pw_status/public/pw_status/status_with_size.h

// Copyright 2020 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#pragma once

#include <cstddef>

#include "pw_status/status.h"

namespace pw {

// StatusWithSize stores a status and an unsigned integer. The integer must not
// exceed StatusWithSize::max_size(), which is 134,217,727 (2**27 - 1) on 32-bit
// systems.
//
// StatusWithSize is useful for reporting the number of bytes read or written in
// an operation along with the status. For example, a function that writes a
// formatted string may want to report both the number of characters written and
// whether it ran out of space.
//
// StatusWithSize is more efficient than its alternatives. It packs a status and
// size into a single word, which can be returned from a function in a register.
// Because they are packed together, the size is limited to max_size().
//
// StatusWithSize's alternatives result in larger code size. For example:
//
//   1. Return status, pass size output as a pointer argument.
//
//      Requires an additional argument and forces the output argument to the
//      stack in order to pass an address, increasing code size.
//
//   2. Return an object with Status and size members.
//
//      At least for ARMv7-M, the returned struct is created on the stack, which
//      increases code size.
//
class StatusWithSize {
 public:
  // Creates a StatusWithSize with Status::OK and the provided size.
  // TODO(hepler): Add debug-only assert that size <= max_size().
  explicit constexpr StatusWithSize(size_t size = 0) : size_(size) {}

  // Creates a StatusWithSize with the provided status and size.
  explicit constexpr StatusWithSize(Status status, size_t size)
      : StatusWithSize(size | (static_cast<size_t>(status) << kStatusShift)) {}

  // Allow implicit conversions from status.
  constexpr StatusWithSize(Status status)
      : StatusWithSize(static_cast<size_t>(status) << kStatusShift) {}
  constexpr StatusWithSize(Status::Code status)
      : StatusWithSize(Status(status)) {}

  constexpr StatusWithSize(const StatusWithSize&) = default;
  constexpr StatusWithSize& operator=(const StatusWithSize&) = default;

  constexpr operator Status() const { return status(); }

  // Returns the size. The size is always present, even if status() is an error.
  constexpr size_t size() const { return size_ & kSizeMask; }

  // The maximum valid value for size.
  static constexpr size_t max_size() { return kSizeMask; }

  // True if status() == Status::OK.
  constexpr bool ok() const { return (size_ & kStatusMask) == 0u; }

  constexpr Status status() const {
    return static_cast<Status::Code>((size_ & kStatusMask) >> kStatusShift);
  }

 private:
  static constexpr size_t kStatusBits = 5;
  static constexpr size_t kSizeMask = ~static_cast<size_t>(0) >> kStatusBits;
  static constexpr size_t kStatusMask = ~kSizeMask;
  static constexpr size_t kStatusShift = sizeof(size_t) * 8 - kStatusBits;

  size_t size_;
};

}  // namespace pw