Expose Bro's linear congruence PRNG as utility function.

It was previously not possible to crank the wheel on the PRNG in a
deterministic way without affecting the globally unique seed. The new extra
utility function bro_prng takes a state in the form of a long int and returns
the new PRNG state, now allowing arbitrary code parts to use the random number
functionality.

This commit also fixes a problem in the H3 constructor, which requires use
of multiple seeds. The single seed passed in now serves as seed to crank out as
many value needed using bro_prng.

src/H3.h

@@ -72,6 +72,7 @@ public:
 		for ( size_t bit = 0; bit < N * CHAR_BIT; bit++ )
 			{
 			bit_lookup[bit] = 0;
+			seed = bro_prng(seed);
 			for ( size_t i = 0; i < sizeof(T)/2; i++ )
 				// assume random() returns at least 16 random bits
 				bit_lookup[bit] = (bit_lookup[bit] << 16) | (seed & 0xFFFF);

src/util.cc

@@ -829,22 +829,29 @@ bool have_random_seed()
 	return bro_rand_determistic;
 	}
 
+long int bro_prng(long int state)
+  {
+	// Use our own simple linear congruence PRNG to make sure we are
+	// predictable across platforms.
+	static const long int m = 2147483647;
+	static const long int a = 16807;
+	const long int q = m / a;
+	const long int r = m % a;
+
+	state = a * ( state % q ) - r * ( state / q );
+
+	if ( state <= 0 )
+		state += m;
+
+	return state;
+  }
+
 long int bro_random()
 	{
 	if ( ! bro_rand_determistic )
 		return random(); // Use system PRNG.
 
-	// Use our own simple linear congruence PRNG to make sure we are
+  bro_rand_state = bro_prng(bro_rand_state);
-	// predictable across platforms.
-	const long int m = 2147483647;
-	const long int a = 16807;
-	const long int q = m / a;
-	const long int r = m % a;
-
-	bro_rand_state = a * ( bro_rand_state % q ) - r * ( bro_rand_state / q );
-
-	if ( bro_rand_state <= 0 )
-		bro_rand_state += m;
 
 	return bro_rand_state;
 	}

src/util.h

@@ -173,9 +173,12 @@ unsigned int initial_seed();
 // Returns true if the user explicitly set a seed via init_random_seed();
 extern bool have_random_seed();
 
+// A simple linear congruence PRNG. It takes its state as argument and returns
+// a new random value, which can serve as state for subsequent calls.
+long int bro_prng(long int state);
+
 // Replacement for the system random(), to which is normally falls back
-// except when a seed has been given. In that case, we use our own
+// except when a seed has been given. In that case, the function bro_prng.
-// predictable PRNG.
 long int bro_random();
 
 // Calls the system srandom() function with the given seed if not running