Uses Hash values for sparse histograms for InputMethod.ID statistics.

chrome/browser/chromeos/input_method/input_method_manager_impl.cc

Index: chrome/browser/chromeos/input_method/input_method_manager_impl.cc
diff --git a/chrome/browser/chromeos/input_method/input_method_manager_impl.cc b/chrome/browser/chromeos/input_method/input_method_manager_impl.cc
index bff428cf90affa2c97936a29952c3c3809a76663..accbbccae08c41ab9181614ff55811917a12ebcb 100644
--- a/chrome/browser/chromeos/input_method/input_method_manager_impl.cc
+++ b/chrome/browser/chromeos/input_method/input_method_manager_impl.cc
@@ -10,6 +10,7 @@
 
 #include "base/basictypes.h"
 #include "base/bind.h"
+#include "base/hash.h"
 #include "base/location.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
@@ -863,22 +864,11 @@ InputMethodManagerImpl::InputMethodManagerImpl(
   util_.ResetInputMethods(descriptors);
 
   // Initializes the stat id map.
-  std::map<int, std::vector<std::string> > buckets;
   for (InputMethodDescriptors::const_iterator it = descriptors.begin();
        it != descriptors.end(); ++it) {
-    char first_char;
-    int cat_id = static_cast<int>(
-        GetInputMethodCategory(it->id(), &first_char));
-    int key = cat_id * 1000 + first_char;
-    buckets[key].push_back(it->id());
-  }
-  for (std::map<int, std::vector<std::string>>::iterator i =
-       buckets.begin(); i != buckets.end(); ++i) {
-    std::sort(i->second.begin(), i->second.end());
-    for (size_t j = 0; j < i->second.size() && j < 100; ++j) {
-      int key = i->first * 100 + j;
-      stat_id_map_[i->second[j]] = key;
-    }
+    std::string id = it->id();
+    stat_id_map_[id] =
+        static_cast<int>(base::SuperFastHash(id.c_str(), id.length()));

[Ilya Sherman, 2015/07/17 01:43:58]

nit: Rather than static_cast<int>, please use base::strict_cast, and cast to
int32_t or base::HistogramBase::Sample.

[Ilya Sherman, 2015/07/17 01:43:58]

Do you actually need this map anymore?  I think you could just directly hash at
the histogram logging site.

[Ilya Sherman, 2015/07/17 01:43:58]

nit: Any reason not to call this as "base::Hash(id)"?  If the lack of
BASE_EXPORT is the problem, I bet it's fine to add it -- I see no reason why the
convenience wrappers would not be exported.

[Shu Chen, 2015/07/17 03:55:50]

There will be compiling errors as below. I want to cast uint32 to int32 here.

../../base/numerics/safe_conversions.h:70:3: error: static_assert failed "The
numeric conversion is out of range for this type. You should probably use one of
the following conversion mechanisms on the value you want to pass:\n-
base::checked_cast\n- base::saturated_cast\n- base::CheckedNumeric"
  static_assert((internal::StaticDstRangeRelationToSrcRange<Dst, Src>::value ==
  ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../chrome/browser/chromeos/input_method/input_method_manager_impl.cc:886:56:
note: in instantiation of function template specialization
'base::strict_cast<int, unsigned int>' requested here
  UMA_HISTOGRAM_SPARSE_SLOWLY("InputMethod.ID2", base::strict_cast<int32_t>(
...

[Shu Chen, 2015/07/17 03:55:50]

Done.

[Shu Chen, 2015/07/17 03:55:50]

The reason was simply because I'd like it be super fast. :)
I've now changed to use base::Hash.

   }
 }
 
@@ -892,8 +882,7 @@ void InputMethodManagerImpl::RecordInputMethodUsage(
   UMA_HISTOGRAM_ENUMERATION("InputMethod.Category",
                             GetInputMethodCategory(input_method_id),
                             INPUT_METHOD_CATEGORY_MAX);
-  UMA_HISTOGRAM_SPARSE_SLOWLY("InputMethod.ID",
-                              stat_id_map_[input_method_id]);
+  UMA_HISTOGRAM_SPARSE_SLOWLY("InputMethod.ID2", stat_id_map_[input_method_id]);
 }
 
 void InputMethodManagerImpl::AddObserver(