package auth

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

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

// Argon2 参数（DEVELOPMENT 第 12 节 / OWASP 最低配置）。
const (
	ArgonMemoryKiB = 19 * 1024 // 19 MiB
	ArgonTime      = 2
	ArgonThreads   = 1
	ArgonKeyLen    = 32
	ArgonSaltLen   = 16
)

// ErrInvalidPHC 表示 PHC 字符串无法解析或参数不支持。
var ErrInvalidPHC = errors.New("auth: invalid phc")

// HashPassword 用 argon2id 生成 PHC 字符串（不含并发池；池见 HashPool）。
func HashPassword(password string) (string, error) {
	salt := make([]byte, ArgonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	hash := argon2.IDKey([]byte(password), salt, ArgonTime, ArgonMemoryKiB, ArgonThreads, ArgonKeyLen)
	return encodePHC(salt, hash), nil
}

// VerifyPassword 常量时间比较密码与 PHC；不匹配时 ok=false 且 err=nil。
func VerifyPassword(password, phc string) (bool, error) {
	salt, hash, err := decodePHC(phc)
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, ArgonTime, ArgonMemoryKiB, ArgonThreads, ArgonKeyLen)
	if subtle.ConstantTimeCompare(got, hash) == 1 {
		return true, nil
	}
	return false, nil
}

func encodePHC(salt, hash []byte) string {
	return fmt.Sprintf(
		"$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		ArgonMemoryKiB,
		ArgonTime,
		ArgonThreads,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(hash),
	)
}

func decodePHC(phc string) (salt, hash []byte, err error) {
	// $argon2id$v=19$m=19456,t=2,p=1$salt$hash
	parts := strings.Split(phc, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return nil, nil, ErrInvalidPHC
	}
	var version int
	if _, scanErr := fmt.Sscanf(parts[2], "v=%d", &version); scanErr != nil || version != argon2.Version {
		return nil, nil, ErrInvalidPHC
	}
	var m, t, p int
	if _, scanErr := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &m, &t, &p); scanErr != nil {
		return nil, nil, ErrInvalidPHC
	}
	if m != ArgonMemoryKiB || t != ArgonTime || p != ArgonThreads {
		return nil, nil, ErrInvalidPHC
	}
	salt, err = base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return nil, nil, ErrInvalidPHC
	}
	hash, err = base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return nil, nil, ErrInvalidPHC
	}
	if len(salt) == 0 || len(hash) == 0 {
		return nil, nil, ErrInvalidPHC
	}
	return salt, hash, nil
}