使用特定长度的随机字节生成NSData对象的最佳方法？

Question

如果我使用dataWithBytes:length:创建一个特定大小的新NSData对象,那么创建输入字节(20 Mb值)随机字符的最有效方法是什么,最好不要从文件中读取数据？我每次都需要一个特定大小的独特缓冲区.

谢谢

Answer 1

你可以创建一个20*2 ^ 20b的NSData对象,然后在它上面添加一个随机的4字节整数20*2 ^ 20/4次arc4random().我相信你需要包括stdlib.h(通过在Objective-C中生成随机数).

#include <stdlib.h>

-(NSData*)create20mbRandomNSData
{
  int twentyMb           = 20971520;
  NSMutableData* theData = [NSMutableData dataWithCapacity:twentyMb];
  for( unsigned int i = 0 ; i < twentyMb/4 ; ++i )
  { 
    u_int32_t randomBits = arc4random();
    [theData appendBytes:(void*)&randomBits length:4];
  }
  return theData;
}

Answer 2

void * bytes = malloc(numberOfBytes);
NSData * data = [NSData dataWithBytes:bytes length:numberOfBytes];
free(bytes);

这些字节不是"随机",而是包含垃圾值(在运行之前堆上的任何内容).其优点是速度快,代码简洁.

Answer 3

这是一个3线程的快速版本:

斯威夫特2

let length = 2048
let bytes = [UInt32](count: length, repeatedValue: 0).map { _ in arc4random() }
let data = NSData(bytes: bytes, length: bytes.count * sizeof(UInt32))

斯威夫特3

let bytes = [UInt32](repeating: 0, count: length).map { _ in arc4random() }
let data = Data(bytes: bytes, count: length)

Answer 4

您可以考虑使用CCRandomGenerateBytes函数from CommonCrypto来生成随机数据.喜欢:

func generateBytes(length : Int) throws -> NSData? {
    var bytes = [UInt8](count: length, repeatedValue: UInt8(0))
    let statusCode = CCRandomGenerateBytes(&bytes, bytes.count)
    if statusCode != CCRNGStatus(kCCSuccess) {
        return nil
    }
    return NSData(bytes: bytes, length: bytes.count)
}

Answer 5

斯威夫特 3：

import Security

func randomBytes(length: Int) -> Data {
    var data = Data(capacity: length)
    data.withUnsafeMutableBytes { (bytes: UnsafeMutablePointer<UInt8>) -> Void in
        let _ = SecRandomCopyBytes(kSecRandomDefault, length, bytes)
    }
    return data
}

斯威夫特 5 更新：

func randomBytes(length: Int) -> Data? {
    var data = Data(count: length)
    let result = data.withUnsafeMutableBytes {
        SecRandomCopyBytes(kSecRandomDefault, 10, $0.baseAddress!)
    }
    guard result == errSecSuccess else {
        return nil
    }
    return data
}

感谢这篇文章。