Tweak the new evutil_weakrand_() code

Make its state actually get seeded.

Document it more thoroughly.

Turn its state into a structure.

Fix a bug in evutil_weakrand_range_() where it could return the top of
the range.

Change its return type to ev_int32_t.

Add a quick unit test to make sure that the value of
evutil_weakrand_range_() is in range.

1 files changed, 39 insertions, 10 deletions

diff --git a/evutil.c b/evutil.c
index a87b5246..9307d259 100644
--- a/evutil.c
+++ b/evutil.c
@@ -2274,21 +2274,50 @@ evutil_getenv_(const char *varname)
 }
 
 ev_uint32_t
-evutil_weakrand_(ev_uint32_t* seed)
+evutil_weakrand_seed_(struct evutil_weakrand_state *state, ev_uint32_t seed)
 {
-	*seed = ((*seed) * 1103515245 + 12345) & 0x7fffffff;
-	return (*seed);
+	if (seed == 0) {
+		struct timeval tv;
+		evutil_gettimeofday(&tv, NULL);
+		seed = (ev_uint32_t)tv.tv_sec + (ev_uint32_t)tv.tv_usec;
+#ifdef _WIN32
+		seed += (ev_uint32_t) _getpid();
+#else
+		seed += (ev_uint32_t) getpid();
+#endif
+	}
+	state->seed = seed;
+	return seed;
 }
 
-ev_uint32_t
-evutil_weakrand_range_(ev_uint32_t* seed, ev_uint32_t top)
+ev_int32_t
+evutil_weakrand_(struct evutil_weakrand_state *state)
+{
+	/* This RNG implementation is a linear congruential generator, with
+	 * modulus 2^31, multiplier 1103515245, and addend 12345.  It's also
+	 * used by OpenBSD, and by Glibc's TYPE_0 RNG.
+	 *
+	 * The linear congruential generator is not an industrial-strength
+	 * RNG!  It's fast, but it can have higher-order patterns.  Notably,
+	 * the low bits tend to have periodicity.
+	 */
+	state->seed = ((state->seed) * 1103515245 + 12345) & 0x7fffffff;
+	return (ev_int32_t)(state->seed);
+}
+
+ev_int32_t
+evutil_weakrand_range_(struct evutil_weakrand_state *state, ev_int32_t top)
 {
-	ev_uint32_t divisor, result;
+	ev_int32_t divisor, result;
 
-	divisor = EV_INT32_MAX / top;
-	do
-		result = evutil_weakrand_(seed) / divisor;
-	while (result > top);
+	/* We can't just do weakrand() % top, since the low bits of the LCG
+	 * are less random than the high ones.  (Specifically, since the LCG
+	 * modulus is 2^N, every 2^m for m<N will divide the modulus, and so
+	 * therefore the low m bits of the LCG will have period 2^m.) */
+	divisor = EVUTIL_WEAKRAND_MAX / top;
+	do {
+		result = evutil_weakrand_(state) / divisor;
+	} while (result >= top);
 	return result;
 }