package crypto

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"strings"

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

const (
	prefix    = "enc:"
	infoLabel = "veola-v1"
	keyLen    = 32
	nonceLen  = 12
)

// DeriveKey derives a 32-byte AES key from raw key material via HKDF-SHA256.
func DeriveKey(rawKey []byte) ([]byte, error) {
	if len(rawKey) < 32 {
		return nil, errors.New("raw encryption key must be at least 32 bytes")
	}
	r := hkdf.New(sha256.New, rawKey, nil, []byte(infoLabel))
	out := make([]byte, keyLen)
	if _, err := io.ReadFull(r, out); err != nil {
		return nil, fmt.Errorf("derive key: %w", err)
	}
	return out, nil
}

func Encrypt(key []byte, plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, nonceLen)
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	ct := gcm.Seal(nil, nonce, []byte(plaintext), nil)
	buf := make([]byte, 0, len(nonce)+len(ct))
	buf = append(buf, nonce...)
	buf = append(buf, ct...)
	return prefix + base64.StdEncoding.EncodeToString(buf), nil
}

func Decrypt(key []byte, value string) (string, error) {
	if value == "" {
		return "", nil
	}
	if !IsEncrypted(value) {
		// Plaintext passthrough lets us decrypt rows that pre-date encryption
		// without a separate migration.
		return value, nil
	}
	raw, err := base64.StdEncoding.DecodeString(strings.TrimPrefix(value, prefix))
	if err != nil {
		return "", fmt.Errorf("decode ciphertext: %w", err)
	}
	if len(raw) < nonceLen {
		return "", errors.New("ciphertext too short")
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce, ct := raw[:nonceLen], raw[nonceLen:]
	pt, err := gcm.Open(nil, nonce, ct, nil)
	if err != nil {
		return "", fmt.Errorf("decrypt: %w", err)
	}
	return string(pt), nil
}

func IsEncrypted(value string) bool {
	return strings.HasPrefix(value, prefix)
}