grow_calculator.h

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/// @file
 
#ifndef MYSQL_CONTAINERS_BUFFERS_GROW_CALCULATOR_H
#define MYSQL_CONTAINERS_BUFFERS_GROW_CALCULATOR_H
 
#include "mysql/containers/buffers/grow_constraint.h"  // Grow_constraint
 
#include <algorithm>  // std::min
#include <limits>     // std::numeric_limits
#include <string>     // std::string
#ifdef NDEBUG
#include <sstream>  // std::stringstream
#endif
 
/// @addtogroup GroupLibsMysqlContainers
/// @{
 
namespace mysql::containers::buffers {
 
/// Description of a heuristic to determine how much memory to allocate.
///
/// This may be used in diverse contexts such as growing a memory
/// buffer, or growing a pool of objects.
///
/// This encapsulates several common heuristics for growth:
///
/// - The growth rate can be exponential.  This is useful for cases
///   such as contiguous memory buffers, where each size increment may
///   copy all the existing data, e.g., using 'realloc'.  Or, more
///   generally, any data structure where size growth has a cost that
///   is linear in the total size.  In such cases an exponential
///   growth rate ensures that execution time is not quadratic in the
///   number of grow operations.
///
/// - The growth rate can be linear.  This is useful for cases such as
///   linked lists, where each size increment is linear in the
///   increment size.
///
/// - There can be an upper bound on the size.  This is useful
///   e.g. when there are configurable memory limits.
///
/// - The size can be specified to be a multiple of a given number.
///   This can potentially be useful if there is a way to align
///   allocated objects to page sizes, or similar.
class Grow_calculator : public Grow_constraint {
 public:
  using Grow_constraint::Size_t;
  /// Return type for compute_new_size.
  using Grow_constraint::Result_t;
  /// By default, limit memory to 1 GiB.
  static constexpr Size_t default_max_size =
      Size_t(1024) * Size_t(1024) * Size_t(1024);
  /// By default, double the size in each allocation.
  static constexpr double default_grow_factor = 2.0;
  /// By default, allocate at least 1 KiB more in each call.
  static constexpr Size_t default_grow_increment = 1024;
  /// By default, allocate multiples of 1 KiB.
  static constexpr Size_t default_block_size = 1024;
 
  Grow_calculator();
 
  /// Compute the new size.
  ///
  /// This follows the following rules:
  ///
  /// - It returns exceeds_max_size if the requested size, or the
  ///   existing size, exceeds the configured max size.
  ///
  /// - It never shrinks.  If the request is smaller than the existing
  ///   size, it just returns the existing size.
  ///
  /// - It multiplies the old size by the grow_factor, and if needed
  ///   increments the size further until it has grown by the
  ///   grow_increment, and then rounds up to the nearest multiple of
  ///   the block_size.  If the result of these operations exceeds the
  ///   max size, the result is reduced to the max size.
  ///
  /// @param old_size The existing size.
  ///
  /// @param requested_size The total size needed.
  ///
  /// @retval A pair.  The first component is `bool` and contains the
  /// error status: `false` means success, i.e., the requested size
  /// does not exceed the maximum size.  It also counts as success if
  /// the request is less than the existing size, or if the request is
  /// zero.  `true` means error, i.e., the requested size exceeds the
  /// maximum size.  The second component is the new size.  If the
  /// first component is `true` for error, the second component is
  /// zero.
  ///
  /// @retval other value The new size.
  Result_t compute_new_size(Size_t old_size, Size_t requested_size) const;
};
 
}  // namespace mysql::containers::buffers
 
/// @}
 
#endif /* MYSQL_CONTAINERS_BUFFERS_GROW_CALCULATOR_H */