badrand: a looks_random functions for a simple sanity check

kern/libkern/badrand.c

@@ -1,4 +1,5 @@
 #include "badrand.h"
+#include "stdio.h"
 #include <stdint.h>
 #include <string.h>
 
@@ -49,4 +50,30 @@ void sbadrand(uint64_t s) {
     }
 }
 
+/* Simple but dumb sanity check for randomness */
+int looks_random(char *buf, size_t len) {
+    int counts[256] = {0};
+    int run = 1, max_run = 1;
+
+    for (size_t i = 0; i < len; i++) {
+        counts[(unsigned int)buf[i]]++;
+        if (i > 0 && buf[i] == buf[i - 1]) {
+            if (++run > max_run)
+                max_run = run;
+        } else {
+            run = 1;
+        }
+    }
+
+    for (int i = 0; i < 256; i++) {
+        if (counts[i] > (int)(len * 1 / 10))
+            return 0;
+    }
+
+    if (max_run > 16)
+        return 0;
+
+    return 1;
+}
+
 #undef PRAND_BUILD_SEED

kern/libkern/badrand.h

@@ -45,4 +45,6 @@ uint64_t badrand_range(uint64_t min, uint64_t max);
  */
 void badrand_buf(char *buf, size_t len);
 
+int looks_random(char *buf, size_t len);
+
 #endif // BADRAND_H