pw_hdlc/public/pw_hdlc/internal/protocol.h

// Copyright 2021 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_varint/varint.h"

namespace pw::hdlc {

inline constexpr std::byte kFlag = std::byte{0x7E};
inline constexpr std::byte kEscape = std::byte{0x7D};
inline constexpr std::byte kEscapeConstant = std::byte{0x20};

inline constexpr std::array<std::byte, 2> kEscapedFlag = {kEscape,
                                                          std::byte{0x5E}};
inline constexpr std::array<std::byte, 2> kEscapedEscape = {kEscape,
                                                            std::byte{0x5D}};

inline constexpr varint::Format kAddressFormat =
    varint::Format::kOneTerminatedLeastSignificant;

constexpr bool NeedsEscaping(std::byte b) {
  return (b == kFlag || b == kEscape);
}

constexpr std::byte Escape(std::byte b) { return b ^ kEscapeConstant; }

// Class that manages the 1-byte control field of an HDLC U-frame.
class UFrameControl {
 public:
  static constexpr UFrameControl UnnumberedInformation() {
    return UFrameControl(kUnnumberedInformation);
  }

  constexpr std::byte data() const { return data_; }

 private:
  // Types of HDLC U-frames and their bit patterns.
  enum Type : uint8_t {
    kUnnumberedInformation = 0x00,
  };

  constexpr UFrameControl(Type type)
      : data_(kUFramePattern | std::byte{type}) {}

  // U-frames are identified by having the bottom two control bits set.
  static constexpr std::byte kUFramePattern = std::byte{0x03};

  std::byte data_;
};

}  // namespace pw::hdlc