package utils

import (
	"crypto/rand"
	"encoding/base64"
	"math/big"
	"time"
)

const (
	// 字符集
	alphanumeric = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	letters      = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
	numbers      = "0123456789"
)

// GenerateRandomString 生成指定长度的随机字符串
func GenerateRandomString(length int) string {
	return generateRandomFromCharset(length, alphanumeric)
}

// GenerateRandomLetters 生成指定长度的随机字母字符串
func GenerateRandomLetters(length int) string {
	return generateRandomFromCharset(length, letters)
}

// GenerateRandomNumbers 生成指定长度的随机数字字符串
func GenerateRandomNumbers(length int) string {
	return generateRandomFromCharset(length, numbers)
}

// generateRandomFromCharset 从指定字符集生成随机字符串
func generateRandomFromCharset(length int, charset string) string {
	result := make([]byte, length)
	charsetLen := big.NewInt(int64(len(charset)))
	
	for i := 0; i < length; i++ {
		randomIndex, err := rand.Int(rand.Reader, charsetLen)
		if err != nil {
			// 如果加密随机数生成失败，回退到时间种子
			return generateRandomFallback(length, charset)
		}
		result[i] = charset[randomIndex.Int64()]
	}
	
	return string(result)
}

// generateRandomFallback 回退的随机生成方法
func generateRandomFallback(length int, charset string) string {
	// 使用时间作为种子的简单随机生成
	seed := time.Now().UnixNano()
	result := make([]byte, length)
	
	for i := 0; i < length; i++ {
		seed = seed*1103515245 + 12345
		result[i] = charset[(seed/65536)%int64(len(charset))]
	}
	
	return string(result)
}

// GenerateSecureToken 生成安全令牌
func GenerateSecureToken(length int) (string, error) {
	bytes := make([]byte, length)
	if _, err := rand.Read(bytes); err != nil {
		return "", err
	}
	return base64.URLEncoding.EncodeToString(bytes), nil
}

// GenerateID 生成唯一ID
func GenerateID() string {
	timestamp := time.Now().UnixNano()
	random := GenerateRandomString(8)
	return base64.URLEncoding.EncodeToString([]byte(string(timestamp) + random))[:16]
}