Best way to generate NSData object with random byt

Question

If I create a new NSData object of a specific size using dataWithBytes:length:, what is the most efficient way to create the input bytes (20 Mb worth) of random characters, preferably without reading the data in from a file? I need a unique buffer of a specific size each time.

Thanks

Answer 1

The original version has a bug but mine takes care of that and hopefully doesn't introduce any new one. Hope it helps.

- (NSData *)randomDataWithBytes: (NSUInteger)length {
    NSMutableData *mutableData = [NSMutableData dataWithCapacity: length];
    for (unsigned int i = 0; i < size; i++) { 
        NSInteger randomBits = arc4random();
        [mutableData appendBytes: (void *) &randomBits length: 1];
    } return mutableData;
}

Here is its unit test:

NSInteger givenLength = INT16_MAX;
NSData *randomData = [self randomDataWithBytes: givenLength];
STAssertTrue([randomData length] == givenLength,
             @"RandomDataWithBytes Failed Expected size %d and got %d", 
             givenLength, [randomData length]);

Answer 2

I've open sourced my JFRandom class over at github which can do exactly this. Here's a blog post demonstrating how to obtain/use it to achieve your goal...

http://jayfuerstenberg.com/devblog/generating-random-numbers-strings-and-data-in-objective-c

Answer 3

You can create a 20*2^20b NSData object, then append a random 4 byte integer to it 20*2^20/4 times with arc4random(). I believe you need to include stdlib.h (via Generating random numbers in Objective-C).

#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 4

Here's a 3-liner swift version:

Swift 2

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

Swift 3

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

Answer 5

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

The bytes are not 'random', but will contain garbage values (whatever was on the heap before this was run). The advantage being its fast and the code is concise.

Answer 6

Use arc4random_buf to fill the buffer with random bytes

Obj-C

+ (nullable NSData *)radomDataOfSize:(size_t)sizeInBytes
{
    void *buff = malloc(sizeInBytes);
    if (buff == NULL) {
        return nil;
    }
    arc4random_buf(buff, sizeInBytes);

    return [NSData dataWithBytesNoCopy:buff length:sizeInBytes freeWhenDone:YES];
}