Fold LIBC_RAND into LIBC_STDIO/TINYMATH/INTRIN

libc/tinymath/measureentropy.c

@@ -0,0 +1,54 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/math.h"
+#include "libc/stdio/rand.h"
+#include "libc/str/str.h"
+
+/**
+ * Returns Shannon entropy of array.
+ *
+ * This gives you an idea of the density of information. Cryptographic
+ * random should be in the ballpark of 7.9 whereas plaintext will be
+ * more like 4.5.
+ *
+ * @param p is treated as binary octets
+ * @param n should be at least 1000
+ * @return number between 0 and 8
+ */
+double MeasureEntropy(const char *p, size_t n) {
+  size_t i;
+  double e, x;
+  long h[256];
+  e = 0;
+  if (n) {
+    bzero(h, sizeof(h));
+    for (i = 0; i < n; ++i) {
+      ++h[p[i] & 255];
+    }
+    for (i = 0; i < 256; i++) {
+      if (h[i]) {
+        x = h[i];
+        x /= n;
+        e += x * log(x);
+      }
+    }
+    e = -(e / M_LN2);
+  }
+  return e;
+}