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 template <typename Key, typename Value>
21 class LruCache {
22    public:
LruCache(std::size_t maxSize)23     LruCache(std::size_t maxSize) : m_maxSize(maxSize) { m_table.reserve(maxSize); }
24 
get(const Key & key)25     Value* get(const Key& key) {
26         auto tableIt = m_table.find(key);
27         if (tableIt == m_table.end()) {
28             return nullptr;
29         }
30 
31         // Move to front.
32         auto elementsIt = tableIt->second;
33         m_elements.splice(elementsIt, m_elements, m_elements.begin());
34         return &elementsIt->value;
35     }
36 
set(const Key & key,Value && value)37     void set(const Key& key, Value&& value) {
38         auto tableIt = m_table.find(key);
39         if (tableIt == m_table.end()) {
40             if (m_table.size() >= m_maxSize) {
41                 auto& kv = m_elements.back();
42                 m_table.erase(kv.key);
43                 m_elements.pop_back();
44             }
45         } else {
46             auto elementsIt = tableIt->second;
47             m_elements.erase(elementsIt);
48         }
49         m_elements.emplace_front(KeyValue{
50             key,
51             std::forward<Value>(value),
52         });
53         m_table[key] = m_elements.begin();
54     }
55 
remove(const Key & key)56     void remove(const Key& key) {
57         auto tableIt = m_table.find(key);
58         if (tableIt == m_table.end()) {
59             return;
60         }
61         auto elementsIt = tableIt->second;
62         m_elements.erase(elementsIt);
63         m_table.erase(tableIt);
64     }
65 
clear()66     void clear() {
67         m_elements.clear();
68         m_table.clear();
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