Simple PRNG implementation

prand.c

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+#include "prand.h"
+#include <stdint.h>
+#include <stdio.h>
+#include <time.h>
+
+#define BUILD_SEED                                                             \
+    ((uint64_t)(__TIME__[0]) * (uint64_t)(__TIME__[1]) *                       \
+     (uint64_t)(__TIME__[3]) * (uint64_t)(__TIME__[4]) *                       \
+     (uint64_t)(__TIME__[6]) * (uint64_t)(__TIME__[7]))
+
+#define PRNG_SAVE_INTERVAL 50 // Save every 1000 calls to prand()
+
+static uint64_t seed = BUILD_SEED;
+
+uint64_t prand() {
+    seed = seed * 6364136223846793005ULL + 1;
+    return seed;
+}
+
+inline uint64_t prand_range(uint64_t min, uint64_t max) {
+    return min + (prand() % (max - min + 1));
+}
+
+void sprand(uint64_t s) {
+    if (s) {
+        seed = s;
+    } else {
+        rand_reseed();
+    }
+}
+
+void rand_reseed() { seed = BUILD_SEED; }
+
+#define PRAND_MAIN
+#ifdef PRAND_MAIN
+int main() {
+    // time(NULL) provides a somewhat unique seed at runtime
+    sprand(time(NULL));
+
+
+    uint64_t rn1 = prand();
+    uint64_t rn2 = prand();
+
+    printf("rn1 = %lu\n", rn1);
+    printf("rn2 = %lu\n", rn2);
+}
+#endif

prand.h

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+#pragma once
+#include <stdint.h>
+
+/*
+ * This is a PRNG for non-cryptographic use. May be used on micros given that
+ * the seed is periodically saved to EEPROM or flash.
+ */
+
+/**
+ * @brief Sets the seed for the PRNG.
+ *
+ * @param s The specific seed value or zero. If zero is passed, it will call
+ * rand_reseed().
+ */
+void sprand(uint64_t s);
+
+/**
+ * @brief Generates a pseudo-random 64-bit number.
+ *
+ * Saves PRNG state to flash periodically.
+ *
+ * Uses a simple Linear Congruential Generator (LCG) to produce
+ * a sequence of pseudo-random numbers.
+ *
+ * @return A pseudo-random 64-bit unsigned integer.
+ */
+uint64_t prand();
+
+/**
+ * @brief Generates a random number within a specified range.
+ *
+ * Produces a random number in the inclusive range [min, max].
+ * Ensures uniform distribution by applying a modulo operation.
+ *
+ * @param min The lower bound of the range (inclusive).
+ * @param max The upper bound of the range (inclusive).
+ * @return A random number between min and max.
+ */
+inline uint64_t prand_range(uint64_t min, uint64_t max);
+
+/**
+ * @brief Re-seeds the PRNG seed state from BUILD_SEED.
+ */
+void rand_reseed();