Performant util/histogram.

Summary:
Earlier way to record in histogram=>
Linear search BucketLimit array to find the bucket and increment the
counter
Current way to record in histogram=>
Store a HistMap statically which points the buckets of each value in the
range [kFirstValue, kLastValue);

In the proccess use vectors instead of array's and refactor some code to
HistogramHelper class.

Test Plan:
run db_bench with histogram=1 and see a histogram being
printed.

Reviewers: dhruba, chip, heyongqiang

Reviewed By: chip

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8265

util/histogram_test.cc

@@ -0,0 +1,57 @@
+#include "util/histogram.h"
+
+#include "util/testharness.h"
+
+namespace leveldb {
+
+class HistogramTest { };
+
+TEST(HistogramTest, BasicOperation) {
+
+  Histogram histogram;
+  for (uint64_t i = 1; i <= 100; i++) {
+    histogram.Add(i);
+  }
+
+  {
+    double median = histogram.Median();
+    // ASSERT_LE(median, 50);
+    ASSERT_GT(median, 0);
+  }
+
+  {
+    double percentile100 = histogram.Percentile(100.0);
+    ASSERT_LE(percentile100, 100.0);
+    ASSERT_GT(percentile100, 0.0);
+    double percentile99 = histogram.Percentile(99.0);
+    double percentile85 = histogram.Percentile(85.0);
+    ASSERT_LE(percentile99, 99.0);
+    ASSERT_TRUE(percentile99 >= percentile85);
+  }
+
+  ASSERT_EQ(histogram.Average(), 50.5); // avg is acurately caluclated.
+}
+
+TEST(HistogramTest, EmptyHistogram) {
+  Histogram histogram;
+  ASSERT_EQ(histogram.Median(), 0.0);
+  ASSERT_EQ(histogram.Percentile(85.0), 0.0);
+  ASSERT_EQ(histogram.Average(), 0.0);
+}
+
+TEST(HistogramTest, ClearHistogram) {
+  Histogram histogram;
+  for (uint64_t i = 1; i <= 100; i++) {
+    histogram.Add(i);
+  }
+  histogram.Clear();
+  ASSERT_EQ(histogram.Median(), 0);
+  ASSERT_EQ(histogram.Percentile(85.0), 0);
+  ASSERT_EQ(histogram.Average(), 0);
+}
+
+}  // namespace leveldb
+
+int main(int argc, char** argv) {
+  return leveldb::test::RunAllTests();
+}