package wireguard

import (
	"bytes"
	"crypto/rand"
	"encoding/base64"
	"errors"

	"golang.org/x/crypto/curve25519"
)

const (
	// PrivateKeySize is the length in bytes of a Wireguard Private Key
	PrivateKeySize = 32
	// PublicKeySize is the length in bytes of a Wireguard Public Key
	PublicKeySize = 32
)

var (
	// ErrInvalidKeySize indicates the key size is wrong
	ErrInvalidKeySize = errors.New("invalid key size")
)

type (
	// PrivateKey is a binary Wireguard Private Key
	PrivateKey []byte
	// PublicKey is a binary Wireguard Public Key
	PublicKey []byte
)

func (key PrivateKey) String() string {
	return encodeKey(key)
}

func (pub PublicKey) String() string {
	return encodeKey(pub)
}

// IsZero tells if the key hasn't been set
func (key PrivateKey) IsZero() bool {
	return len(key) == 0
}

// IsZero tells if the key hasn't been set
func (pub PublicKey) IsZero() bool {
	return len(pub) == 0
}

// Equal checks if two private keys are identical
func (key PrivateKey) Equal(alter PrivateKey) bool {
	return bytes.Equal(key, alter)
}

// Equal checks if two public keys are identical
func (pub PublicKey) Equal(alter PublicKey) bool {
	return bytes.Equal(pub, alter)
}

// PrivateKeyFromBase64 decodes a base64-based string into
// a [PrivateKey]
func PrivateKeyFromBase64(data string) (PrivateKey, error) {
	b, err := decodeKey(data, PrivateKeySize)
	return b, err
}

// PublicKeyFromBase64 decodes a base64-based string into
// a [PublicKey]
func PublicKeyFromBase64(data string) (PublicKey, error) {
	b, err := decodeKey(data, PublicKeySize)
	return b, err
}

func encodeKey(b []byte) string {
	switch {
	case len(b) == 0:
		return ""
	default:
		return base64.StdEncoding.EncodeToString(b)
	}
}

func decodeKey(data string, size int) ([]byte, error) {
	b, err := base64.StdEncoding.DecodeString(data)
	switch {
	case err != nil:
		return []byte{}, err
	case len(b) != size:
		err = ErrInvalidKeySize
		return []byte{}, err
	default:
		return b, nil
	}
}

// NewPrivateKey creates a new PrivateKey
func NewPrivateKey() (PrivateKey, error) {
	var s [PrivateKeySize]byte

	_, err := rand.Read(s[:])
	if err != nil {
		return []byte{}, err
	}

	// apply same clamping as wireguard-go/device/noise-helpers.go
	s[0] &= 0xf8
	s[31] = (s[31] & 0x7f) | 0x40

	return s[:], nil
}

// Public generates the corresponding PublicKey
func (key PrivateKey) Public() PublicKey {
	if len(key) != PrivateKeySize {
		return []byte{}
	}

	out := [PublicKeySize]byte{}
	in := (*[PrivateKeySize]byte)(key)

	curve25519.ScalarBaseMult(&out, in)
	return out[:]
}

// KeyPair holds a Key pair
type KeyPair struct {
	PrivateKey PrivateKey
	PublicKey  PublicKey
}

// NewKeyPair creates a new KeyPair for Wireguard
func NewKeyPair() (*KeyPair, error) {
	key, err := NewPrivateKey()
	if err != nil {
		return nil, err
	}

	out := &KeyPair{
		PrivateKey: key,
		PublicKey:  key.Public(),
	}
	return out, nil
}