using System.Buffers;
using System.Collections.Specialized;
using System.Net.Sockets;
using System.Text;
using System.Web;

namespace Uwaa.HTTP;

class HttpStream : IDisposable
{
    /// <summary>
    /// The underlying TCP stream.
    /// </summary>
    readonly Stream Stream;

    /// <summary>
    /// The read/write buffer.
    /// </summary>
    readonly BufferedStream Buffer;

    /// <summary>
    /// Text decoder.
    /// </summary>
    readonly Decoder Decoder;

    /// <summary>
    /// The maximum time the socket may be inactive before it is presumed dead and closed.
    /// </summary>
    public TimeSpan Timeout;

    public HttpStream(Stream stream, TimeSpan timeout) : base()
    {
        Stream = stream;
        Timeout = timeout;
        Buffer = new BufferedStream(stream);
        Decoder = Encoding.ASCII.GetDecoder();
        
    }

    public async ValueTask<string> ReadLine()
    {
        CancellationTokenSource cancelSrc = new CancellationTokenSource();
        cancelSrc.CancelAfter(Timeout);

        const int maxChars = 4096;
        byte[] dataBuffer = ArrayPool<byte>.Shared.Rent(1);
        char[] charBuffer = ArrayPool<char>.Shared.Rent(maxChars);
        try
        {
            int charBufferIndex = 0;
            while (true)
            {
                if (await Buffer.ReadAsync(dataBuffer.AsMemory(0, 1), cancelSrc.Token) == 0)
                    if (charBufferIndex == 0)
                        throw new SocketException((int)SocketError.ConnectionReset);
                    else
                        break;

                if (charBufferIndex >= maxChars)
                    throw new HttpException("Header is too large");

                charBufferIndex += Decoder.GetChars(dataBuffer, 0, 1, charBuffer, charBufferIndex, false);

                if (charBufferIndex >= 2 && charBuffer[charBufferIndex - 1] == '\n' && charBuffer[charBufferIndex - 2] == '\r')
                {
                    charBufferIndex -= 2;
                    break;
                }
            }
            Decoder.Reset();
            return new string(charBuffer, 0, charBufferIndex);
        }
        catch (IOException e)
        {
            if (e.InnerException is SocketException se)
                throw se;
            else
                throw;
        }
        finally
        {
            //Clearing the array is unnecessary but it is good security just in case.
            ArrayPool<char>.Shared.Return(charBuffer, true);
            ArrayPool<byte>.Shared.Return(dataBuffer);
        }
    }

    public async ValueTask<int> Read(Memory<byte> buffer)
    {
        CancellationTokenSource cancelSrc = new CancellationTokenSource();
        cancelSrc.CancelAfter(Timeout);

        try
        {
            int index = 0;
            while (index < buffer.Length)
            {
                int count = await Buffer.ReadAsync(buffer[index..], cancelSrc.Token);
                if (count == 0)
                    break;

                index += count;
            }
            return index;
        }
        catch (IOException e)
        {
            if (e.InnerException is SocketException se)
                throw se;
            else
                throw;
        }
    }


    public ValueTask Write(string text)
    {
        CancellationTokenSource cancelSrc = new CancellationTokenSource();
        cancelSrc.CancelAfter(Timeout);

        byte[] data = Encoding.ASCII.GetBytes(text);
        return Buffer.WriteAsync(data, cancelSrc.Token);
    }

    public ValueTask WriteLine(string text)
    {
        return Write(text + "\r\n");
    }

    public ValueTask WriteLine()
    {
        return Write("\r\n");
    }

    public ValueTask Write(ReadOnlyMemory<byte> bytes)
    {
        CancellationTokenSource cancelSrc = new CancellationTokenSource();
        cancelSrc.CancelAfter(Timeout);
        try
        {
            return Buffer.WriteAsync(bytes, cancelSrc.Token);
        }
        catch (IOException e)
        {
            if (e.InnerException is SocketException se)
                throw se;
            else
                throw;
        }
    }

    public async Task Flush()
    {
        CancellationTokenSource cancelSrc = new CancellationTokenSource();
        cancelSrc.CancelAfter(Timeout);
        try
        {
            await Buffer.FlushAsync(cancelSrc.Token);
            await Stream.FlushAsync(cancelSrc.Token);
        }
        catch (IOException e)
        {
            if (e.InnerException is SocketException se)
                throw se;
            else
                throw;
        }
    }


    public async ValueTask<HttpFields> ReadFields()
    {
        HttpFields fields = new HttpFields();
        while (true)
        {
            string? headerStr = await ReadLine();

            if (string.IsNullOrWhiteSpace(headerStr))
                break; //End of headers

            if (fields.Misc != null && fields.Misc.Count >= 30)
                throw new HttpException("Too many headers");

            int splitPoint = headerStr.IndexOf(':');
            if (splitPoint == -1)
                throw new HttpException("A header is invalid");

            string name = headerStr.Remove(splitPoint).Trim();
            string value = headerStr.Substring(splitPoint + 1).Trim();
            fields[name] = value;
        }
        return fields;
    }

    public async ValueTask<HttpContent?> ReadContent(HttpFields headers)
    {
        if (!headers.ContentLength.HasValue)
            return null;

        if (!headers.ContentType.HasValue)
            throw new HttpException("Content length was sent but no content type");

        if (headers.ContentLength.Value > 10_000_000)
            throw new HttpException("Too much content (max: 10 MB)");

        byte[] data = new byte[headers.ContentLength.Value];
        await Read(data);
        return new HttpContent(headers.ContentType.Value, data);
    }

    public async ValueTask<(HttpMethod Method, string Path)> ReadRequestHeader()
    {
        //Read initial header
        string header = await ReadLine();

        string[] parts = header.Split(' ', 3, StringSplitOptions.RemoveEmptyEntries);
        if (parts.Length < 2) //breaks specification, must require 3, but impl genuinely only needs 2
            throw new HttpException("Invalid initial header");

        //Method
        if (!Enum.TryParse(parts[0], true, out HttpMethod method))
            throw new HttpException("Unknown HTTP method");

        //Path
        string path = parts[1].Replace("\\", "/");

        return (method, path);
    }

    public async ValueTask<(int Code, string Message)> ReadResponseHeader()
    {
        string responseHeader = await ReadLine();
        string[] parts = responseHeader.Split(' ', 3, StringSplitOptions.RemoveEmptyEntries);
        if (parts.Length != 3)
            throw new HttpException("Invalid initial header");

        if (!parts[0].Equals("HTTP/1.0", StringComparison.OrdinalIgnoreCase) && !parts[0].Equals("HTTP/1.1", StringComparison.OrdinalIgnoreCase))
            throw new HttpException("Unsupported HTTP version");

        if (!int.TryParse(parts[1], out int statusCode))
            throw new HttpException("Invalid status code");

        return (statusCode, parts[2]);
    }

    public async Task<HttpRequest> ReadRequest()
    {
        try
        {
            (HttpMethod method, string path) = await ReadRequestHeader();
            HttpFields fields = await ReadFields();
            HttpContent? content = await ReadContent(fields);
            return new HttpRequest(method, path, fields, content);
        }
        catch (FormatException e)
        {
            throw new HttpException(e.Message, e);
        }
    }

    public async Task<HttpResponse> ReadResponse()
    {
        try
        {
            (int statusCode, string statusMessage) = await ReadResponseHeader();
            HttpFields fields = await ReadFields();
            HttpContent? content = await ReadContent(fields);
            return new HttpResponse(statusCode, statusMessage, fields, content);
        }
        catch (FormatException e)
        {
            throw new HttpException(e.Message, e);
        }
    }

    /// <summary>
    /// Disposes the underlying stream.
    /// </summary>
    public void Dispose()
    {
        ((IDisposable)Stream).Dispose();
    }
}