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#include "random.h"
#include <stdbool.h>
#include <stdint.h>
#include "debug.h"
/* RC4-based pseudo-random number generator (PRNG).
RC4 is a stream cipher. We're not using it here for its
cryptographic properties, but because it is easy to implement
and its output is plenty random for non-cryptographic
purposes.
See http://en.wikipedia.org/wiki/RC4_(cipher) for information
on RC4.*/
/* RC4 state. */
static uint8_t s[256]; /* S[]. */
static uint8_t s_i, s_j; /* i, j. */
/* Already initialized? */
static bool inited;
/* Swaps the bytes pointed to by A and B. */
static inline void
swap_byte (uint8_t *a, uint8_t *b)
{
uint8_t t = *a;
*a = *b;
*b = t;
}
/* Initializes or reinitializes the PRNG with the given SEED. */
void
random_init (unsigned seed)
{
uint8_t *seedp = (uint8_t *) &seed;
int i;
uint8_t j;
for (i = 0; i < 256; i++)
s[i] = i;
for (i = j = 0; i < 256; i++)
{
j += s[i] + seedp[i % sizeof seed];
swap_byte (s + i, s + j);
}
s_i = s_j = 0;
inited = true;
}
/* Writes SIZE random bytes into BUF. */
void
random_bytes (void *buf_, size_t size)
{
uint8_t *buf;
if (!inited)
random_init (0);
for (buf = buf_; size-- > 0; buf++)
{
uint8_t s_k;
s_i++;
s_j += s[s_i];
swap_byte (s + s_i, s + s_j);
s_k = s[s_i] + s[s_j];
*buf = s[s_k];
}
}
/* Returns a pseudo-random unsigned long.
Use random_ulong() % n to obtain a random number in the range
0...n (exclusive). */
unsigned long
random_ulong (void)
{
unsigned long ul;
random_bytes (&ul, sizeof ul);
return ul;
}
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