package aep

import "fmt"

type Function uint8

const (
	EchoRequest Function = 1
	EchoReply   Function = 2
)

// Packet represents an AEP packet.
type Packet struct {
	Function Function
	Data     []byte
}

// Marshal marshals an AEP packet.
func (p *Packet) Marshal() ([]byte, error) {
	if p.Function < 1 || p.Function > 2 {
		return nil, fmt.Errorf("invalid AEP function %d", p.Function)
	}
	if len(p.Data) == 0 {
		return nil, fmt.Errorf("empty AEP packet")
	}
	return append([]byte{byte(p.Function)}, p.Data...), nil
}

// Unmarshal unmarshals an AEP packet.
func Unmarshal(data []byte) (*Packet, error) {
	if len(data) < 1 {
		return nil, fmt.Errorf("insufficient input length %d for AEP packet", len(data))
	}
	return &Packet{
		Function: Function(data[0]),
		Data:     data[1:],
	}, nil
}