include/ling3/state_cache.h

#pragma once
#include <cstddef>
#include <list>
#include <string>
#include <unordered_map>
#include <utility>

namespace ling3 {
// Caller serializes access. Values should own immutable snapshots, so an active
// reader can safely retain a copy even after the cache releases its reference.
template<class T> class StateCache {
    struct Entry { std::string key; T value; std::size_t bytes; };
    using Iterator = typename std::list<Entry>::iterator;
    std::list<Entry> entries_;
    std::unordered_map<std::string, Iterator> index_;
    std::size_t limit_, bytes_ = 0, evictions_ = 0;
public:
    explicit StateCache(std::size_t limit) : limit_(limit) {}
    StateCache(const StateCache &) = delete;
    StateCache & operator=(const StateCache &) = delete;
    StateCache(StateCache &&) = default;
    StateCache & operator=(StateCache &&) = default;
    const T * Get(const std::string & key) {
        auto found = index_.find(key);
        if (found == index_.end()) return nullptr;
        entries_.splice(entries_.begin(), entries_, found->second);
        return &found->second->value;
    }
    bool Erase(const std::string & key) {
        auto found = index_.find(key);
        if (found == index_.end()) return false;
        bytes_ -= found->second->bytes; entries_.erase(found->second); index_.erase(found);
        return true;
    }
    bool Put(std::string key, T value, std::size_t bytes) {
        // An oversized update must not leave an old version under the same key.
        Erase(key);
        if (!limit_ || bytes > limit_) return false;
        while (!entries_.empty() && bytes_ > limit_-bytes) {
            Erase(entries_.back().key); ++evictions_;
        }
        entries_.push_front({std::move(key), std::move(value), bytes});
        try { index_.emplace(entries_.front().key, entries_.begin()); }
        catch (...) { entries_.pop_front(); throw; }
        bytes_ += bytes; return true;
    }
    void Clear() { index_.clear(); entries_.clear(); bytes_ = 0; }
    std::size_t bytes() const { return bytes_; }
    std::size_t limit() const { return limit_; }
    std::size_t size() const { return entries_.size(); }
    std::size_t evictions() const { return evictions_; }
};
} // namespace ling3