package tio

import (
	"io"
)

// QueueSet holds one or many Queue objects and helps close them in an orderly way.
type QueueSet interface {
	io.Closer
	Queues() []Queue

	// Add takes a tun fd, adds it to the set, and prepares it for use as a Queue.
	Add(fd int) error
}

// Capabilities advertises which kernel offload features a Queue successfully negotiated.
// Callers consult this to decide which coalescers to wire onto the write path.
type Capabilities struct {
	// TSO means the FD was opened with IFF_VNET_HDR and the kernel agreed to TUN_F_TSO4|TSO6,
	// and WriteGSO with GSOProtoTCP is safe.
	TSO bool
	// USO means the kernel additionally agreed to TUN_F_USO4|USO6,
	// so WriteGSO with GSOProtoUDP is safe. Linux ≥ 6.2.
	USO bool
}

// Queue is a readable/writable Poll queue.
// Concurrency contract: a single read goroutine drives Read; plain Write is safe for concurrent callers;
// WriteGSO (on Queues that implement GSOWriter) is single-writer per queue.
//
// Close on an individual Queue does NOT unblock a Read parked in poll — closing an fd
// never wakes its pollers. Orderly teardown goes through the owning QueueSet's Close,
// which first signals a shared shutdown eventfd every reader polls alongside its own fd.
// That eventfd is a set-wide kill switch: once signaled, every Queue in the set returns
// os.ErrClosed from Read, so it cannot be used to stop a single Queue.
type Queue interface {
	io.Closer

	// Read returns one or more packets.
	// The returned Packet.Bytes slices are borrowed from the Queue's internal buffer and are only valid
	// until the next Read or Close on this Queue.
	// A Packet may carry a GSO/USO superpacket (see GSOInfo)
	// Single-reader only: not safe for concurrent Reads (it reuses per-queue rx scratch each call).
	Read() ([]Packet, error)

	// Write emits a single packet on the plaintext (outside→inside) delivery path.
	// Safe for concurrent use.
	Write(p []byte) (int, error)
}

// Packet is the unit Queue.Read returns.
// Bytes points into the queue's internal buffer and is only valid until the next Read or Close on the queue that produced it.
// GSO is the zero value for an already-segmented IP datagram;
// when non-zero it describes a kernel-supplied TSO/USO superpacket the caller must segment before consuming.
type Packet struct {
	Bytes []byte
	GSO   GSOInfo
}

// GSOInfo describes a kernel-supplied superpacket sitting in Packet.Bytes.
// The zero value means Bytes is one regular IP datagram and no segmentation is required.
type GSOInfo struct {
	// Size is the GSO segment size: max payload bytes per segment
	// (== TCP MSS for TSO, == UDP payload chunk for USO). Zero means not a superpacket.
	Size uint16
	// HdrLen is the total L3+L4 header length within Bytes (already corrected via correctHdrLen, so safe to slice on).
	HdrLen uint16
	// CsumStart is the L4 header offset inside Bytes (== L3 header length).
	CsumStart uint16
	// Proto picks the L4 protocol (TCP or UDP) so the segmenter knows which checksum/header layout to apply.
	Proto GSOProto
}

// IsSuperpacket reports whether g describes a multi-segment GSO/USO
// superpacket that needs segmentation before its bytes can be encrypted and sent on the wire.
func (g GSOInfo) IsSuperpacket() bool { return g.Size > 0 }

// Clone returns a Packet whose Bytes is a freshly allocated copy of p.Bytes,
// safe to retain past the next Read or Close on the originating Queue.
// GSO metadata is copied verbatim.
// Use this only when a caller needs the data to outlive the borrowed-slice contract.
func (p Packet) Clone() Packet {
	if p.Bytes == nil {
		return p
	}
	cp := make([]byte, len(p.Bytes))
	copy(cp, p.Bytes)
	return Packet{Bytes: cp, GSO: p.GSO}
}

// CapsProvider is an optional interface implemented by Queues that negotiate kernel offload features at open time.
// Callers pick a write-path coalescer based on the result.
// Queues that don't implement it are treated as having no offload capability.
type CapsProvider interface {
	Capabilities() Capabilities
}

// GSOProto selects the L4 protocol for a GSO superpacket.
// Determines which VIRTIO_NET_HDR_GSO_* type the writer stamps and which checksum offset
// inside the transport header virtio NEEDS_CSUM expects.
type GSOProto uint8

const (
	GSOProtoUnknown GSOProto = iota
	GSOProtoTCP
	GSOProtoUDP
)

// GSOWriter is implemented by Queues that can emit a TCP or UDP superpacket
// assembled from a header prefix plus one or more borrowed payload fragments,
// in a single vectored write (writev with a leading virtio_net_hdr).
// This lets the coalescer avoid copying payload bytes between the caller's decrypt buffer and the TUN.
// Backends without GSO support do not implement this interface and coalescing is skipped.
//
// hdr contains the IPv4/IPv6 header prefix (mutable: callers will have filled in total length and IP csum).
// transportHdr is the TCP or UDP header
// (mutable: the L4 checksum field must hold the pseudo-header partial, single-fold not inverted, per virtio NEEDS_CSUM semantics).
// pays are non-overlapping payload fragments whose concatenation is the full superpacket payload.
// They are read-only from the writer's perspective and must remain valid until the call returns.
// Every segment in pays except possibly the last must be exactly the same size.
// proto picks the L4 protocol so the writer knows which gsoType / CsumOffset to set.
//
// Callers should also consult CapsProvider (via SupportsGSO) for the per-protocol negotiated capability:
// USO may not have been negotiated even when TSO was.
type GSOWriter interface {
	io.Writer
	CapsProvider
	WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, proto GSOProto) error
}

// SupportsGSO reports whether w implements GSOWriter and the underlying
// queue advertises the negotiated capability for `want`.
func SupportsGSO(w io.Writer, want GSOProto) (GSOWriter, bool) {
	gw, ok := w.(GSOWriter)
	if !ok {
		return nil, false
	}
	caps := gw.Capabilities()
	switch want {
	case GSOProtoTCP:
		return gw, caps.TSO
	case GSOProtoUDP:
		return gw, caps.USO
	default:
		return gw, false
	}
}