Prevent calling internal metrics code with invalid values.

base/metrics/histogram.h

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 
 // It supports calls to accumulate either time intervals (which are processed
 // as integral number of milliseconds), or arbitrary integral units.
 
@@ skipping 411 matching lines @@
     // samples we've accumulated into all of our buckets.  We can compare this
     // count to the sum of the counts in all buckets, and detect problems.  Note
     // that due to races in histogram accumulation (if a histogram is indeed
     // updated on several threads simultaneously), the tallies might mismatch,
     // and also the snapshotting code may asynchronously get a mismatch (though
     // generally either race based mismatch cause is VERY rare).
     int64 redundant_count_;
   };
 
   //----------------------------------------------------------------------------
-  // minimum should start from 1. 0 is invalid as a minimum. 0 is an implicit
-  // default underflow bucket.
+  // For a valid histogram, input should follow these restrictions:
+  // minimum > 0 (if a minimum below 1 is specified, it will implicitly be
+  //              normalized up to 1)
+  // maximum > minimum
+  // buckets > 2 [minimum buckets needed: underflow, overflow and the range]
+  // Additionally,
+  // buckets <= (maximum - minimum + 2) - this is to ensure that we don't have
+  // more buckets than the range of numbers; having more buckets than 1 per
+  // value in the range would be nonsensical.
   static Histogram* FactoryGet(const std::string& name,
                                Sample minimum,
                                Sample maximum,
                                size_t bucket_count,
                                Flags flags);
   static Histogram* FactoryTimeGet(const std::string& name,
                                    base::TimeDelta minimum,
                                    base::TimeDelta maximum,
                                    size_t bucket_count,
                                    Flags flags);
@@ skipping 440 matching lines @@
   // of our data, and to quickly see if some other CachedRanges instance is
   // possibly Equal() to this instance.
   uint32 range_checksum_;
 
   DISALLOW_COPY_AND_ASSIGN(CachedRanges);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_H_