# HG changeset patch
# User Markus Mützel <markus.muetzel@gmx.de>
# Date 1595507086 -7200
# Node ID a61b651914d67a81fa8f4d209dd1df3288547495
# Parent  b81238f3ecdb0be80d3777163441ecf951572123
Use C++11 functions to gather entropy from system (bug #58800).

* randmtzig.cc (octave::init_mersenne_twister): Gather entropy for algorithm
from random_device if available. Use PID as additional entropy source.

diff -r b81238f3ecdb -r a61b651914d6 liboctave/numeric/randmtzig.cc
--- a/liboctave/numeric/randmtzig.cc	Sat Jul 25 11:33:55 2020 +0200
+++ b/liboctave/numeric/randmtzig.cc	Thu Jul 23 14:24:46 2020 +0200
@@ -162,7 +162,9 @@
 #include <cstdio>
 
 #include <algorithm>
+#include <random>
 
+#include "oct-syscalls.h"
 #include "oct-time.h"
 #include "randmtzig.h"
 
@@ -259,36 +261,50 @@
     uint32_t entropy[MT_N];
     int n = 0;
 
-    /* Look for entropy in /dev/urandom */
-    FILE *urandom = std::fopen ("/dev/urandom", "rb");
-    if (urandom)
-      {
-        while (n < MT_N)
-          {
-            unsigned char word[4];
-            if (std::fread (word, 4, 1, urandom) != 1)
-              break;
-            entropy[n++] = word[0] + (word[1]<<8) + (word[2]<<16)
-                           + (static_cast<uint32_t> (word[3])<<24);
-          }
-        std::fclose (urandom);
-      }
-
-    /* If there isn't enough entropy, gather some from various sources */
+    // Gather some entropy from various sources
 
     sys::time now;
 
+    // Current time in seconds
     if (n < MT_N)
-      entropy[n++] = now.unix_time (); /* Current time in seconds */
+      entropy[n++] = now.unix_time ();
+
+    // CPU time used (usec)
+    if (n < MT_N)
+      entropy[n++] = clock ();
+
+    // Fractional part of current time
+    if (n < MT_N)
+      entropy[n++] = now.usec ();
+
+    // Include the PID to make sure that several processes reaching here at the
+    // same time use different random numbers.
+    if (n < MT_N)
+      entropy[n++] = sys::getpid ();
 
     if (n < MT_N)
-      entropy[n++] = clock ();    /* CPU time used (usec) */
+      {
+        try
+          {
+            // The standard doesn't *guarantee* that random_device provides
+            // non-deterministic random numbers. So add entropy from this
+            // source last to make sure we gathered at least some entropy from
+            // the earlier sources.
+            std::random_device rd;
+            std::uniform_int_distribution<uint32_t> dist;
+            // Add 1024 bit of "true" entropy
+            int n_max = std::min (n + 32, MT_N);
+            while (n < n_max)
+              entropy[n++] = dist (rd);
+          }
+        catch (std::exception& e)
+          {
+            // Just ignore any exception and skip that source of entropy.
+          }
+      }
 
-    if (n < MT_N)
-      entropy[n++] = now.usec ();   /* Fractional part of current time */
-
-    /* Send all the entropy into the initial state vector */
-    init_mersenne_twister (entropy,n);
+    // Send all the entropy into the initial state vector
+    init_mersenne_twister (entropy, n);
   }
 
   void set_mersenne_twister_state (const uint32_t *save)