// Package crypto 密码哈希（argon2id）与对称加密（AES-GCM）。
package crypto

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

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

// PasswordHasher argon2id 参数（来自配置）。
type PasswordHasher struct {
	Time    uint32
	Memory  uint32
	Threads uint8
	KeyLen  uint32
	SaltLen int
}

func NewPasswordHasher(time, memory uint32, threads uint8, keyLen uint32, saltLen int) *PasswordHasher {
	return &PasswordHasher{Time: time, Memory: memory, Threads: threads, KeyLen: keyLen, SaltLen: saltLen}
}

// HashPassword 编码为 $argon2id$v=19$m=...,t=...,p=...$salt$hash
func (h *PasswordHasher) HashPassword(password string) (string, error) {
	salt := make([]byte, h.SaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(password), salt, h.Time, h.Memory, h.Threads, h.KeyLen)
	enc := base64.RawStdEncoding
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s",
		h.Memory, h.Time, h.Threads, enc.EncodeToString(salt), enc.EncodeToString(key)), nil
}

// VerifyPassword 校验密码，常数时间比较。
func (h *PasswordHasher) VerifyPassword(encoded, password string) (bool, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, errors.New("invalid hash format")
	}
	var memory, time uint32
	var threads uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time, &threads); err != nil {
		return false, err
	}
	enc := base64.RawStdEncoding
	salt, err := enc.DecodeString(parts[4])
	if err != nil {
		return false, err
	}
	want, err := enc.DecodeString(parts[5])
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, time, memory, threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

// ---------------------------------------------------------------------------
// AES-GCM 渠道密钥加密

// Encryptor 用主密钥加解密渠道上游 key。
type Encryptor struct {
	key []byte
}

// NewEncryptor 主密钥必须为 16/24/32 字节；不足时用 SHA-256 派生固定 32 字节。
func NewEncryptor(master string) *Encryptor {
	key := []byte(master)
	switch len(key) {
	case 16, 24, 32:
	default:
		key = sha256Sum(master)
	}
	return &Encryptor{key: key}
}

// Encrypt 输出 base64(nonce || ciphertext)
func (e *Encryptor) Encrypt(plain string) (string, error) {
	block, err := aes.NewCipher(e.key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return "", err
	}
	ct := gcm.Seal(nil, nonce, []byte(plain), nil)
	return base64.StdEncoding.EncodeToString(append(nonce, ct...)), nil
}

// Decrypt 解析 Encrypt 的输出。
func (e *Encryptor) Decrypt(enc string) (string, error) {
	raw, err := base64.StdEncoding.DecodeString(enc)
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(e.key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	if len(raw) < gcm.NonceSize() {
		return "", errors.New("ciphertext too short")
	}
	nonce, ct := raw[:gcm.NonceSize()], raw[gcm.NonceSize():]
	plain, err := gcm.Open(nil, nonce, ct, nil)
	if err != nil {
		return "", err
	}
	return string(plain), nil
}