struct data {

Key key;

std::any value;

std::int64_t index;

};

using shared_list = std::shared_ptr<std::vector<data>>;

std::int64_t index_;

shared_list list_;

public:

static flat_impl make_root() {

flat_impl i;

i.index_ = -1;

i.list_ = std::make_shared<std::vector<data>>();

return i;

}

static flat_impl make(Key key, std::any value, const flat_impl& parent) {

flat_impl impl;

data d{std::move(key), std::move(value), parent.index_};

parent.list_->emplace_back(std::move(d));

impl.list_ = parent.list_;

impl.index_ = parent.list_->size() - 1;

assert(impl.index_ > parent.index_);

return impl;

}

std::any lookup(const Key& key) const {

auto i = index_;

while (i >= 0) {

const auto& d = (*list_)[i];

if (KeyEqual{}(d.key, key)) {

return d.value;

}

i = d.index;

}

return std::any{};

}