xorshift

lookup3.c

@@ -1,5 +1,7 @@
 #include <stdint.h>
+#include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 #define rot(x, k) (((x) << (k)) | ((x) >> (32 - (k))))
 
@@ -61,3 +63,12 @@ uint32_t lookup3(const void *key, size_t length, uint32_t initval) {
     final(a, b, c);
     return c;
 }
+
+int main(void) {
+    char *a = "Hello!\0";
+    int   len = strlen(a);
+
+    uint32_t hash = lookup3(a, len, 0);
+
+  printf("%.8X\n", hash);
+}

prand.h

@@ -4,6 +4,9 @@
 /*
  * This is a PRNG for non-cryptographic use. May be used on micros given that
  * the seed is periodically saved to EEPROM or flash.
+ *
+ * See:
+ *      https://en.wikipedia.org/wiki/Linear_congruential_generator
  */
 
 /**

xorshift.c

@@ -0,0 +1,60 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <time.h>
+
+struct xorshift32_state {
+    uint32_t a;
+};
+
+/* The state must be initialized to non-zero */
+uint32_t xorshift32(struct xorshift32_state *state) {
+    /* Algorithm "xor" from p. 4 of Marsaglia, "Xorshift RNGs" */
+    uint32_t x = state->a;
+    x ^= x << 13;
+    x ^= x >> 17;
+    x ^= x << 5;
+    return state->a = x;
+}
+
+struct xorshift64_state {
+    uint64_t a;
+};
+
+/* The state must be initialized to non-zero */
+uint64_t xorshift64(struct xorshift64_state *state) {
+    uint64_t x = state->a;
+    x ^= x << 13;
+    x ^= x >> 7;
+    x ^= x << 17;
+    return state->a = x;
+}
+
+/* struct xorshift128_state can alternatively be defined as a pair
+   of uint64_t or a uint128_t where supported */
+struct xorshift128_state {
+    uint32_t x[4];
+};
+
+/* The state must be initialized to non-zero */
+uint32_t xorshift128(struct xorshift128_state *state) {
+    /* Algorithm "xor128" from p. 5 of Marsaglia, "Xorshift RNGs" */
+    uint32_t t = state->x[3];
+
+    uint32_t s = state->x[0]; /* Perform a contrived 32-bit shift. */
+    state->x[3] = state->x[2];
+    state->x[2] = state->x[1];
+    state->x[1] = s;
+
+    t ^= t << 11;
+    t ^= t >> 8;
+    return state->x[0] = t ^ s ^ (s >> 19);
+}
+
+int main(void) {
+    struct xorshift32_state s = {};
+    s.a = time(0);
+
+    for (int a = 0; a < 10; a++) {
+        printf("Number %d: %d\n", a, xorshift32(&s));
+    }
+}