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1 | // -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*- |
2 | // vim: ts=8 sw=2 smarttab | |
3 | ||
4 | #pragma once | |
5 | ||
6 | #include <list> | |
7 | #include <map> | |
8 | #include <optional> | |
9 | #include <type_traits> | |
10 | #include <unordered_map> | |
11 | ||
12 | template <class Key, class Value, bool Ordered> | |
13 | class SimpleLRU { | |
14 | static_assert(std::is_default_constructible_v<Value>); | |
15 | using list_type = std::list<Key>; | |
16 | template<class K, class V> | |
17 | using map_t = std::conditional_t<Ordered, | |
18 | std::map<K, V>, | |
19 | std::unordered_map<K, V>>; | |
20 | using map_type = map_t<Key, std::pair<Value, typename list_type::iterator>>; | |
21 | list_type lru; | |
22 | map_type cache; | |
23 | const size_t max_size; | |
24 | ||
25 | public: | |
26 | SimpleLRU(size_t size = 20) | |
27 | : cache(size), | |
28 | max_size(size) | |
29 | {} | |
30 | size_t size() const { | |
31 | return cache.size(); | |
32 | } | |
33 | size_t capacity() const { | |
34 | return max_size; | |
35 | } | |
36 | using insert_return_type = std::pair<Value, bool>; | |
37 | insert_return_type insert(const Key& key, Value value); | |
38 | std::optional<Value> find(const Key& key); | |
39 | std::optional<std::enable_if<Ordered, Value>> lower_bound(const Key& key); | |
40 | void erase(const Key& key); | |
41 | void clear(); | |
42 | private: | |
43 | // bump the item to the front of the lru list | |
44 | Value _lru_add(typename map_type::iterator found); | |
45 | // evict the last element of most recently used list | |
46 | void _evict(); | |
47 | }; | |
48 | ||
49 | template <class Key, class Value, bool Ordered> | |
50 | typename SimpleLRU<Key,Value,Ordered>::insert_return_type | |
51 | SimpleLRU<Key,Value,Ordered>::insert(const Key& key, Value value) | |
52 | { | |
53 | if constexpr(Ordered) { | |
54 | auto found = cache.lower_bound(key); | |
55 | if (found != cache.end() && found->first == key) { | |
56 | // already exists | |
57 | return {found->second.first, true}; | |
58 | } else { | |
59 | if (size() >= capacity()) { | |
60 | _evict(); | |
61 | } | |
62 | lru.push_front(key); | |
63 | // use lower_bound as hint to save the lookup | |
64 | cache.emplace_hint(found, key, std::make_pair(value, lru.begin())); | |
65 | return {std::move(value), false}; | |
66 | } | |
67 | } else { | |
68 | // cache is not ordered | |
69 | auto found = cache.find(key); | |
70 | if (found != cache.end()) { | |
71 | // already exists | |
72 | return {found->second.first, true}; | |
73 | } else { | |
74 | if (size() >= capacity()) { | |
75 | _evict(); | |
76 | } | |
77 | lru.push_front(key); | |
78 | cache.emplace(key, std::make_pair(value, lru.begin())); | |
79 | return {std::move(value), false}; | |
80 | } | |
81 | } | |
82 | } | |
83 | ||
84 | template <class Key, class Value, bool Ordered> | |
85 | std::optional<Value> SimpleLRU<Key,Value,Ordered>::find(const Key& key) | |
86 | { | |
87 | if (auto found = cache.find(key); found != cache.end()){ | |
88 | return _lru_add(found); | |
89 | } else { | |
90 | return {}; | |
91 | } | |
92 | } | |
93 | ||
94 | template <class Key, class Value, bool Ordered> | |
95 | std::optional<std::enable_if<Ordered, Value>> | |
96 | SimpleLRU<Key,Value,Ordered>::lower_bound(const Key& key) | |
97 | { | |
98 | if (auto found = cache.lower_bound(key); found != cache.end()) { | |
99 | return _lru_add(found); | |
100 | } else { | |
101 | return {}; | |
102 | } | |
103 | } | |
104 | ||
105 | template <class Key, class Value, bool Ordered> | |
106 | void SimpleLRU<Key,Value,Ordered>::clear() | |
107 | { | |
108 | lru.clear(); | |
109 | cache.clear(); | |
110 | } | |
111 | ||
112 | template <class Key, class Value, bool Ordered> | |
113 | void SimpleLRU<Key,Value,Ordered>::erase(const Key& key) | |
114 | { | |
115 | if (auto found = cache.find(key); found != cache.end()) { | |
9f95a23c | 116 | lru.erase(found->second.second); |
11fdf7f2 TL |
117 | cache.erase(found); |
118 | } | |
119 | } | |
120 | ||
121 | template <class Key, class Value, bool Ordered> | |
122 | Value SimpleLRU<Key,Value,Ordered>::_lru_add( | |
123 | typename SimpleLRU<Key,Value,Ordered>::map_type::iterator found) | |
124 | { | |
125 | auto& [value, in_lru] = found->second; | |
126 | if (in_lru != lru.begin()){ | |
127 | // move item to the front | |
128 | lru.splice(lru.begin(), lru, in_lru); | |
129 | } | |
130 | // the item is already at the front | |
131 | return value; | |
132 | } | |
133 | ||
134 | template <class Key, class Value, bool Ordered> | |
135 | void SimpleLRU<Key,Value,Ordered>::_evict() | |
136 | { | |
137 | // evict the last element of most recently used list | |
138 | auto last = --lru.end(); | |
139 | cache.erase(*last); | |
140 | lru.erase(last); | |
141 | } |