From 45afd3e243c075963129faad8ceccc13748553d7 Mon Sep 17 00:00:00 2001
From: apio <blobs.trading@gmail.com>
Date: Sat, 15 Oct 2022 16:46:54 +0200
Subject: [PATCH] Replace the rand() which reads from /dev/random with a
 Mersenne PRNG.

Much better now.

Also, it is seeded by reading from /dev/random at startup. Not sure if that's the best idea, but we'll see.
---
 apps/src/init.c            | 20 +-----------
 libs/libc/include/stdlib.h |  2 +-
 libs/libc/src/init.cpp     | 29 +++++++++++++++++
 libs/libc/src/rand.cpp     | 67 ++++++++++++++++++++++++++------------
 4 files changed, 78 insertions(+), 40 deletions(-)

diff --git a/apps/src/init.c b/apps/src/init.c
index fe54d161..e4b2fad1 100644
--- a/apps/src/init.c
+++ b/apps/src/init.c
@@ -117,25 +117,7 @@ int main()
 
     if (fclose(config) < 0) { perror("fclose"); }
 
-    FILE* fp = fopen("/dev/random", "rw");
-    if (!fp)
-    {
-        perror("fopen");
-        return 1;
-    }
-
-    long int random = 0;
-
-    fread(&random, sizeof(random), 1, fp);
-    if (ferror(fp))
-    {
-        perror("fread");
-        return 1;
-    }
-
-    fclose(fp);
-
-    printf("\n\nGot random number %ld\n\n", random);
+    printf("\n\nGot random number %d\n\n", rand());
 
     sleep(2);
 
diff --git a/libs/libc/include/stdlib.h b/libs/libc/include/stdlib.h
index 90a5ac48..647a9b04 100644
--- a/libs/libc/include/stdlib.h
+++ b/libs/libc/include/stdlib.h
@@ -53,7 +53,7 @@ extern "C"
     /* Returns a random number. */
     int rand(void);
 
-    /* Does nothing. */
+    /* Seeds the random number generator with the specified seed. */
     void srand(unsigned int seed);
 
 #ifdef __cplusplus
diff --git a/libs/libc/src/init.cpp b/libs/libc/src/init.cpp
index 279b8d1e..7352cee9 100644
--- a/libs/libc/src/init.cpp
+++ b/libs/libc/src/init.cpp
@@ -10,6 +10,33 @@ static void terminate_libc()
     fclose(stderr);
 }
 
+static void initialize_random()
+{
+    unsigned int seed = 3735928559U;
+
+    FILE* fp = fopen("/dev/random", "rw");
+    if (!fp)
+    {
+        errno = 0;
+        goto failed_to_read_dev_random;
+    }
+
+    fread(&seed, sizeof(seed), 1, fp);
+    if (ferror(fp))
+    {
+        errno = 0;
+        fclose(fp);
+        goto failed_to_read_dev_random;
+    }
+
+    fclose(fp);
+
+failed_to_read_dev_random:
+    srand(seed); // If we failed, this will be seeded with a known value. Else, it will be seeded with a random value
+                 // from the kernel.
+    return;
+}
+
 static void check_for_file(int fd, FILE** target_stream, const char* path, const char* mode)
 {
     if (lseek(fd, 0, SEEK_CUR) < 0)
@@ -27,5 +54,7 @@ extern "C" void initialize_libc()
 {
     check_for_file(STDOUT_FILENO, &stdout, "/dev/console", "rw");
     check_for_file(STDERR_FILENO, &stderr, "/dev/console", "rw");
+
+    initialize_random();
     atexit(terminate_libc);
 }
\ No newline at end of file
diff --git a/libs/libc/src/rand.cpp b/libs/libc/src/rand.cpp
index eea3ae4e..8398928d 100644
--- a/libs/libc/src/rand.cpp
+++ b/libs/libc/src/rand.cpp
@@ -1,37 +1,64 @@
 #include <assert.h>
-#include <stdio.h>
+#include <stddef.h>
+#include <stdint.h>
 #include <stdlib.h>
 
-static FILE* randfp = nullptr;
+typedef uint32_t word_t;
+#define STATE_SIZE 624
+#define MIDDLE 397
+#define INIT_SHIFT 30
+#define INIT_FACT 1812433253
+#define TWIST_MASK 0x9908b0df
+#define SHIFT1 11
+#define MASK1 0xffffffff
+#define SHIFT2 7
+#define MASK2 0x9d2c5680
+#define SHIFT3 15
+#define MASK3 0xefc60000
+#define SHIFT4 18
 
-void close_randfp()
+#define LOWER_MASK 0x7fffffff
+#define UPPER_MASK (~(word_t)LOWER_MASK)
+
+static word_t state[STATE_SIZE];
+static size_t index = STATE_SIZE + 1;
+
+static void twist()
 {
-    fclose(randfp);
+    for (size_t i = 0; i < STATE_SIZE; i++)
+    {
+        word_t x = (state[i] & UPPER_MASK) | (state[(i + 1) % STATE_SIZE] & LOWER_MASK);
+        x = (x >> 1) ^ (x & 1 ? TWIST_MASK : 0);
+        state[i] = state[(i + MIDDLE) % STATE_SIZE] ^ x;
+    }
+    index = 0;
 }
 
 extern "C"
 {
-    int rand() // FIXME: This function is a dirty and extremely hacky way to get going, /dev/random should only be used
-               // for initial seeding, userspace should do their own random calculations.
+    int rand()
     {
-        if (!randfp)
+        if (index >= STATE_SIZE)
         {
-            randfp = fopen("/dev/random", "rw");
-            assert(randfp != nullptr);
-            atexit(close_randfp);
+            assert(index == STATE_SIZE && "Mersenne generator was never seeded");
+            twist();
         }
-        int result;
-        fread(&result, sizeof(result), 1, randfp);
-        if (ferror(randfp))
-        {
-            perror("read(randfp)");
-            abort();
-        }
-        return result;
+
+        word_t y = state[index];
+        y ^= (y >> SHIFT1) & MASK1;
+        y ^= (y << SHIFT2) & MASK2;
+        y ^= (y << SHIFT3) & MASK3;
+        y ^= y >> SHIFT4;
+
+        index++;
+        return y;
     }
 
-    void srand(unsigned int)
+    void srand(unsigned int seed)
     {
-        return;
+        index = STATE_SIZE;
+        state[0] = seed;
+        for (size_t i = 1; i < STATE_SIZE; i++)
+            state[i] = (word_t)((INIT_FACT * (state[i - 1] ^ (state[i - 1] >> INIT_SHIFT))) + i);
     }
 }
\ No newline at end of file