>Compress a string.</summary>
        /// <param name="text">The text to compress.</param>
        /// <remarks>Derived from <a href="https://stackoverflow.com/a/17993002/262123"/>.</remarks>
        public string CompressString(string text)
        {
            // get raw bytes
            byte[] buffer = Encoding.UTF8.GetBytes(text);

            // compressed
            byte[] compressedData;
            using (MemoryStream stream = new())
            {
                using (GZipStream zipStream = new(stream, CompressionLevel.Optimal, leaveOpen: true))
                    zipStream.Write(buffer, 0, buffer.Length);

                stream.Position = 0;
                compressedData = new byte[stream.Length];
                stream.Read(compressedData, 0, compressedData.Length);
            }

            // prefix length
            byte[] zipBuffer = new byte[compressedData.Length + 4];
            Buffer.BlockCopy(compressedData, 0, zipBuffer, 4, compressedData.Length);
            Buffer.BlockCopy(BitConverter.GetBytes(buffer.Length), 0, zipBuffer, 0, 4);

            // return string representation
            return Convert.ToBase64String(zipBuffer);
        }

        /// <summary>Decompress a string.</summary>
        /// <param name="rawText">The compressed text.</param>
        /// <remarks>Derived from <a href="https://stackoverflow.com/a/17993002/262123"/>.</remarks>
        [return: NotNullIfNotNull("rawText")]
        public string? DecompressString(string? rawText)
        {
            if (rawText is null)
                return rawText;

            // get raw bytes
            byte[] zipBuffer;
            try
            {
                zipBuffer = Convert.FromBase64String(rawText);
            }
            catch
            {
                return rawText; // not valid base64, wasn't compressed by the log parser
            }

            // skip if not gzip
            if (BitConverter.ToUInt16(zipBuffer, 4) != GzipHelper.GzipLeadBytes)
                return rawText;

            // decompress
            using MemoryStream memoryStream = new();
            {
                // read length prefix
                int dataLength = BitConverter.ToInt32(zipBuffer, 0);
                memoryStream.Write(zipBuffer, 4, zipBuffer.Length - 4);

                // read data
                byte[] buffer = new byte[dataLength];
                memoryStream.Position = 0;
                using (GZipStream gZipStream = new(memoryStream, CompressionMode.Decompress))
                    gZipStream.Read(buffer, 0, buffer.Length);

                // return original string
                return Encoding.UTF8.GetString(buffer);
            }
        }
    }
}