From 9e4aa1f98c1e47b3ccd7599d57d26eb31d8a5fe1 Mon Sep 17 00:00:00 2001
From: Alexander Block <ablock84@gmail.com>
Date: Mon, 11 Mar 2019 06:33:46 +0100
Subject: [PATCH] Implement unordered_lru_cache

---
 src/Makefile.am           |   1 +
 src/unordered_lru_cache.h | 110 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 111 insertions(+)
 create mode 100644 src/unordered_lru_cache.h

diff --git a/src/Makefile.am b/src/Makefile.am
index e87247f8b49f4..25818bc2029c9 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -227,6 +227,7 @@ BITCOIN_CORE_H = \
   txmempool.h \
   ui_interface.h \
   undo.h \
+  unordered_lru_cache.h \
   util.h \
   utilmoneystr.h \
   utiltime.h \
diff --git a/src/unordered_lru_cache.h b/src/unordered_lru_cache.h
new file mode 100644
index 0000000000000..df685e260fe61
--- /dev/null
+++ b/src/unordered_lru_cache.h
@@ -0,0 +1,110 @@
+// Copyright (c) 2019 The Dash Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UNORDERED_LRU_CACHE_H
+#define BITCOIN_UNORDERED_LRU_CACHE_H
+
+#include <unordered_map>
+
+template<typename Key, typename Value, typename Hasher, size_t MaxSize = 0, size_t TruncateThreshold = 0>
+class unordered_lru_cache
+{
+private:
+    typedef std::unordered_map<Key, std::pair<Value, int64_t>, Hasher> MapType;
+
+    MapType cacheMap;
+    size_t maxSize;
+    size_t truncateThreshold;
+    int64_t accessCounter{0};
+
+public:
+    unordered_lru_cache(size_t _maxSize = MaxSize, size_t _truncateThreshold = TruncateThreshold) :
+        maxSize(_maxSize),
+        truncateThreshold(_truncateThreshold == 0 ? _maxSize * 2 : _truncateThreshold)
+    {
+        // either specify maxSize through template arguments or the contructor and fail otherwise
+        assert(_maxSize != 0);
+    }
+
+
+    template<typename Value2>
+    void _emplace(const Key& key, Value2&& v)
+    {
+        truncate_if_needed();
+        auto it = cacheMap.find(key);
+        if (it == cacheMap.end()) {
+            cacheMap.emplace(key, std::make_pair(std::forward<Value2>(v), accessCounter++));
+        } else {
+            it->second.first = std::forward<Value2>(v);
+            it->second.second = accessCounter++;
+        }
+    }
+
+    void emplace(const Key& key, Value&& v)
+    {
+        _emplace(key, v);
+    }
+
+    void insert(const Key& key, const Value& v)
+    {
+        _emplace(key, v);
+    }
+
+    bool get(const Key& key, Value& value)
+    {
+        auto it = cacheMap.find(key);
+        if (it != cacheMap.end()) {
+            it->second.second = accessCounter++;
+            value = it->second.first;
+            return true;
+        }
+        return false;
+    }
+
+    bool exists(const Key& key)
+    {
+        auto it = cacheMap.find(key);
+        if (it != cacheMap.end()) {
+            it->second.second = accessCounter++;
+            return true;
+        }
+        return false;
+    }
+
+    void erase(const Key& key)
+    {
+        cacheMap.erase(key);
+    }
+
+    void clear()
+    {
+        cacheMap.clear();
+    }
+
+private:
+    void truncate_if_needed()
+    {
+        typedef typename MapType::iterator Iterator;
+
+        if (cacheMap.size() <= truncateThreshold) {
+            return;
+        }
+
+        std::vector<Iterator> vec;
+        vec.reserve(cacheMap.size());
+        for (auto it = cacheMap.begin(); it != cacheMap.end(); ++it) {
+            vec.emplace_back(it);
+        }
+        // sort by last access time (descending order)
+        std::sort(vec.begin(), vec.end(), [](const Iterator& it1, const Iterator& it2) {
+            return it1->second.second > it2->second.second;
+        });
+
+        for (size_t i = maxSize; i < vec.size(); i++) {
+            cacheMap.erase(vec[i]);
+        }
+    }
+};
+
+#endif // BITCOIN_UNORDERED_LRU_CACHE_H