Cpu usage optimization:
1/ Avoid unnecessary field/dimension proto construction.
2/ use unordered_map for slicing.
3/ Use dimension fields to compare dimension keys.

Test: all statsd tests passed.
Change-Id: I2f74f78589b7f6ecd0803a2ead822b8d0399f334
diff --git a/cmds/statsd/src/HashableDimensionKey.cpp b/cmds/statsd/src/HashableDimensionKey.cpp
index 288ebe9..857a6dd 100644
--- a/cmds/statsd/src/HashableDimensionKey.cpp
+++ b/cmds/statsd/src/HashableDimensionKey.cpp
@@ -69,18 +69,17 @@
 
 using std::string;
 
-
 string HashableDimensionKey::toString() const {
     string flattened;
     DimensionsValueToString(getDimensionsValue(), &flattened);
     return flattened;
 }
 
-bool compareDimensionsValue(const DimensionsValue& s1, const DimensionsValue& s2) {
+bool EqualsTo(const DimensionsValue& s1, const DimensionsValue& s2) {
     if (s1.field() != s2.field()) {
         return false;
     }
-    if (s1.value_case() != s1.value_case()) {
+    if (s1.value_case() != s2.value_case()) {
         return false;
     }
     switch (s1.value_case()) {
@@ -102,8 +101,8 @@
                 }
                 bool allMatched = true;
                 for (int i = 0; allMatched && i < s1.value_tuple().dimensions_value_size(); ++i) {
-                    allMatched &= compareDimensionsValue(s1.value_tuple().dimensions_value(i),
-                                                    s2.value_tuple().dimensions_value(i));
+                    allMatched &= EqualsTo(s1.value_tuple().dimensions_value(i),
+                                           s2.value_tuple().dimensions_value(i));
                 }
                 return allMatched;
             }
@@ -113,12 +112,54 @@
     }
 }
 
+bool LessThan(const DimensionsValue& s1, const DimensionsValue& s2) {
+    if (s1.field() != s2.field()) {
+        return s1.field() < s2.field();
+    }
+    if (s1.value_case() != s2.value_case()) {
+        return s1.value_case() < s2.value_case();
+    }
+    switch (s1.value_case()) {
+        case DimensionsValue::ValueCase::kValueStr:
+            return s1.value_str() < s2.value_str();
+        case DimensionsValue::ValueCase::kValueInt:
+            return s1.value_int() < s2.value_int();
+        case DimensionsValue::ValueCase::kValueLong:
+            return s1.value_long() < s2.value_long();
+        case DimensionsValue::ValueCase::kValueBool:
+            return (int)s1.value_bool() < (int)s2.value_bool();
+        case DimensionsValue::ValueCase::kValueFloat:
+            return s1.value_float() < s2.value_float();
+        case DimensionsValue::ValueCase::kValueTuple:
+            {
+                if (s1.value_tuple().dimensions_value_size() !=
+                        s2.value_tuple().dimensions_value_size()) {
+                    return s1.value_tuple().dimensions_value_size() <
+                        s2.value_tuple().dimensions_value_size();
+                }
+                for (int i = 0;  i < s1.value_tuple().dimensions_value_size(); ++i) {
+                    if (EqualsTo(s1.value_tuple().dimensions_value(i),
+                                 s2.value_tuple().dimensions_value(i))) {
+                        continue;
+                    } else {
+                        return LessThan(s1.value_tuple().dimensions_value(i),
+                                        s2.value_tuple().dimensions_value(i));
+                    }
+                }
+                return false;
+            }
+        case DimensionsValue::ValueCase::VALUE_NOT_SET:
+        default:
+            return false;
+    }
+}
+
 bool HashableDimensionKey::operator==(const HashableDimensionKey& that) const {
-    return compareDimensionsValue(getDimensionsValue(), that.getDimensionsValue());
+    return EqualsTo(getDimensionsValue(), that.getDimensionsValue());
 };
 
 bool HashableDimensionKey::operator<(const HashableDimensionKey& that) const {
-    return toString().compare(that.toString()) < 0;
+    return LessThan(getDimensionsValue(), that.getDimensionsValue());
 };
 
 }  // namespace statsd