text: /**
   *    Copyright (C) 2018-present MongoDB, Inc.
   *
   *    This program is free software: you can redistribute it and/or modify
   *    it under the terms of the Server Side Public License, version 1,
   *    as published by MongoDB, Inc.
   *
   *    This program is distributed in the hope that it will be useful,
   *    but WITHOUT ANY WARRANTY; without even the implied warranty of
   *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   *    Server Side Public License for more details.
   *
   *    You should have received a copy of the Server Side Public License
   *    along with this program. If not, see
   *    <http://www.mongodb.com/licensing/server-side-public-license>.
   *
   *    As a special exception, the copyright holders give permission to link the
   *    code of portions of this program with the OpenSSL library under certain
   *    conditions as described in each individual source file and distribute
   *    linked combinations including the program with the OpenSSL library. You
   *    must comply with the Server Side Public License in all respects for
   *    all of the code used other than as permitted herein. If you modify file(s)
   *    with this exception, you may extend this exception to your version of the
   *    file(s), but you are not obligated to do so. If you do not wish to do so,
   *    delete this exception statement from your version. If you delete this
   *    exception statement from all source files in the program, then also delete
   *    it in the license file.
   */
  
  #include "mongo/platform/basic.h"
  
  #include "mongo/platform/stack_locator.h"
  
  #include "mongo/util/assert_util.h"
  
  namespace mongo {
  
  boost::optional<std::size_t> StackLocator::available() const {
      if (!begin() || !end())
          return boost::none;
  
      // Technically, it is undefined behavior to compare or subtract
      // two pointers that do not point into the same
      // aggregate. However, we know that these are both pointers within
      // the same stack, and it seems unlikely that the compiler will
      // see that it can elide the comparison here.
  
      const auto cbegin = reinterpret_cast<const char*>(begin());
      const auto cthis = reinterpret_cast<const char*>(this);
      const auto cend = reinterpret_cast<const char*>(end());
  
      // TODO: Assumes that stack grows downward
      invariant(cthis <= cbegin);
      invariant(cthis > cend);
  
      std::size_t avail = cthis - cend;
  
      return avail;
  }
  
  boost::optional<size_t> StackLocator::size() const {
      if (!begin() || !end())
          return boost::none;
  
      const auto cbegin = reinterpret_cast<const char*>(begin());
      const auto cend = reinterpret_cast<const char*>(end());
  
      // TODO: Assumes that stack grows downward
      invariant(cbegin > cend);
  
      return static_cast<size_t>(cbegin - cend);
  }
  
  }  // namespace mongo
  