Luis de la Rosa

Generating random numbers in Cocoa

I'm writing an application to pick prize winners for the RubyNation conference that's coming up here in the Washington DC area. As part of that, I have to generate random numbers. So I went looking for how to generate random numbers in Cocoa. I was looking for something reasonably fast, built-in to Mac OS X, had an easy to use API, and had the most randomness. I defined a random number generation algorithm as being more random if it had a larger range of numbers that it could generate.

The basic usage of a random number generator is something like this:

  1. Seed (initialize) the random number generator
  2. Generate a random number
  3. Modulo the random number against your maximum number to get a number from 0 to your maximum number - 1.

Alright, first off I looked for Objective-C libraries. You know, maybe there was an NSRandom or something like that. There isn't really one, so I went to look for C functions instead.

Digging into my memory banks, I remembered from my early C days that the standard C library call is srand() with a seed to initialize and then rand() to get the random number. So I looked up srand in the Apple Developer Connection (also known as ADC) and came up with the laughable description:

rand, rand_r, srand, sranddev -- bad random number generator ... These interfaces are obsoleted by random(3).

Alright, so onto random. I looked up random in the ADC to see its Mac OS X Manual page. Alright, so it can generate numbers from 0 to (2**31) - 1. You can seed it with srandom() - which could be useful because the same seed will generate the same sequence of random numbers - useful for replaying a game sequence. A better seeding is to use srandomdev() which creates a state array which can't be guessed by attackers and effectively uses the /dev/random device.

OK that seemed pretty good, but I read on and found this intriguing line:

Applications requiring cryptographic quality randomness should use arc4random(3).

Alright, so now I look up arc4random on ADC. arc4random uses the Alleged RC4 cipher, hence the ARC4. It has a range of 0 to (2**32 - 1), which is twice the range of random. It uses the /dev/urandom device. And best of all, in my opinion, it doesn't require seeding/initializing since it initializes itself.

So the winner in my book for generating random numbers in Cocoa (really any Objective-C or C program on Mac OS X) is arc4random.