libcxx/src/include/tzdb/tzdb_list_private.h

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

// For information see https://libcxx.llvm.org/DesignDocs/TimeZone.html

#ifndef _LIBCPP_SRC_INCLUDE_TZDB_TZDB_LIST_PRIVATE_H
#define _LIBCPP_SRC_INCLUDE_TZDB_TZDB_LIST_PRIVATE_H

#include <__mutex/unique_lock.h>
#include <forward_list>

// When threads are not available the locking is not required.
// When threads are available, we use std::mutex over std::shared_mutex
// due to the increased overhead of std::shared_mutex.
// See shared_mutex_vs_mutex.bench.cpp
#ifndef _LIBCPP_HAS_NO_THREADS
#  include <mutex>
#endif

#include "types_private.h"
#include "tzdb_private.h"

_LIBCPP_BEGIN_NAMESPACE_STD

namespace chrono {

//===----------------------------------------------------------------------===//
//                          Private API
//===----------------------------------------------------------------------===//

// The tzdb_list stores a list of "tzdb" entries.
//
// The public tzdb database does not store the RULE entries of the IANA
// database. These entries are considered an implementation detail. Since most
// of the tzdb_list interface is exposed as "a list of tzdb entries" it's not
// possible to use a helper struct that stores a tzdb and the RULE database.
// Instead this class stores these in parallel forward lists.
//
// Since the nodes of a forward_list are stable it's possible to store pointers
// and references to these nodes.
class tzdb_list::__impl {
public:
  __impl() { __load_no_lock(); }

  [[nodiscard]] const tzdb& __load() {
#ifndef _LIBCPP_HAS_NO_THREADS
    unique_lock __lock{__mutex_};
#endif
    __load_no_lock();
    return __tzdb_.front();
  }

  using const_iterator = tzdb_list::const_iterator;

  const tzdb& __front() const noexcept {
#ifndef _LIBCPP_HAS_NO_THREADS
    unique_lock __lock{__mutex_};
#endif
    return __tzdb_.front();
  }

  const_iterator __erase_after(const_iterator __p) {
#ifndef _LIBCPP_HAS_NO_THREADS
    unique_lock __lock{__mutex_};
#endif

    __rules_.erase_after(std::next(__rules_.cbegin(), std::distance(__tzdb_.cbegin(), __p)));
    return __tzdb_.erase_after(__p);
  }

  const_iterator __begin() const noexcept {
#ifndef _LIBCPP_HAS_NO_THREADS
    unique_lock __lock{__mutex_};
#endif
    return __tzdb_.begin();
  }
  const_iterator __end() const noexcept {
    //  forward_list<T>::end does not access the list, so no need to take a lock.
    return __tzdb_.end();
  }

private:
  // Loads the tzdbs
  // pre: The caller ensures the locking, if needed, is done.
  void __load_no_lock() { chrono::__init_tzdb(__tzdb_.emplace_front(), __rules_.emplace_front()); }

#ifndef _LIBCPP_HAS_NO_THREADS
  mutable mutex __mutex_;
#endif
  forward_list<tzdb> __tzdb_;

  forward_list<__tz::__rules_storage_type> __rules_;
};

} // namespace chrono

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP_SRC_INCLUDE_TZDB_TZDB_LIST_PRIVATE_H