package lorawan

import (
	"bytes"
	"crypto/aes"
	"errors"
)

// Otaa is used to store Over The Air Activation data of a LoRaWAN session
type Otaa struct {
	DevEUI   [8]uint8
	AppEUI   [8]uint8
	AppKey   [16]uint8
	DevNonce [2]uint8
	AppNonce [3]uint8
	NetID    [3]uint8
}

// Set configures the Otaa AppEUI, DevEUI, AppKey for the device
func (o *Otaa) Set(appEUI [8]uint8, devEUI [8]uint8, appKey [16]uint8) {
	o.AppEUI = appEUI
	o.DevEUI = devEUI
	o.AppKey = appKey
}

// GenerateJoinRequest Generates a LoraWAN Join request
func (o *Otaa) GenerateJoinRequest(buf []uint8) error {
	// TODO: Add checks
	buf = append(buf, 0x00)
	buf = append(buf, revertByteArray(o.AppEUI[:])...)
	buf = append(buf, revertByteArray(o.DevEUI[:])...)
	buf = append(buf, revertByteArray(o.DevNonce[:])...)
	mic := genPayloadMIC(buf, o.AppKey)
	buf = append(buf, mic[:]...)
	return nil
}

// DecodeJoinAccept Decodes a Lora Join Accept packet
func (o *Otaa) DecodeJoinAccept(phyPload []uint8, s *Session) error {
	if len(phyPload) < 12 {
		return errors.New("Bad packet")
	}
	data := phyPload[1:] // Remove trailing 0x20

	// Prepare AES Cipher
	block, err := aes.NewCipher(o.AppKey[:])
	if err != nil {
		return errors.New("Lora Cipher error 1")
	}
	buf := make([]byte, len(data))
	for k := 0; k < len(data)/aes.BlockSize; k++ {
		block.Encrypt(buf[k*aes.BlockSize:], data[k*aes.BlockSize:])
	}

	copy(o.AppNonce[:], buf[0:3])
	copy(o.NetID[:], buf[3:6])
	copy(s.DevAddr[:], buf[6:10])
	s.DLSettings = buf[10]
	s.RXDelay = buf[11]

	if len(buf) > 16 {
		copy(s.CFList[:], buf[12:28])
	}
	rxMic := buf[len(buf)-4:]

	dataMic := []byte{}
	dataMic = append(dataMic, phyPload[0])
	dataMic = append(dataMic, o.AppNonce[:]...)
	dataMic = append(dataMic, o.NetID[:]...)
	dataMic = append(dataMic, s.DevAddr[:]...)
	dataMic = append(dataMic, s.DLSettings)
	dataMic = append(dataMic, s.RXDelay)
	dataMic = append(dataMic, s.CFList[:]...)
	computedMic := genPayloadMIC(dataMic[:], o.AppKey)
	if !bytes.Equal(computedMic[:], rxMic[:]) {
		return errors.New("invalid Mic")
	}
	// Generate NwkSKey
	// NwkSKey = aes128_encrypt(AppKey, 0x01|AppNonce|NetID|DevNonce|pad16)
	sKey := []byte{}
	sKey = append(sKey, 0x01)
	sKey = append(sKey, o.AppNonce[:]...)
	sKey = append(sKey, o.NetID[:]...)
	sKey = append(sKey, o.DevNonce[:]...)
	for i := 0; i < 7; i++ {
		sKey = append(sKey, 0x00) // PAD to 16
	}
	block.Encrypt(buf, sKey)
	copy(s.NwkSKey[:], buf[0:16])

	// Generate AppSKey
	// AppSKey = aes128_encrypt(AppKey, 0x02|AppNonce|NetID|DevNonce|pad16)
	sKey[0] = 0x02
	block.Encrypt(buf, sKey)
	copy(s.AppSKey[:], buf[0:16])

	// Reset counters
	s.FCntDown = 0
	s.FCntUp = 0

	return nil
}