Avoid modulo bias when generating random numbers.

Add Q_rand_uniform() function that generates random numbers modulo N,
using the trick from OpenBSD implementation of arc4random_uniform().

inc/shared/shared.h

@@ -338,6 +338,7 @@ static inline int Q_gcd(int a, int b)
 
 void Q_srand(uint32_t seed);
 uint32_t Q_rand(void);
+uint32_t Q_rand_uniform(uint32_t n);
 
 #define clamp(a,b,c)    ((a)<(b)?(a)=(b):(a)>(c)?(a)=(c):(a))
 #define cclamp(a,b,c)   ((b)>(c)?clamp(a,c,b):clamp(a,b,c))

src/baseq2/g_items.c

@@ -125,7 +125,7 @@ void DoRespawn(edict_t *ent)
         for (count = 0, ent = master; ent; ent = ent->chain, count++)
             ;
 
-        choice = Q_rand() % count;
+        choice = Q_rand_uniform(count);
 
         for (count = 0, ent = master; count < choice; ent = ent->chain, count++)
             ;

src/baseq2/g_utils.c

@@ -136,7 +136,7 @@ edict_t *G_PickTarget(char *targetname)
         return NULL;
     }
 
-    return choice[Q_rand() % num_choices];
+    return choice[Q_rand_uniform(num_choices)];
 }
 
 

src/baseq2/p_client.c

@@ -744,7 +744,7 @@ edict_t *SelectRandomDeathmatchSpawnPoint(void)
     } else
         count -= 2;
 
-    selection = Q_rand() % count;
+    selection = Q_rand_uniform(count);
 
     spot = NULL;
     do {

src/shared/shared.c

@@ -873,6 +873,13 @@ void Q_setenv(const char *name, const char *value)
 static uint32_t mt_state[N];
 static uint32_t mt_index;
 
+/*
+==================
+Q_srand
+
+Seed PRNG with initial value
+==================
+*/
 void Q_srand(uint32_t seed)
 {
     mt_index = N;
@@ -881,6 +888,13 @@ void Q_srand(uint32_t seed)
         mt_state[i] = seed = 1812433253U * (seed ^ seed >> 30) + i;
 }
 
+/*
+==================
+Q_rand
+
+Generate random integer in range [0, 2^32)
+==================
+*/
 uint32_t Q_rand(void)
 {
     uint32_t x, y;
@@ -913,6 +927,28 @@ uint32_t Q_rand(void)
     return y;
 }
 
+/*
+==================
+Q_rand_uniform
+
+Generate random integer in range [0, n) avoiding modulo bias
+==================
+*/
+uint32_t Q_rand_uniform(uint32_t n)
+{
+    uint32_t r, m;
+
+    if (n < 2)
+        return 0;
+
+    m = -n % n; // 2^32 mod n
+    do {
+        r = Q_rand();
+    } while (r < m);
+
+    return r % n;
+}
+
 /*
 =====================================================================