Github user zuyu commented on a diff in the pull request:
https://github.com/apache/incubator-quickstep/pull/309#discussion_r143522055
--- Diff: query_execution/ProbabilityStore.hpp ---
@@ -0,0 +1,274 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ **/
+
+#ifndef QUICKSTEP_QUERY_EXECUTION_PROBABILITY_STORE_HPP_
+#define QUICKSTEP_QUERY_EXECUTION_PROBABILITY_STORE_HPP_
+
+#include <algorithm>
+#include <cstddef>
+#include <random>
+#include <unordered_map>
+#include <vector>
+
+#include "utility/Macros.hpp"
+
+#include "glog/logging.h"
+
+namespace quickstep {
+
+/**
+ * @brief A class that stores the probabilities of objects. We use an
integer field
+ * called "key" to identify each object.
+ * A probability is expressed as a fraction. All the objects share
a common denominator.
+ **/
+class ProbabilityStore {
+ public:
+ /**
+ * @brief Constructor.
+ **/
+ ProbabilityStore()
+ : common_denominator_(1.0) {}
+
+ /**
+ * @brief Get the number of objects in the store.
+ **/
+ inline const std::size_t getNumObjects() const {
+ DCHECK_EQ(individual_probabilities_.size(),
cumulative_probabilities_.size());
+ return individual_probabilities_.size();
+ }
+
+ /**
+ * @brief Get the common denominator.
+ */
+ inline const std::size_t getDenominator() const {
--- End diff --
Remove `const` in the return type.
---
