// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package channel provides the implemention of channel-based data-link layer
// endpoints. Such endpoints allow injection of inbound packets and store
// outbound packets in a channel.
package channel

import (
	"gvisor.googlesource.com/gvisor/pkg/tcpip"
	"gvisor.googlesource.com/gvisor/pkg/tcpip/buffer"
	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
)

// PacketInfo holds all the information about an outbound packet.
type PacketInfo struct {
	Header  buffer.View
	Payload buffer.View
	Proto   tcpip.NetworkProtocolNumber
}

// Endpoint is link layer endpoint that stores outbound packets in a channel
// and allows injection of inbound packets.
type Endpoint struct {
	dispatcher stack.NetworkDispatcher
	mtu        uint32
	linkAddr   tcpip.LinkAddress

	// C is where outbound packets are queued.
	C chan PacketInfo
}

// New creates a new channel endpoint.
func New(size int, mtu uint32, linkAddr tcpip.LinkAddress) (tcpip.LinkEndpointID, *Endpoint) {
	e := &Endpoint{
		C:        make(chan PacketInfo, size),
		mtu:      mtu,
		linkAddr: linkAddr,
	}

	return stack.RegisterLinkEndpoint(e), e
}

// Drain removes all outbound packets from the channel and counts them.
func (e *Endpoint) Drain() int {
	c := 0
	for {
		select {
		case <-e.C:
			c++
		default:
			return c
		}
	}
}

// Inject injects an inbound packet.
func (e *Endpoint) Inject(protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
	e.InjectLinkAddr(protocol, "", vv)
}

// InjectLinkAddr injects an inbound packet with a remote link address.
func (e *Endpoint) InjectLinkAddr(protocol tcpip.NetworkProtocolNumber, remote tcpip.LinkAddress, vv buffer.VectorisedView) {
	e.dispatcher.DeliverNetworkPacket(e, remote, "" /* local */, protocol, vv.Clone(nil))
}

// Attach saves the stack network-layer dispatcher for use later when packets
// are injected.
func (e *Endpoint) Attach(dispatcher stack.NetworkDispatcher) {
	e.dispatcher = dispatcher
}

// IsAttached implements stack.LinkEndpoint.IsAttached.
func (e *Endpoint) IsAttached() bool {
	return e.dispatcher != nil
}

// MTU implements stack.LinkEndpoint.MTU. It returns the value initialized
// during construction.
func (e *Endpoint) MTU() uint32 {
	return e.mtu
}

// Capabilities implements stack.LinkEndpoint.Capabilities.
func (*Endpoint) Capabilities() stack.LinkEndpointCapabilities {
	return 0
}

// MaxHeaderLength returns the maximum size of the link layer header. Given it
// doesn't have a header, it just returns 0.
func (*Endpoint) MaxHeaderLength() uint16 {
	return 0
}

// LinkAddress returns the link address of this endpoint.
func (e *Endpoint) LinkAddress() tcpip.LinkAddress {
	return e.linkAddr
}

// WritePacket stores outbound packets into the channel.
func (e *Endpoint) WritePacket(_ *stack.Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) *tcpip.Error {
	p := PacketInfo{
		Header:  hdr.View(),
		Proto:   protocol,
		Payload: payload.ToView(),
	}

	select {
	case e.C <- p:
	default:
	}

	return nil
}