Use strings for keys in the histogram map.

base/metrics/statistics_recorder.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // StatisticsRecorder holds all Histograms and BucketRanges that are used by
 // Histograms in the system. It provides a general place for
 // Histograms/BucketRanges to register, and supports a global API for accessing
 // (i.e., dumping, or graphing) the data.
 
 #ifndef BASE_METRICS_STATISTICS_RECORDER_H_
@@ skipping 14 matching lines @@
 #include "base/metrics/histogram_base.h"
 #include "base/strings/string_piece.h"
 
 namespace base {
 
 class BucketRanges;
 class Lock;
 
 class BASE_EXPORT StatisticsRecorder {
  public:
-  typedef std::map<uint64_t, HistogramBase*> HistogramMap;  // Key is name-hash.
+  // A class used as a key for the histogram map below. It always references
+  // a string owned outside of this class, likely in the value of the map.
+  class StringKey : public StringPiece {
+   public:
+    // Constructs the StringKey using various sources. The source must live
+    // at least as long as the created object.
+    StringKey(const std::string& str) : StringPiece(str) {}
+    StringKey(StringPiece str) : StringPiece(str) {}
+
+    // Though StringPiece is better passed by value than by reference, in
+    // this case it's being passed many times and likely already been stored
+    // in memory (not just registers) so the benefit of pass-by-value is
+    // negated.
+    bool operator<(const StringKey& rhs) const {
+      // Since order is unimportant in the map and string comparisons can be
+      // slow, use the length as the primary sort value.
+      if (length() < rhs.length())
+        return true;
+      if (length() > rhs.length())
+        return false;
+
+      // Fall back to an actual string comparison. The lengths are the same
+      // so a simple memory-compare is sufficient. This is slightly more
+      // efficient than calling operator<() for StringPiece which would
+      // again have to check lengths before calling wordmemcmp().
+      return wordmemcmp(data(), rhs.data(), length()) < 0;
+    }
+  };
+
+  typedef std::map<StringKey, HistogramBase*> HistogramMap;
   typedef std::vector<HistogramBase*> Histograms;
 
   // A class for iterating over the histograms held within this global resource.
   class BASE_EXPORT HistogramIterator {
    public:
     HistogramIterator(const HistogramMap::iterator& iter,
                       bool include_persistent);
     HistogramIterator(const HistogramIterator& rhs);  // Must be copyable.
     ~HistogramIterator();
 
@@ skipping 133 matching lines @@
 
   // Lock protects access to above maps.
   static base::Lock* lock_;
 
   DISALLOW_COPY_AND_ASSIGN(StatisticsRecorder);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_STATISTICS_RECORDER_H_