using System.Runtime.InteropServices;
using System.Text;
using Microsoft.Extensions.Logging;
using ZstdSharp;

namespace MesETL.App.Services.ETL;

/// <summary>
/// 解压ZST文件，从中读取CSV数据
/// </summary>
public class ZstReader : CsvReader
{
    protected new readonly Lazy<StreamReader> Reader;
    private Stream? _stream;

    private readonly List<char> _str = new(1024);
    private readonly char[] _charBuffer = new char[1024];
    private int _charLen = 0;
    private int _charPos = 0;
    
    
    public ZstReader(string filePath, string tableName, string[] headers, string delimiter = ",", char quoteChar = '\"', ILogger? logger = null) 
        : base(filePath, tableName, headers, delimiter, quoteChar, logger)
    {
        Reader = new Lazy<StreamReader>(() =>
        {
             _stream = new DecompressionStream(File.OpenRead(filePath));
            return new StreamReader(_stream);
        }, false);
        ReadBuffer();
    }
        
    public ZstReader(Stream stream, string tableName, string[] headers, string delimiter = ",", char quoteChar = '\"', ILogger? logger = null) 
        : base(stream, tableName, headers, delimiter, quoteChar, logger)
    {
        var ds = new DecompressionStream(stream);
        Reader = new Lazy<StreamReader>(() => new StreamReader(ds), false);
        ReadBuffer();
    }

    private int ReadBuffer()
    {
        _charLen = _charPos = 0;
        _charLen = Reader.Value.ReadBlock(_charBuffer);
        return _charLen;
    }

    public override async ValueTask<bool> ReadAsync()
    {
        // 缓冲区已经读取完毕，并且流状态为EOF
        if (_charPos == _charLen && ReadBuffer() == 0)
            return false;

        do
        {
            // 读取缓冲区
            var span = _charBuffer.AsSpan(_charPos, _charLen - _charPos);
            var newLineIdx = span.IndexOfAny('\r', '\n');
            // 读取到行，结合当前构建字符串转换进行转换
            if (newLineIdx >= 0)
            {
                if (_str.Count == 0) // => 可以提高一点性能...
                {
                    var fields = ParseRowFaster(span[..newLineIdx], QuoteChar, Delimiter[0]);
                    Current = new DataRecord(fields, TableName, Headers);
                }
                else
                {
                    _str.AddRange(span[..newLineIdx]);
                    var fields = ParseRowFaster(CollectionsMarshal.AsSpan(_str), QuoteChar, Delimiter[0]);
                    Current = new DataRecord(fields, TableName, Headers);
                }
                _str.Clear();

                var ch = span[newLineIdx];
                _charPos += newLineIdx + 1;
                if (ch == '\r' && (_charPos < _charLen || ReadBuffer() > 0) && _charBuffer[_charPos] == '\n') // 跳过CRLF
                    ++_charPos;
                return true;
            }
            // 未读取到行，将缓冲区插入构建字符串
            _str.AddRange(span);
        } while (ReadBuffer() > 0);

        var f = ParseRowFaster(CollectionsMarshal.AsSpan(_str), QuoteChar, Delimiter[0]);
        Current = new DataRecord(f, TableName, Headers);
        _str.Clear();
        return true;
    }

    public override void Dispose()
    {
        base.Dispose();
        if (Reader.IsValueCreated)
        {
            Reader.Value.Dispose();
            _stream?.Dispose();
        }
    }

}