#include "random.h"
+#include <stdbool.h>
#include <stdint.h>
+#include "debug.h"
-/* RC4-based pseudo-random state. */
-static uint8_t s[256];
-static uint8_t s_i, s_j;
+/* 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)
{
*b = t;
}
-static uint8_t
-key_byte (int idx)
-{
- return idx ^ 0xff;
-}
-
+/* Initializes the PRNG with the given SEED.
+ Does nothing if the PRNG has already been initialized. */
void
-random_init (void)
+random_init (unsigned seed)
{
+ uint8_t *seedp = (uint8_t *) &seed;
int i;
uint8_t j;
+ if (inited)
+ return;
+
for (i = 0; i < 256; i++)
s[i] = i;
for (i = j = 0; i < 256; i++)
{
- j += s[i] + key_byte (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;
+ ASSERT (inited);
for (buf = buf_; size-- > 0; buf++)
{
uint8_t s_k;
}
}
-/* Returns a pseudo-random unsigned long. */
+/* 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)
{