package shadowaead

import (
	"crypto/rand"
	"errors"
	"io"

	"github.com/go-gost/go-shadowsocks2/core"
)

// ErrShortPacket means that the packet is too short for a valid encrypted packet.
var ErrShortPacket = errors.New("short packet")

var _zerononce [128]byte // read-only. 128 bytes is more than enough.

// Pack encrypts plaintext using Cipher with a randomly generated salt and
// returns a slice of dst containing the encrypted packet and any error occurred.
// Ensure len(dst) >= ciph.SaltSize() + len(plaintext) + aead.Overhead().
func Pack(dst, plaintext []byte, ciph core.ShadowCipher) ([]byte, error) {
	saltSize := ciph.SaltSize()
	salt := dst[:saltSize]
	if _, err := io.ReadFull(rand.Reader, salt); err != nil {
		return nil, err
	}

	aead, err := ciph.Encrypter(nil, salt)
	if err != nil {
		return nil, err
	}
	AddSalt(salt)

	if len(dst) < saltSize+len(plaintext)+aead.Overhead() {
		return nil, io.ErrShortBuffer
	}
	b := aead.Seal(dst[saltSize:saltSize], _zerononce[:aead.NonceSize()], plaintext, nil)
	return dst[:saltSize+len(b)], nil
}

// Unpack decrypts pkt using Cipher and returns a slice of dst containing the decrypted payload and any error occurred.
// Ensure len(dst) >= len(pkt) - aead.SaltSize() - aead.Overhead().
func Unpack(dst, pkt []byte, ciph core.ShadowCipher) ([]byte, error) {
	saltSize := ciph.SaltSize()
	if len(pkt) < saltSize {
		return nil, ErrShortPacket
	}
	salt := pkt[:saltSize]
	aead, err := ciph.Decrypter(nil, salt)
	if err != nil {
		return nil, err
	}
	if CheckSalt(salt) {
		return nil, ErrRepeatedSalt
	}
	if len(pkt) < saltSize+aead.Overhead() {
		return nil, ErrShortPacket
	}
	if saltSize+len(dst)+aead.Overhead() < len(pkt) {
		return nil, io.ErrShortBuffer
	}
	b, err := aead.Open(dst[:0], _zerononce[:aead.NonceSize()], pkt[saltSize:], nil)

	return b, err
}