// Package crypto 提供口令哈希（argon2id）与字段加密（AES-256-GCM，enc:v1: 前缀）原语。
package crypto

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"io"
	"math/big"
	"os"
	"path/filepath"
	"strings"
	"sync"

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

const encPrefix = "enc:v1:"

var (
	encKey     []byte
	encKeyOnce sync.Once
	encKeyErr  error
)

// InitEncryptionKey 初始化字段加密密钥：
// 优先环境变量 FLOWSTACK_ENCRYPTION_KEY，否则首次启动生成 32 字节随机密钥
// 写入 dataDir/encryption.key（0600），重启复用。
func InitEncryptionKey(dataDir string) error {
	encKeyOnce.Do(func() {
		if env := os.Getenv("FLOWSTACK_ENCRYPTION_KEY"); env != "" {
			encKey = []byte(env)
			return
		}
		keyFile := filepath.Join(dataDir, "encryption.key")
		if data, err := os.ReadFile(keyFile); err == nil && len(data) == 32 {
			encKey = data
			return
		}
		key := make([]byte, 32)
		if _, err := rand.Read(key); err != nil {
			encKeyErr = err
			return
		}
		if err := os.WriteFile(keyFile, key, 0o600); err != nil {
			encKeyErr = err
			return
		}
		encKey = key
	})
	return encKeyErr
}

func gcm() (cipher.AEAD, error) {
	if encKey == nil {
		return nil, errors.New("加密密钥未初始化")
	}
	block, err := aes.NewCipher(encKey)
	if err != nil {
		return nil, err
	}
	return cipher.NewGCM(block)
}

// EncryptField AES-256-GCM 加密，返回 enc:v1:base64；空串原样返回
func EncryptField(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	aead, err := gcm()
	if err != nil {
		return "", err
	}
	nonce := make([]byte, aead.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	ciphertext := aead.Seal(nonce, nonce, []byte(plaintext), nil)
	return encPrefix + base64.StdEncoding.EncodeToString(ciphertext), nil
}

// DecryptField 解密 enc:v1: 前缀密文；非该格式原样返回（向后兼容）
func DecryptField(encrypted string) (string, error) {
	if !strings.HasPrefix(encrypted, encPrefix) {
		return encrypted, nil
	}
	data, err := base64.StdEncoding.DecodeString(strings.TrimPrefix(encrypted, encPrefix))
	if err != nil {
		return "", err
	}
	aead, err := gcm()
	if err != nil {
		return "", err
	}
	if len(data) < aead.NonceSize() {
		return "", errors.New("ciphertext too short")
	}
	nonce, ciphertext := data[:aead.NonceSize()], data[aead.NonceSize():]
	plain, err := aead.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plain), nil
}

// IsEncrypted 判断字段是否为加密格式
func IsEncrypted(v string) bool {
	return strings.HasPrefix(v, encPrefix)
}

// Mask 敏感值脱敏展示（保留前后各 1/4，中间星号，最长 24 位）
func Mask(s string) string {
	if s == "" {
		return ""
	}
	r := []rune(s)
	if len(r) <= 4 {
		return strings.Repeat("*", len(r))
	}
	keep := len(r) / 4
	if keep > 4 {
		keep = 4
	}
	return string(r[:keep]) + strings.Repeat("*", 6) + string(r[len(r)-keep:])
}

// HashPassword 生成 argon2id 口令哈希（salt+hash base64 编码）
func HashPassword(password string) (string, error) {
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	hash := argon2.IDKey([]byte(password), salt, 3, 64*1024, 4, 32)
	combined := append(salt, hash...)
	return base64.StdEncoding.EncodeToString(combined), nil
}

// VerifyPassword 常数时间比对口令哈希
func VerifyPassword(password, hashStr string) bool {
	combined, err := base64.StdEncoding.DecodeString(hashStr)
	if err != nil || len(combined) < 16 {
		return false
	}
	salt := combined[:16]
	hash := combined[16:]
	newHash := argon2.IDKey([]byte(password), salt, 3, 64*1024, 4, 32)
	return subtle.ConstantTimeCompare(hash, newHash) == 1
}

// passwordAlphabet 去掉易混淆字符（0O1lI）的密码字母表
const passwordAlphabet = "23456789abcdefghjkmnpqrstuvwxyzABCDEFGHJKMNPQRSTUVWXYZ"

// RandomPassword 生成 n 位随机密码（crypto/rand，无易混淆字符）
func RandomPassword(n int) (string, error) {
	max := big.NewInt(int64(len(passwordAlphabet)))
	buf := make([]byte, n)
	for i := range buf {
		idx, err := rand.Int(rand.Reader, max)
		if err != nil {
			return "", err
		}
		buf[i] = passwordAlphabet[idx.Int64()]
	}
	return string(buf), nil
}