pw_hdlc_lite/decoder.cc

// 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.

#include "pw_hdlc_lite/decoder.h"

#include "pw_assert/assert.h"
#include "pw_bytes/endian.h"
#include "pw_checksum/crc32.h"
#include "pw_hdlc_lite_private/protocol.h"
#include "pw_log/log.h"

using std::byte;

namespace pw::hdlc_lite {
namespace {

constexpr byte kUnescapeConstant = byte{0x20};

}  // namespace

Result<Frame> Decoder::Process(const byte new_byte) {
  switch (state_) {
    case State::kInterFrame: {
      if (new_byte == kFlag) {
        state_ = State::kFrame;

        // Report an error if non-flag bytes were read between frames.
        if (current_frame_size_ != 0u) {
          current_frame_size_ = 0;
          return Status::DATA_LOSS;
        }
      } else {
        // Count bytes to track how many are discarded.
        current_frame_size_ += 1;
      }
      return Status::UNAVAILABLE;  // Report error when starting a new frame.
    }
    case State::kFrame: {
      if (new_byte == kFlag) {
        const Status status = CheckFrame();

        state_ = State::kFrame;
        const size_t completed_frame_size = current_frame_size_;
        current_frame_size_ = 0;

        if (status.ok()) {
          return Frame(buffer_.first(completed_frame_size));
        }
        return status;
      }

      if (new_byte == kEscape) {
        state_ = State::kFrameEscape;
      } else {
        AppendByte(new_byte);
      }
      return Status::UNAVAILABLE;
    }
    case State::kFrameEscape: {
      // The flag character cannot be escaped; return an error.
      if (new_byte == kFlag) {
        state_ = State::kFrame;
        current_frame_size_ = 0;
        return Status::DATA_LOSS;
      }

      if (new_byte == kEscape) {
        // Two escape characters in a row is illegal -- invalidate this frame.
        // The frame is reported abandoned when the next flag byte appears.
        state_ = State::kInterFrame;

        // Count the escape byte so that the inter-frame state detects an error.
        current_frame_size_ += 1;
      } else {
        state_ = State::kFrame;
        AppendByte(new_byte ^ kUnescapeConstant);
      }
      return Status::UNAVAILABLE;
    }
  }
  PW_CRASH("Bad decoder state");
}

void Decoder::AppendByte(byte new_byte) {
  if (current_frame_size_ < max_size()) {
    buffer_[current_frame_size_] = new_byte;
  }

  // Always increase size: if it is larger than the buffer, overflow occurred.
  current_frame_size_ += 1;
}

Status Decoder::CheckFrame() const {
  // Empty frames are not an error; repeated flag characters are okay.
  if (current_frame_size_ == 0u) {
    return Status::UNAVAILABLE;
  }

  if (current_frame_size_ < Frame::kMinSizeBytes) {
    PW_LOG_ERROR("Received %lu-byte frame; frame must be at least 6 bytes",
                 static_cast<unsigned long>(current_frame_size_));
    return Status::DATA_LOSS;
  }

  if (current_frame_size_ > max_size()) {
    PW_LOG_ERROR("Frame size [%lu] exceeds the maximum buffer size [%lu]",
                 static_cast<unsigned long>(current_frame_size_),
                 static_cast<unsigned long>(max_size()));
    return Status::RESOURCE_EXHAUSTED;
  }

  if (!VerifyFrameCheckSequence()) {
    PW_LOG_ERROR("Frame check sequence verification failed");
    return Status::DATA_LOSS;
  }

  return Status::OK;
}

bool Decoder::VerifyFrameCheckSequence() const {
  uint32_t fcs = bytes::ReadInOrder<uint32_t>(
      std::endian::little, buffer_.data() + current_frame_size_ - sizeof(fcs));
  return fcs == checksum::Crc32::Calculate(
                    buffer_.first(current_frame_size_ - sizeof(fcs)));
}

}  // namespace pw::hdlc_lite