package config

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

const keyFileName = "encryption.key"

// keyPath returns the full path to the encryption key file.
func keyPath() string {
	return filepath.Join(ConfigDir(), keyFileName)
}

// loadOrCreateKey reads the 32-byte AES key, creating it if absent.
func loadOrCreateKey() ([]byte, error) {
	path := keyPath()
	data, err := os.ReadFile(path)
	if err == nil && len(data) == 32 {
		return data, nil
	}

	key := make([]byte, 32)
	if _, err := io.ReadFull(rand.Reader, key); err != nil {
		return nil, fmt.Errorf("generate key: %w", err)
	}

	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
		return nil, fmt.Errorf("create config dir: %w", err)
	}
	if err := os.WriteFile(path, key, 0o600); err != nil {
		return nil, fmt.Errorf("write key file: %w", err)
	}
	return key, nil
}

// Encrypt encrypts plaintext using AES-256-GCM and returns a base64 string.
func Encrypt(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	key, err := loadOrCreateKey()
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", fmt.Errorf("new cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", fmt.Errorf("new gcm: %w", err)
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", fmt.Errorf("generate nonce: %w", err)
	}
	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

// Decrypt decrypts a base64 AES-256-GCM ciphertext back to plaintext.
func Decrypt(encoded string) (string, error) {
	if encoded == "" {
		return "", nil
	}
	key, err := loadOrCreateKey()
	if err != nil {
		return "", err
	}
	ciphertext, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", fmt.Errorf("decode base64: %w", err)
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", fmt.Errorf("new cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", fmt.Errorf("new gcm: %w", err)
	}
	nonceSize := gcm.NonceSize()
	if len(ciphertext) < nonceSize {
		return "", fmt.Errorf("ciphertext too short")
	}
	nonce, ct := ciphertext[:nonceSize], ciphertext[nonceSize:]
	plaintext, err := gcm.Open(nil, nonce, ct, nil)
	if err != nil {
		return "", fmt.Errorf("decrypt: %w", err)
	}
	return string(plaintext), nil
}