Clean up sparse_histogram.h header

base/metrics/histogram_macros.h

 // Copyright 2014 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.
 
 #ifndef BASE_METRICS_HISTOGRAM_MACROS_H_
 #define BASE_METRICS_HISTOGRAM_MACROS_H_
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_macros_internal.h"
 #include "base/metrics/histogram_macros_local.h"
 #include "base/time/time.h"
 
+
 // Macros for efficient use of histograms.
 //
 // For best practices on deciding when to emit to a histogram and what form
 // the histogram should take, see
 // https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/histograms/README.md
-//
-// TODO(nikunjb): Move sparse macros to this file.
-//
-// UMA_HISTOGRAM_SPARSE_SLOWLY is defined in sparse_histogram.h as it has
-// different #include dependencies.
 
 // TODO(rkaplow): Link to proper documentation on metric creation once we have
 // it in a good state.
 
 // All of these macros must be called with |name| as a runtime constant - it
 // doesn't have to literally be a constant, but it must be the same string on
 // all calls from a particular call site. If this rule is violated, it is
 // possible the data will be written to the wrong histogram.
 
 //------------------------------------------------------------------------------
@@ skipping 203 matching lines @@
                                               bucket_count)                    \
     INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(                                \
         name, sample, min, max, bucket_count,                                  \
         base::HistogramBase::kUmaStabilityHistogramFlag)
 
 #define UMA_STABILITY_HISTOGRAM_ENUMERATION(name, sample, enum_max)            \
     INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(                                  \
         name, sample, enum_max,                                                \
         base::HistogramBase::kUmaStabilityHistogramFlag)
 
+//------------------------------------------------------------------------------
+// Sparse histograms.
+
+// Sparse histograms are well suited for recording counts of exact sample values
+// that are sparsely distributed over a large range.
+//
+// UMA_HISTOGRAM_SPARSE_SLOWLY is good for sparsely distributed and/or
+// infrequently recorded values since the implementation is slower
+// and takes more memory.
+//
+// For instance, Sqlite.Version.* are sparse because for any given database,
+// there's going to be exactly one version logged.
+#define UMA_HISTOGRAM_SPARSE_SLOWLY(name, sample)                              \
+    INTERNAL_HISTOGRAM_SPARSE_SLOWLY(name, sample)
 
 //------------------------------------------------------------------------------
 // Deprecated histogram macros. Not recommended for current use.
 
 // Legacy name for UMA_HISTOGRAM_COUNTS_1M. Suggest using explicit naming
 // and not using this macro going forward.
 #define UMA_HISTOGRAM_COUNTS(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(        \
     name, sample, 1, 1000000, 50)
 
 // MB-granularity memory metric. This has a short max (1G).
 #define UMA_HISTOGRAM_MEMORY_MB(name, sample)                                  \
     UMA_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 1000, 50)
 
 // For an enum with customized range. In general, sparse histograms should be
 // used instead.
 // Samples should be one of the std::vector<int> list provided via
 // |custom_ranges|. See comments above CustomRanges::FactoryGet about the
 // requirement of |custom_ranges|. You can use the helper function
 // CustomHistogram::ArrayToCustomRanges to transform a C-style array of valid
 // sample values to a std::vector<int>.
 #define UMA_HISTOGRAM_CUSTOM_ENUMERATION(name, sample, custom_ranges)          \
     STATIC_HISTOGRAM_POINTER_BLOCK(name, Add(sample),                          \
         base::CustomHistogram::FactoryGet(name, custom_ranges,                 \
             base::HistogramBase::kUmaTargetedHistogramFlag))
 
 #endif  // BASE_METRICS_HISTOGRAM_MACROS_H_