package backup

import (
	"bytes"
	"crypto/rand"
	"errors"
	"os"
	"path/filepath"
	"syscall"

	"golang.org/x/crypto/argon2"
)

const passwordMagic = "DSHPASS\x01"
const MaxEncryptedBytes int64 = int64(maxArchiveBytes + len(passwordMagic) + 16)

// The format fixes KDF costs; untrusted archives cannot request arbitrary memory.
func passwordKey(password string, salt []byte) ([]byte, error) {
	if len(password) == 0 || len(password) > 1024 {
		return nil, errors.New("backup password required (maximum 1024 bytes)")
	}
	return argon2.IDKey([]byte(password), salt, 3, 64*1024, 2, 32), nil
}

func CreateWithPassword(home, destination, password string, metadata Metadata) error {
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return err
	}
	key, err := passwordKey(password, salt)
	if err != nil {
		return err
	}
	defer clear(key)
	dir, err := os.MkdirTemp(filepath.Dir(destination), ".password-backup-")
	if err != nil {
		return err
	}
	defer os.RemoveAll(dir)
	inner := filepath.Join(dir, "archive")
	if err = CreateWithMetadata(home, inner, key, metadata); err != nil {
		return err
	}
	data, err := os.ReadFile(inner)
	if err != nil {
		return err
	}
	data = append(append([]byte(passwordMagic), salt...), data...)
	root, err := os.OpenRoot(filepath.Dir(destination))
	if err != nil {
		return err
	}
	defer root.Close()
	return replace(root, filepath.Base(destination), data)
}

// Prepare authenticates and validates into a private staging home before the caller
// stops a container or snapshots its live home. Only ciphertext is retained.
func Prepare(archive, password string, legacyKey []byte) (restore func(string) error, cleanup func(), err error) {
	f, err := os.OpenFile(archive, os.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, 0)
	if err != nil {
		return nil, nil, err
	}
	data, err := readBounded(f, MaxEncryptedBytes)
	f.Close()
	if err != nil {
		return nil, nil, err
	}
	key := append([]byte(nil), legacyKey...)
	if bytes.HasPrefix(data, []byte(passwordMagic)) {
		if len(data) < len(passwordMagic)+16 {
			return nil, nil, errors.New("truncated backup")
		}
		key, err = passwordKey(password, data[len(passwordMagic):len(passwordMagic)+16])
		if err != nil {
			return nil, nil, err
		}
		data = data[len(passwordMagic)+16:]
	}
	dir, err := os.MkdirTemp("", "dsh-backup-validate-")
	if err != nil {
		clear(key)
		return nil, nil, err
	}
	cleanup = func() { clear(key); os.RemoveAll(dir) }
	inner := filepath.Join(dir, "archive")
	if err = os.WriteFile(inner, data, 0600); err == nil {
		err = os.Mkdir(filepath.Join(dir, "home"), 0700)
	}
	if err == nil {
		err = Restore(inner, filepath.Join(dir, "home"), key)
	}
	if err != nil {
		cleanup()
		return nil, nil, err
	}
	if err = os.RemoveAll(filepath.Join(dir, "home")); err != nil {
		cleanup()
		return nil, nil, err
	}
	return func(home string) error { return Restore(inner, home, key) }, cleanup, nil
}