read csv lines RFC4180 from file.cs

private enum State
{
    BeforeField,
    InField,
    InQuotedField,
    LineEnd,
}

public static IEnumerable<Memory<char>> TryReadLine(TextReader reader)
{
    var bufferLength = (int)Math.Pow(2, 16);
    var buffer = ArrayPool<char>.Shared.Rent(bufferLength);

    var i = 0;
    var j = 0;
    var state = State.BeforeField;
    int c;

    FillBufferAsync(true);

    int FillBufferAsync(bool initial)
    {
        var len = i - j;
        if (initial == false)
        {
            Array.Copy(buffer, j, buffer, 0, len);
        }

        var totalRead = reader.ReadAsync(buffer, len, bufferLength - len).GetAwaiter().GetResult();
        bufferLength = len + totalRead;

        i = 0;
        j = 0;

        return totalRead;
    }

    int Peek() => i < bufferLength ? buffer[i] : -1;

    var hasBufferToConsume = false;


reloop:

    j = i;
    state = State.BeforeField;

    while (hasBufferToConsume = i < bufferLength)
    {
        c = buffer[i++];

        switch (state)
        {
            case State.BeforeField:

                switch (c)
                {
                    case '"':
                        state = State.InQuotedField;
                        break;
                    case ',':
                        //  fields.Add(string.Empty);
                        break;
                    case '\r':
                        // fields.Add(string.Empty);
                        if (Peek() == '\n')
                        {
                            i++;
                        }
                        state = State.LineEnd;
                        goto afterLoop;

                    case '\n':
                        // fields.Add(string.Empty);
                        state = State.LineEnd;
                        goto afterLoop;

                    default:
                        // builder.Append((char)c);
                        state = State.InField;
                        break;
                }
                break;

            case State.InField:
                switch (c)
                {
                    case ',':
                        //  AddField(fields, builder);
                        state = State.BeforeField;
                        break;
                    case '\r':
                        //  AddField(fields, builder);
                        if (Peek() == '\n')
                        {
                            i++;
                        }
                        state = State.LineEnd;
                        goto afterLoop;

                    case '\n':
                        //    AddField(fields, builder);
                        state = State.LineEnd;
                        goto afterLoop;

                    default:
                        //      builder.Append((char)c);
                        break;
                }
                break;

            case State.InQuotedField:
                switch (c)
                {
                    case '"':
                        var nc = Peek();
                        switch (nc)
                        {
                            case '"':
                                //        builder.Append('"');
                                i++;

                                break;
                            case ',':
                                i++;

                                //           AddField(fields, builder);
                                state = State.BeforeField;
                                break;
                            case '\r':
                                i++;

                                //          AddField(fields, builder);
                                if (Peek() == '\n')
                                {
                                    i++;
                                }
                                state = State.LineEnd;
                                goto afterLoop;

                            case '\n':
                                i++;
                                //          AddField(fields, builder);
                                state = State.LineEnd;
                                goto afterLoop;
                               
                            default:
                                throw new InvalidDataException("Corrupt field found. A double quote is not escaped or there is extra data after a quoted field.");
                        }
                        break;
                    default:
                        //    builder.Append((char)c);
                        break;
                }
                break;                        

            default:
                throw new NotImplementedException();
        }

        //if (state == State.LineEnd)
        //{
        //    if (i == 1)
        //        throw new Exception(); // goto reloop;

        //    break;
        //}
    }

    afterLoop:

    if (hasBufferToConsume == false)
    {
        if (FillBufferAsync(false) == 0)
        {
            ArrayPool<char>.Shared.Return(buffer);
            yield break;
        }

        goto reloop;
    }

    switch (state)
    {
        case State.BeforeField:
            yield return buffer.AsMemory(j, i - j);
            goto reloop;

        case State.LineEnd:
            if ((i == 1 && char.IsWhiteSpace(buffer[0])) == false)
            yield return buffer.AsMemory(j, i - j - 1);
            goto reloop;

        case State.InField:
            yield return buffer.AsMemory(j, i - j);
            goto reloop;

        case State.InQuotedField:
            break;
            throw new InvalidDataException("When the line ends with a quoted field, the last character should be an unescaped double quote.");
    }
}