X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?p=pintos-anon;a=blobdiff_plain;f=src%2Flib%2Frandom.c;h=6a963e21b42ccce71551e66e8dad806edd542d9a;hp=39452a99666246ae33ccd96da58eaf62aefe7b97;hb=HEAD;hpb=750d21936d284127e265d050ccbce76fca1ece1a

diff --git a/src/lib/random.c b/src/lib/random.c
index 39452a9..6a963e2 100644
--- a/src/lib/random.c
+++ b/src/lib/random.c
@@ -1,10 +1,26 @@
 #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) 
 {
@@ -13,34 +29,38 @@ swap_byte (uint8_t *a, uint8_t *b)
   *b = t;
 }
 
-static uint8_t
-key_byte (int idx) 
-{
-  return idx ^ 0xff;
-}
-
+/* Initializes or reinitializes the PRNG with the given SEED. */
 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;
 
+  if (!inited)
+    random_init (0);
+
   for (buf = buf_; size-- > 0; buf++)
     {
       uint8_t s_k;
@@ -54,7 +74,9 @@ random_bytes (void *buf_, size_t size)
     }
 }
 
-/* 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) 
 {