minor refactoring in folly/stats

Summary:
While wrapping my head around the TDigest code, I made a few small
drive-by clean-ups.

Reviewed By: yfeldblum

Differential Revision: D16478284

fbshipit-source-id: aaf289a57ef633f0f1527263beffb7c0edf596d1

folly/stats/QuantileEstimator.cpp

@@ -26,9 +26,9 @@ QuantileEstimates estimatesFromDigest(
   result.quantiles.reserve(quantiles.size());
   result.sum = digest.sum();
   result.count = digest.count();
-  for (auto it = quantiles.begin(); it != quantiles.end(); ++it) {
+  for (auto& quantile : quantiles) {
     result.quantiles.push_back(
-        std::make_pair(*it, digest.estimateQuantile(*it)));
+        std::make_pair(quantile, digest.estimateQuantile(quantile)));
   }
   return result;
 }

folly/stats/TDigest.cpp

@@ -211,8 +211,8 @@ TDigest TDigest::merge(sorted_equivalent_t, Range<const double*> sortedValues)
 
 TDigest TDigest::merge(Range<const TDigest*> digests) {
   size_t nCentroids = 0;
-  for (auto it = digests.begin(); it != digests.end(); it++) {
-    nCentroids += it->centroids_.size();
+  for (const auto& digest : digests) {
+    nCentroids += digest.centroids_.size();
   }
 
   if (nCentroids == 0) {
@@ -232,16 +232,16 @@ TDigest TDigest::merge(Range<const TDigest*> digests) {
   double min = std::numeric_limits<double>::infinity();
   double max = -std::numeric_limits<double>::infinity();
 
-  for (auto it = digests.begin(); it != digests.end(); it++) {
+  for (const auto& digest : digests) {
     starts.push_back(centroids.end());
-    double curCount = it->count();
+    double curCount = digest.count();
     if (curCount > 0) {
-      DCHECK(!std::isnan(it->min_));
-      DCHECK(!std::isnan(it->max_));
-      min = std::min(min, it->min_);
-      max = std::max(max, it->max_);
+      DCHECK(!std::isnan(digest.min_));
+      DCHECK(!std::isnan(digest.max_));
+      min = std::min(min, digest.min_);
+      max = std::max(max, digest.max_);
       count += curCount;
-      for (const auto& centroid : it->centroids_) {
+      for (const auto& centroid : digest.centroids_) {
         centroids.push_back(centroid);
       }
     }

folly/stats/detail/DigestBuilder-defs.h

@@ -50,6 +50,7 @@ DigestT DigestBuilder<DigestT>::build() {
   }
 
   std::vector<DigestT> digests;
+  digests.reserve(digestPtrs.size());
   for (auto& digestPtr : digestPtrs) {
     digests.push_back(std::move(*digestPtr));
   }