1 // Copyright 2022 The Android Open Source Project
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 #pragma once
16 
17 #include <list>
18 #include <unordered_map>
19 
20 namespace android {
21 namespace base {
22 
23 template <typename Key, typename Value>
24 class LruCache {
25   public:
LruCache(std::size_t maxSize)26     LruCache(std::size_t maxSize) : m_maxSize(maxSize) {
27         m_table.reserve(maxSize);
28     }
29 
get(const Key & key)30     Value* get(const Key& key) {
31         auto tableIt = m_table.find(key);
32         if (tableIt == m_table.end()) {
33             return nullptr;
34         }
35 
36         // Move to front.
37         auto elementsIt = tableIt->second;
38         m_elements.splice(elementsIt, m_elements, m_elements.begin());
39         return &elementsIt->value;
40     }
41 
set(const Key & key,Value && value)42     void set(const Key& key, Value&& value) {
43         auto tableIt = m_table.find(key);
44         if (tableIt == m_table.end()) {
45             if (m_table.size() >= m_maxSize) {
46                 auto& kv = m_elements.back();
47                 m_table.erase(kv.key);
48                 m_elements.pop_back();
49             }
50         } else {
51             auto elementsIt = tableIt->second;
52             m_elements.erase(elementsIt);
53         }
54         m_elements.emplace_front(KeyValue{
55             key,
56             std::forward<Value>(value),
57         });
58         m_table[key] = m_elements.begin();
59     }
60 
remove(const Key & key)61     void remove(const Key& key) {
62         auto tableIt = m_table.find(key);
63         if (tableIt == m_table.end()) {
64             return;
65         }
66         auto elementsIt = tableIt->second;
67         m_elements.erase(elementsIt);
68         m_table.erase(tableIt);
69     }
70 
71   private:
72     struct KeyValue {
73         Key key;
74         Value value;
75     };
76 
77     const std::size_t m_maxSize;
78     // Front is the most recently used and back is the least recently used.
79     std::list<KeyValue> m_elements;
80     std::unordered_map<Key, typename std::list<KeyValue>::iterator> m_table;
81 };
82 
83 }  // namespace base
84 }  // namespace android
85