// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Buffers;
using System.Net.Http;
 
namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http3;
 
internal sealed class Http3FrameReader
{
    /* https://quicwg.org/base-drafts/draft-ietf-quic-http.html#frame-layout
         0                   1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                           Type (i)                          ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                          Length (i)                         ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                       Frame Payload (*)                     ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    */
    internal static bool TryReadFrame(ref ReadOnlySequence<byte> readableBuffer, Http3RawFrame frame, out ReadOnlySequence<byte> framePayload)
    {
        framePayload = ReadOnlySequence<byte>.Empty;
        SequencePosition consumed;
 
        var type = VariableLengthIntegerHelper.GetInteger(readableBuffer, out consumed, out _);
        if (type == -1)
        {
            return false;
        }
 
        var firstLengthBuffer = readableBuffer.Slice(consumed);
 
        var length = VariableLengthIntegerHelper.GetInteger(firstLengthBuffer, out consumed, out _);
 
        // Make sure the whole frame is buffered
        if (length == -1)
        {
            return false;
        }
 
        var startOfFramePayload = readableBuffer.Slice(consumed);
        if (startOfFramePayload.Length < length)
        {
            return false;
        }
 
        frame.Length = length;
        frame.Type = (Http3FrameType)type;
 
        // The remaining payload minus the extra fields
        framePayload = startOfFramePayload.Slice(0, length);
        readableBuffer = readableBuffer.Slice(framePayload.End);
 
        return true;
    }
}