#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;
-static bool inited;
+/* 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;
}
+/* Initializes or reinitializes the PRNG with the given SEED. */
void
random_init (unsigned 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++)
inited = true;
}
+/* Writes SIZE random bytes into BUF. */
void
random_bytes (void *buf_, size_t size)
{
uint8_t *buf;
- ASSERT (inited);
+ if (!inited)
+ random_init (0);
+
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)
{