Re: [gentoo-user] OT: Mapping random numbers (PRNG)

[Marc Joliet]

Am Sat, 7 Jun 2014 12:19:11 +0300
schrieb Matti Nykyri <Matti.Nykyri@×××.fi>:

> On Sat, Jun 07, 2014 at 12:03:29AM +0300, Matti Nykyri wrote:
[...]
> unsigned char get_6bits ()
> {
> 	static unsigned int rand = 0; //(sizeof(int) = 32)

Just an itty bitty nitpick: since you already require C99 as per the for loop
below, you might as well use uint32_t (from stdint.h) instead of assuming that
sizeof(int) == 32 :) .

> 	static char bits_avail = 0;
> 	unsigned char result = 0;
>
> 	//get 2 bits 3 times: 32 is devidable by 2
> 	for (int i = 0; i < 3; i++) { // --std=c99
> 		//Fill buffer if it is empty!
> 		if (!bits_avail || bits_avail < 0 ) { //if bits_avail < 0 it is an error!
>         		// Use the correct call for ISAAC instead of rand()
> 			rand = rand();
>
> 			bits_avail = 32;
> 		}
>
> 		result <<= 2; //move two bits to left.
> 		result = result | (rand & 0x3); //add two least signifigant bits to the result.
> 		rand >>= 2; //move two bits to right.
> 		bits_avail -= 2;
> 	}
>
> 	return result; //result has 6 bits of random data...
> }
>
> char next_character()
> {
> 	unsigned char idx = 0;
> 	do {
> 		idx = get_6bits();
> 	} while (idx > 61);
>
> 	return S[idx];
> }
>
> Very simple :)
[...]

-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup