Add test to check PRNG quality.

Credits go to Erik Corry. Taken from:
https://github.com/dart-lang/fletch/blob/master/src/shared/random_test.cc

R=jkummerow@chromium.org
BUG=v8:4566
LOG=N

Review URL: https://codereview.chromium.org/1467133006

Cr-Commit-Position: refs/heads/master@{#32211}

test/cctest/test-random-number-generator.cc

@@ -51,3 +51,104 @@ TEST(RandomSeedFlagIsUsed) {
     i->Dispose();
   }
 }
+
+
+// Chi squared for getting m 0s out of n bits.
+double ChiSquared(int m, int n) {
+  double ys_minus_np1 = (m - n / 2.0);
+  double chi_squared_1 = ys_minus_np1 * ys_minus_np1 * 2.0 / n;
+  double ys_minus_np2 = ((n - m) - n / 2.0);
+  double chi_squared_2 = ys_minus_np2 * ys_minus_np2 * 2.0 / n;
+  return chi_squared_1 + chi_squared_2;
+}
+
+
+// Test for correlations between recent bits from the PRNG, or bits that are
+// biased.
+TEST(RandomBitCorrelations) {
+  FLAG_random_seed = 31415926;
+  LocalContext env;
+  v8::Isolate* isolate = env->GetIsolate();
+  v8::HandleScope scope(isolate);
+  const char* source =
+      "(function() {"
+      "  return (Math.random() * Math.pow(2, 32)) >>> 0;"
+      "})";
+  v8::Local<v8::Context> context = isolate->GetCurrentContext();
+  v8::Local<v8::Function> random_fun = v8::Local<v8::Function>::Cast(
+      CompileRun(context, source).ToLocalChecked());
+  v8::Local<v8::Value> undefined = v8::Undefined(isolate);
+
+#ifdef DEBUG
+  const int kHistory = 2;
+  const int kRepeats = 1000;
+#else
+  const int kHistory = 8;
+  const int kRepeats = 10000;
+#endif
+  uint32_t history[kHistory];
+  // The predictor bit is either constant 0 or 1, or one of the bits from the
+  // history.
+  for (int predictor_bit = -2; predictor_bit < 32; predictor_bit++) {
+    // The predicted bit is one of the bits from the PRNG.
+    for (int random_bit = 0; random_bit < 32; random_bit++) {
+      // The predicted bit is taken from the previous output of the PRNG.
+      for (int ago = 0; ago < kHistory; ago++) {
+        // We don't want to check whether each bit predicts itself.
+        if (ago == 0 && predictor_bit == random_bit) continue;
+
+        // Enter the new random value into the history
+        for (int i = ago; i >= 0; i--) {
+          v8::HandleScope scope(isolate);
+          history[i] = random_fun->Call(context, undefined, 0, NULL)
+                           .ToLocalChecked()
+                           ->Uint32Value(context)
+                           .FromJust();
+        }
+
+        // Find out how many of the bits are the same as the prediction bit.
+        int m = 0;
+        for (int i = 0; i < kRepeats; i++) {
+          v8::HandleScope scope(isolate);
+          uint32_t random = random_fun->Call(context, undefined, 0, NULL)
+                                .ToLocalChecked()
+                                ->Uint32Value(context)
+                                .FromJust();
+          for (int j = ago - 1; j >= 0; j--) history[j + 1] = history[j];
+          history[0] = random;
+
+          int predicted;
+          if (predictor_bit >= 0) {
+            predicted = (history[ago] >> predictor_bit) & 1;
+          } else {
+            predicted = predictor_bit == -2 ? 0 : 1;
+          }
+          int bit = (random >> random_bit) & 1;
+          if (bit == predicted) m++;
+        }
+
+        // Chi squared analysis for k = 2 (2, states: same/not-same) and one
+        // degree of freedom (k - 1).
+        double chi_squared = ChiSquared(m, kRepeats);
+        if (chi_squared > 24) {
+          int percent = static_cast<int>(m * 100.0 / kRepeats);
+          if (predictor_bit < 0) {
+            PrintF("Bit %d is %d %d%% of the time\n", random_bit,
+                   predictor_bit == -2 ? 0 : 1, percent);
+          } else {
+            PrintF("Bit %d is the same as bit %d %d ago %d%% of the time\n",
+                   random_bit, predictor_bit, ago, percent);
+          }
+        }
+
+        // For 1 degree of freedom this corresponds to 1 in a million.  We are
+        // running ~8000 tests, so that would be surprising.
+        CHECK(chi_squared <= 24);
+
+        // If the predictor bit is a fixed 0 or 1 then it makes no sense to
+        // repeat the test with a different age.
+        if (predictor_bit < 0) break;
+      }
+    }
+  }
+}