Random word from file #5

prng.c

@@ -0,0 +1,91 @@
+
+#include "prng.h"
+// Define MT19937 constants (32-bit RNG)
+enum
+{
+    // Assumes W = 32 (omitting this)
+    N = 624,
+    M = 397,
+    R = 31,
+    A = 0x9908B0DF,
+
+    F = 1812433253,
+
+    U = 11,
+    // Assumes D = 0xFFFFFFFF (omitting this)
+
+    S = 7,
+    B = 0x9D2C5680,
+
+    T = 15,
+    C = 0xEFC60000,
+
+    L = 18,
+
+    MASK_LOWER = (1ull << R) - 1,
+    MASK_UPPER = (1ull << R)
+};
+
+static uint32_t  mt[N];
+static uint16_t  index;
+
+// Re-init with a given seed
+void InitializePRNG(const uint32_t  seed)
+{
+    uint32_t  i;
+
+    mt[0] = seed;
+
+    for ( i = 1; i < N; i++ )
+    {
+        mt[i] = (F * (mt[i - 1] ^ (mt[i - 1] >> 30)) + i);
+    }
+
+    index = N;
+}
+
+static void Twist()
+{
+    uint32_t  i, x, xA;
+
+    for ( i = 0; i < N; i++ )
+    {
+        x = (mt[i] & MASK_UPPER) + (mt[(i + 1) % N] & MASK_LOWER);
+
+        xA = x >> 1;
+
+        if ( x & 0x1 )
+            xA ^= A;
+
+        mt[i] = mt[(i + M) % N] ^ xA;
+    }
+
+    index = 0;
+}
+
+// Obtain a 32-bit random number
+uint32_t ExtractU32()
+{
+    uint32_t  y;
+    int       i = index;
+
+    if ( index >= N )
+    {
+        Twist();
+        i = index;
+    }
+
+    y = mt[i];
+    index = i + 1;
+
+    y ^= (mt[i] >> U);
+    y ^= (y << S) & B;
+    y ^= (y << T) & C;
+    y ^= (y >> L);
+
+    return y;
+}
+
+int getrandom(int low, int high) {
+    return ((ExtractU32() % high) + low);
+}