#include "pw_protobuf/stream_decoder.h"

#include <algorithm>
#include <bit>
#include <cstdint>
#include <cstring>
#include <limits>
#include <optional>
#include <span>

#include "pw_assert/assert.h"
#include "pw_assert/check.h"
#include "pw_containers/vector.h"
#include "pw_function/function.h"
#include "pw_protobuf/encoder.h"
#include "pw_protobuf/internal/codegen.h"
#include "pw_protobuf/wire_format.h"
#include "pw_status/status.h"
#include "pw_status/status_with_size.h"
#include "pw_status/try.h"
#include "pw_varint/stream.h"
#include "pw_varint/varint.h"

namespace pw::protobuf {

Status StreamDecoder::BytesReader::DoSeek(ptrdiff_t offset, Whence origin) {
  PW_TRY(status_);
  if (!decoder_.reader_.seekable()) {
    return Status::Unimplemented();
  }

  ptrdiff_t absolute_position = std::numeric_limits<ptrdiff_t>::min();

  switch (origin) {
    case Whence::kBeginning:
      absolute_position = start_offset_ + offset;
      break;

    case Whence::kCurrent:
      absolute_position = decoder_.position_ + offset;
      break;

    case Whence::kEnd:
      absolute_position = end_offset_ + offset;
      break;
  }

  if (absolute_position < 0) {
    return Status::InvalidArgument();
  }

  if (static_cast<size_t>(absolute_position) < start_offset_ ||
      static_cast<size_t>(absolute_position) >= end_offset_) {
    return Status::OutOfRange();
  }

  PW_TRY(decoder_.reader_.Seek(absolute_position, Whence::kBeginning));
  decoder_.position_ = absolute_position;
  return OkStatus();
}

StatusWithSize StreamDecoder::BytesReader::DoRead(ByteSpan destination) {
  if (!status_.ok()) {
    return StatusWithSize(status_, 0);
  }

  size_t max_length = end_offset_ - decoder_.position_;
  if (destination.size() > max_length) {
    destination = destination.first(max_length);
  }

  Result<ByteSpan> result = decoder_.reader_.Read(destination);
  if (!result.ok()) {
    return StatusWithSize(result.status(), 0);
  }

  decoder_.position_ += result.value().size();
  return StatusWithSize(result.value().size());
}

StreamDecoder::~StreamDecoder() {
  if (parent_ != nullptr) {
    parent_->CloseNestedDecoder(*this);
  } else if (stream_bounds_.high < std::numeric_limits<size_t>::max()) {
    if (status_.ok()) {

      PW_CHECK(Advance(stream_bounds_.high).ok());
    }
  }
}

Status StreamDecoder::Next() {
  PW_CHECK(!nested_reader_open_,
           "Cannot use parent decoder while a nested one is open");

  PW_TRY(status_);

  if (!field_consumed_) {
    PW_TRY(SkipField());
  }

  if (position_ >= stream_bounds_.high) {
    return Status::OutOfRange();
  }

  status_ = ReadFieldKey();
  return status_;
}

StreamDecoder::BytesReader StreamDecoder::GetBytesReader() {
  Status status = CheckOkToRead(WireType::kDelimited);

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status.Update(Status::DataLoss());
  }

  nested_reader_open_ = true;

  if (!status.ok()) {
    return BytesReader(*this, status);
  }

  size_t low = position_;
  size_t high = low + delimited_field_size_;

  return BytesReader(*this, low, high);
}

StreamDecoder StreamDecoder::GetNestedDecoder() {
  Status status = CheckOkToRead(WireType::kDelimited);

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status.Update(Status::DataLoss());
  }

  nested_reader_open_ = true;

  if (!status.ok()) {
    return StreamDecoder(reader_, this, status);
  }

  size_t low = position_;
  size_t high = low + delimited_field_size_;

  return StreamDecoder(reader_, this, low, high);
}

Status StreamDecoder::Advance(size_t end_position) {
  if (reader_.seekable()) {
    PW_TRY(reader_.Seek(end_position - position_, stream::Stream::kCurrent));
    position_ = end_position;
    return OkStatus();
  }

  while (position_ < end_position) {
    std::byte b;
    PW_TRY(reader_.Read(std::span(&b, 1)));
    position_++;
  }
  return OkStatus();
}

void StreamDecoder::CloseBytesReader(BytesReader& reader) {
  status_ = reader.status_;
  if (status_.ok()) {

    PW_CHECK(Advance(reader.end_offset_).ok());
  }

  field_consumed_ = true;
  nested_reader_open_ = false;
}

void StreamDecoder::CloseNestedDecoder(StreamDecoder& nested) {
  PW_CHECK_PTR_EQ(nested.parent_, this);

  nested.nested_reader_open_ = true;
  nested.parent_ = nullptr;

  status_ = nested.status_;
  position_ = nested.position_;
  if (status_.ok()) {

    PW_CHECK(Advance(nested.stream_bounds_.high).ok());
  }

  field_consumed_ = true;
  nested_reader_open_ = false;
}

Status StreamDecoder::ReadFieldKey() {
  PW_DCHECK(field_consumed_);

  uint64_t varint = 0;
  PW_TRY_ASSIGN(size_t bytes_read,
                varint::Read(reader_, &varint, RemainingBytes()));
  position_ += bytes_read;

  if (!FieldKey::IsValidKey(varint)) {
    return Status::DataLoss();
  }

  current_field_ = FieldKey(varint);

  if (current_field_.wire_type() == WireType::kDelimited) {

    StatusWithSize sws = varint::Read(reader_, &varint, RemainingBytes());
    if (sws.IsOutOfRange()) {

      return Status::DataLoss();
    }
    if (!sws.ok()) {
      return sws.status();
    }
    position_ += sws.size();

    if (varint > std::numeric_limits<uint32_t>::max()) {
      return Status::DataLoss();
    }

    delimited_field_size_ = varint;
    delimited_field_offset_ = position_;
  }

  field_consumed_ = false;
  return OkStatus();
}

Result<StreamDecoder::Bounds> StreamDecoder::GetLengthDelimitedPayloadBounds() {
  PW_TRY(CheckOkToRead(WireType::kDelimited));
  return StreamDecoder::Bounds{delimited_field_offset_,
                               delimited_field_size_ + delimited_field_offset_};
}

Status StreamDecoder::SkipField() {
  PW_DCHECK(!field_consumed_);

  size_t bytes_to_skip = 0;
  uint64_t value = 0;

  switch (current_field_.wire_type()) {
    case WireType::kVarint: {

      PW_TRY_ASSIGN(size_t bytes_read,
                    varint::Read(reader_, &value, RemainingBytes()));
      position_ += bytes_read;
      break;
    }
    case WireType::kDelimited:
      bytes_to_skip = delimited_field_size_;
      break;

    case WireType::kFixed32:
      bytes_to_skip = sizeof(uint32_t);
      break;

    case WireType::kFixed64:
      bytes_to_skip = sizeof(uint64_t);
      break;
  }

  if (bytes_to_skip > 0) {

    if (reader_.ConservativeReadLimit() < bytes_to_skip) {
      status_ = Status::DataLoss();
      return status_;
    }

    if (RemainingBytes() < bytes_to_skip) {
      status_ = Status::DataLoss();
      return status_;
    }

    PW_TRY(Advance(position_ + bytes_to_skip));
  }

  field_consumed_ = true;
  return OkStatus();
}

Status StreamDecoder::ReadVarintField(std::span<std::byte> out,
                                      VarintType decode_type) {
  PW_CHECK(out.size() == sizeof(bool) || out.size() == sizeof(uint32_t) ||
               out.size() == sizeof(uint64_t),
           "Protobuf varints must only be used with bool, int32_t, uint32_t, "
           "int64_t, or uint64_t");
  PW_TRY(CheckOkToRead(WireType::kVarint));

  const StatusWithSize sws = ReadOneVarint(out, decode_type);
  if (sws.status() != Status::DataLoss())
    field_consumed_ = true;
  return sws.status();
}

StatusWithSize StreamDecoder::ReadOneVarint(std::span<std::byte> out,
                                            VarintType decode_type) {
  uint64_t value;
  StatusWithSize sws = varint::Read(reader_, &value, RemainingBytes());
  if (sws.IsOutOfRange()) {

    status_ = Status::DataLoss();
    return StatusWithSize(status_, sws.size());
  }
  if (!sws.ok()) {
    return sws;
  }

  position_ += sws.size();

  if (out.size() == sizeof(uint64_t)) {
    if (decode_type == VarintType::kUnsigned) {
      std::memcpy(out.data(), &value, out.size());
    } else {
      const int64_t signed_value = decode_type == VarintType::kZigZag
                                       ? varint::ZigZagDecode(value)
                                       : static_cast<int64_t>(value);
      std::memcpy(out.data(), &signed_value, out.size());
    }
  } else if (out.size() == sizeof(uint32_t)) {
    if (decode_type == VarintType::kUnsigned) {
      if (value > std::numeric_limits<uint32_t>::max()) {
        return StatusWithSize(Status::FailedPrecondition(), sws.size());
      }
      std::memcpy(out.data(), &value, out.size());
    } else {
      const int64_t signed_value = decode_type == VarintType::kZigZag
                                       ? varint::ZigZagDecode(value)
                                       : static_cast<int64_t>(value);
      if (signed_value > std::numeric_limits<int32_t>::max() ||
          signed_value < std::numeric_limits<int32_t>::min()) {
        return StatusWithSize(Status::FailedPrecondition(), sws.size());
      }
      std::memcpy(out.data(), &signed_value, out.size());
    }
  } else if (out.size() == sizeof(bool)) {
    PW_CHECK(decode_type == VarintType::kUnsigned,
             "Protobuf bool can never be signed");
    std::memcpy(out.data(), &value, out.size());
  }

  return sws;
}

Status StreamDecoder::ReadFixedField(std::span<std::byte> out) {
  WireType expected_wire_type =
      out.size() == sizeof(uint32_t) ? WireType::kFixed32 : WireType::kFixed64;
  PW_TRY(CheckOkToRead(expected_wire_type));

  if (reader_.ConservativeReadLimit() < out.size()) {
    status_ = Status::DataLoss();
    return status_;
  }

  if (RemainingBytes() < out.size()) {
    status_ = Status::DataLoss();
    return status_;
  }

  PW_TRY(reader_.Read(out));
  position_ += out.size();
  field_consumed_ = true;

  if (std::endian::native != std::endian::little) {
    std::reverse(out.begin(), out.end());
  }

  return OkStatus();
}

StatusWithSize StreamDecoder::ReadDelimitedField(std::span<std::byte> out) {
  if (Status status = CheckOkToRead(WireType::kDelimited); !status.ok()) {
    return StatusWithSize(status, 0);
  }

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status_ = Status::DataLoss();
    return StatusWithSize(status_, 0);
  }

  if (out.size() < delimited_field_size_) {

    return StatusWithSize::ResourceExhausted();
  }

  Result<ByteSpan> result = reader_.Read(out.first(delimited_field_size_));
  if (!result.ok()) {
    return StatusWithSize(result.status(), 0);
  }

  position_ += result.value().size();
  field_consumed_ = true;
  return StatusWithSize(result.value().size());
}

StatusWithSize StreamDecoder::ReadPackedFixedField(std::span<std::byte> out,
                                                   size_t elem_size) {
  if (Status status = CheckOkToRead(WireType::kDelimited); !status.ok()) {
    return StatusWithSize(status, 0);
  }

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status_ = Status::DataLoss();
    return StatusWithSize(status_, 0);
  }

  if (out.size() < delimited_field_size_) {

    return StatusWithSize::ResourceExhausted();
  }

  Result<ByteSpan> result = reader_.Read(out.first(delimited_field_size_));
  if (!result.ok()) {
    return StatusWithSize(result.status(), 0);
  }

  position_ += result.value().size();
  field_consumed_ = true;

  if (std::endian::native != std::endian::little) {
    for (auto out_start = out.begin(); out_start != out.end();
         out_start += elem_size) {
      std::reverse(out_start, out_start + elem_size);
    }
  }

  return StatusWithSize(result.value().size() / elem_size);
}

StatusWithSize StreamDecoder::ReadPackedVarintField(std::span<std::byte> out,
                                                    size_t elem_size,
                                                    VarintType decode_type) {
  PW_CHECK(elem_size == sizeof(bool) || elem_size == sizeof(uint32_t) ||
               elem_size == sizeof(uint64_t),
           "Protobuf varints must only be used with bool, int32_t, uint32_t, "
           "int64_t, or uint64_t");

  if (Status status = CheckOkToRead(WireType::kDelimited); !status.ok()) {
    return StatusWithSize(status, 0);
  }

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status_ = Status::DataLoss();
    return StatusWithSize(status_, 0);
  }

  size_t bytes_read = 0;
  size_t number_out = 0;
  while (bytes_read < delimited_field_size_ && !out.empty()) {
    const StatusWithSize sws = ReadOneVarint(out.first(elem_size), decode_type);
    if (!sws.ok()) {
      return StatusWithSize(sws.status(), number_out);
    }

    bytes_read += sws.size();
    out = out.subspan(elem_size);
    ++number_out;
  }

  if (bytes_read < delimited_field_size_) {
    return StatusWithSize(Status::ResourceExhausted(), number_out);
  }

  field_consumed_ = true;
  return StatusWithSize(OkStatus(), number_out);
}

Status StreamDecoder::CheckOkToRead(WireType type) {
  PW_CHECK(!nested_reader_open_,
           "Cannot read from a decoder while a nested decoder is open");
  PW_CHECK(!field_consumed_,
           "Attempting to read from protobuf decoder without first calling "
           "Next()");

  if (current_field_.wire_type() != type) {
    status_ = Status::NotFound();
  }

  return status_;
}

Status StreamDecoder::Read(std::span<std::byte> message,
                           std::span<const MessageField> table) {
  PW_TRY(status_);

  while (Next().ok()) {

    const auto field =
        std::find(table.begin(), table.end(), current_field_.field_number());
    if (field == table.end()) {

      continue;
    }

    const auto out =
        message.subspan(field->field_offset(), field->field_size());
    PW_CHECK(out.begin() >= message.begin() && out.end() <= message.end());

    if (field->use_callback()) {
      const Callback<StreamEncoder, StreamDecoder>* callback =
          reinterpret_cast<const Callback<StreamEncoder, StreamDecoder>*>(
              out.data());
      PW_TRY(callback->Decode(*this));
      continue;
    }

    switch (field->wire_type()) {
      case WireType::kFixed64:
      case WireType::kFixed32: {

        PW_CHECK(field->elem_size() == (field->wire_type() == WireType::kFixed32
                                            ? sizeof(uint32_t)
                                            : sizeof(uint64_t)),
                 "Mismatched message field type and size");
        if (field->is_fixed_size()) {
          PW_CHECK(field->is_repeated(), "Non-repeated fixed size field");
          PW_TRY(ReadPackedFixedField(out, field->elem_size()));
        } else if (field->is_repeated()) {

          if (field->elem_size() == sizeof(uint64_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint64_t>*>(out.data());
            PW_TRY(ReadRepeatedFixedField(*vector));
          } else if (field->elem_size() == sizeof(uint32_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint32_t>*>(out.data());
            PW_TRY(ReadRepeatedFixedField(*vector));
          }
        } else if (field->is_optional()) {

          if (field->elem_size() == sizeof(uint64_t)) {
            uint64_t value = 0;
            PW_TRY(
                ReadFixedField(std::as_writable_bytes(std::span(&value, 1))));
            auto* optional =
                reinterpret_cast<std::optional<uint64_t>*>(out.data());
            *optional = value;
          } else if (field->elem_size() == sizeof(uint32_t)) {
            uint32_t value = 0;
            PW_TRY(
                ReadFixedField(std::as_writable_bytes(std::span(&value, 1))));
            auto* optional =
                reinterpret_cast<std::optional<uint32_t>*>(out.data());
            *optional = value;
          }
        } else {
          PW_CHECK(out.size() == field->elem_size(),
                   "Mismatched message field type and size");
          PW_TRY(ReadFixedField(out));
        }
        break;
      }
      case WireType::kVarint: {

        PW_CHECK(field->elem_size() == sizeof(uint64_t) ||
                     field->elem_size() == sizeof(uint32_t) ||
                     field->elem_size() == sizeof(bool),
                 "Mismatched message field type and size");
        if (field->is_fixed_size()) {
          PW_CHECK(field->is_repeated(), "Non-repeated fixed size field");
          PW_TRY(ReadPackedVarintField(
              out, field->elem_size(), field->varint_type()));
        } else if (field->is_repeated()) {

          if (field->elem_size() == sizeof(uint64_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint64_t>*>(out.data());
            PW_TRY(ReadRepeatedVarintField(*vector, field->varint_type()));
          } else if (field->elem_size() == sizeof(uint32_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint32_t>*>(out.data());
            PW_TRY(ReadRepeatedVarintField(*vector, field->varint_type()));
          } else if (field->elem_size() == sizeof(bool)) {
            auto* vector = reinterpret_cast<pw::Vector<bool>*>(out.data());
            PW_TRY(ReadRepeatedVarintField(*vector, field->varint_type()));
          }
        } else if (field->is_optional()) {

          if (field->elem_size() == sizeof(uint64_t)) {
            uint64_t value = 0;
            PW_TRY(ReadVarintField(std::as_writable_bytes(std::span(&value, 1)),
                                   field->varint_type()));
            auto* optional =
                reinterpret_cast<std::optional<uint64_t>*>(out.data());
            *optional = value;
          } else if (field->elem_size() == sizeof(uint32_t)) {
            uint32_t value = 0;
            PW_TRY(ReadVarintField(std::as_writable_bytes(std::span(&value, 1)),
                                   field->varint_type()));
            auto* optional =
                reinterpret_cast<std::optional<uint32_t>*>(out.data());
            *optional = value;
          } else if (field->elem_size() == sizeof(bool)) {
            bool value = false;
            PW_TRY(ReadVarintField(std::as_writable_bytes(std::span(&value, 1)),
                                   field->varint_type()));
            auto* optional = reinterpret_cast<std::optional<bool>*>(out.data());
            *optional = value;
          }
        } else {
          PW_CHECK(out.size() == field->elem_size(),
                   "Mismatched message field type and size");
          PW_TRY(ReadVarintField(out, field->varint_type()));
        }
        break;
      }
      case WireType::kDelimited: {

        PW_CHECK(!field->is_repeated(),
                 "Repeated delimited messages always require a callback");
        if (field->nested_message_fields()) {

          auto nested_decoder = GetNestedDecoder();
          PW_TRY(nested_decoder.Read(out, *field->nested_message_fields()));
        } else if (field->is_fixed_size()) {

          PW_CHECK(field->elem_size() == sizeof(std::byte),
                   "Mismatched message field type and size");
          PW_TRY(ReadDelimitedField(out));
        } else {

          PW_CHECK(field->elem_size() == sizeof(std::byte),
                   "Mismatched message field type and size");
          auto* vector = reinterpret_cast<pw::Vector<std::byte>*>(out.data());
          if (vector->capacity() < delimited_field_size_) {
            return Status::ResourceExhausted();
          }
          vector->resize(vector->capacity());
          const auto sws =
              ReadDelimitedField(std::span(vector->data(), vector->size()));
          vector->resize(sws.size());
          PW_TRY(sws);
        }
        break;
      }
    }
  }

  if (status_ == Status::OutOfRange()) {
    return OkStatus();
  }

  return status_;
}

}
// 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.

#include "pw_protobuf/stream_decoder.h"

#include <algorithm>
#include <bit>
#include <cstdint>
#include <cstring>
#include <limits>
#include <optional>
#include <span>

#include "pw_assert/assert.h"
#include "pw_assert/check.h"
#include "pw_containers/vector.h"
#include "pw_function/function.h"
#include "pw_protobuf/encoder.h"
#include "pw_protobuf/internal/codegen.h"
#include "pw_protobuf/wire_format.h"
#include "pw_status/status.h"
#include "pw_status/status_with_size.h"
#include "pw_status/try.h"
#include "pw_varint/stream.h"
#include "pw_varint/varint.h"

namespace pw::protobuf {

Status StreamDecoder::BytesReader::DoSeek(ptrdiff_t offset, Whence origin) {
  PW_TRY(status_);
  if (!decoder_.reader_.seekable()) {
    return Status::Unimplemented();
  }

  ptrdiff_t absolute_position = std::numeric_limits<ptrdiff_t>::min();

  // Convert from the position within the bytes field to the position within the
  // proto stream.
  switch (origin) {
    case Whence::kBeginning:
      absolute_position = start_offset_ + offset;
      break;

    case Whence::kCurrent:
      absolute_position = decoder_.position_ + offset;
      break;

    case Whence::kEnd:
      absolute_position = end_offset_ + offset;
      break;
  }

  if (absolute_position < 0) {
    return Status::InvalidArgument();
  }

  if (static_cast<size_t>(absolute_position) < start_offset_ ||
      static_cast<size_t>(absolute_position) >= end_offset_) {
    return Status::OutOfRange();
  }

  PW_TRY(decoder_.reader_.Seek(absolute_position, Whence::kBeginning));
  decoder_.position_ = absolute_position;
  return OkStatus();
}

StatusWithSize StreamDecoder::BytesReader::DoRead(ByteSpan destination) {
  if (!status_.ok()) {
    return StatusWithSize(status_, 0);
  }

  // Bound the read buffer to the size of the bytes field.
  size_t max_length = end_offset_ - decoder_.position_;
  if (destination.size() > max_length) {
    destination = destination.first(max_length);
  }

  Result<ByteSpan> result = decoder_.reader_.Read(destination);
  if (!result.ok()) {
    return StatusWithSize(result.status(), 0);
  }

  decoder_.position_ += result.value().size();
  return StatusWithSize(result.value().size());
}

StreamDecoder::~StreamDecoder() {
  if (parent_ != nullptr) {
    parent_->CloseNestedDecoder(*this);
  } else if (stream_bounds_.high < std::numeric_limits<size_t>::max()) {
    if (status_.ok()) {
      // Advance the stream to the end of the bounds.
      PW_CHECK(Advance(stream_bounds_.high).ok());
    }
  }
}

Status StreamDecoder::Next() {
  PW_CHECK(!nested_reader_open_,
           "Cannot use parent decoder while a nested one is open");

  PW_TRY(status_);

  if (!field_consumed_) {
    PW_TRY(SkipField());
  }

  if (position_ >= stream_bounds_.high) {
    return Status::OutOfRange();
  }

  status_ = ReadFieldKey();
  return status_;
}

StreamDecoder::BytesReader StreamDecoder::GetBytesReader() {
  Status status = CheckOkToRead(WireType::kDelimited);

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status.Update(Status::DataLoss());
  }

  nested_reader_open_ = true;

  if (!status.ok()) {
    return BytesReader(*this, status);
  }

  size_t low = position_;
  size_t high = low + delimited_field_size_;

  return BytesReader(*this, low, high);
}

StreamDecoder StreamDecoder::GetNestedDecoder() {
  Status status = CheckOkToRead(WireType::kDelimited);

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status.Update(Status::DataLoss());
  }

  nested_reader_open_ = true;

  if (!status.ok()) {
    return StreamDecoder(reader_, this, status);
  }

  size_t low = position_;
  size_t high = low + delimited_field_size_;

  return StreamDecoder(reader_, this, low, high);
}

Status StreamDecoder::Advance(size_t end_position) {
  if (reader_.seekable()) {
    PW_TRY(reader_.Seek(end_position - position_, stream::Stream::kCurrent));
    position_ = end_position;
    return OkStatus();
  }

  while (position_ < end_position) {
    std::byte b;
    PW_TRY(reader_.Read(std::span(&b, 1)));
    position_++;
  }
  return OkStatus();
}

void StreamDecoder::CloseBytesReader(BytesReader& reader) {
  status_ = reader.status_;
  if (status_.ok()) {
    // Advance the stream to the end of the bytes field.
    // The BytesReader already updated our position_ field as bytes were read.
    PW_CHECK(Advance(reader.end_offset_).ok());
  }

  field_consumed_ = true;
  nested_reader_open_ = false;
}

void StreamDecoder::CloseNestedDecoder(StreamDecoder& nested) {
  PW_CHECK_PTR_EQ(nested.parent_, this);

  nested.nested_reader_open_ = true;
  nested.parent_ = nullptr;

  status_ = nested.status_;
  position_ = nested.position_;
  if (status_.ok()) {
    // Advance the stream to the end of the nested message field.
    PW_CHECK(Advance(nested.stream_bounds_.high).ok());
  }

  field_consumed_ = true;
  nested_reader_open_ = false;
}

Status StreamDecoder::ReadFieldKey() {
  PW_DCHECK(field_consumed_);

  uint64_t varint = 0;
  PW_TRY_ASSIGN(size_t bytes_read,
                varint::Read(reader_, &varint, RemainingBytes()));
  position_ += bytes_read;

  if (!FieldKey::IsValidKey(varint)) {
    return Status::DataLoss();
  }

  current_field_ = FieldKey(varint);

  if (current_field_.wire_type() == WireType::kDelimited) {
    // Read the length varint of length-delimited fields immediately to simplify
    // later processing of the field.
    StatusWithSize sws = varint::Read(reader_, &varint, RemainingBytes());
    if (sws.IsOutOfRange()) {
      // Out of range indicates the end of the stream. As a value is expected
      // here, report it as a data loss and terminate the decode operation.
      return Status::DataLoss();
    }
    if (!sws.ok()) {
      return sws.status();
    }
    position_ += sws.size();

    if (varint > std::numeric_limits<uint32_t>::max()) {
      return Status::DataLoss();
    }

    delimited_field_size_ = varint;
    delimited_field_offset_ = position_;
  }

  field_consumed_ = false;
  return OkStatus();
}

Result<StreamDecoder::Bounds> StreamDecoder::GetLengthDelimitedPayloadBounds() {
  PW_TRY(CheckOkToRead(WireType::kDelimited));
  return StreamDecoder::Bounds{delimited_field_offset_,
                               delimited_field_size_ + delimited_field_offset_};
}

// Consumes the current protobuf field, advancing the stream to the key of the
// next field (if one exists).
Status StreamDecoder::SkipField() {
  PW_DCHECK(!field_consumed_);

  size_t bytes_to_skip = 0;
  uint64_t value = 0;

  switch (current_field_.wire_type()) {
    case WireType::kVarint: {
      // Consume the varint field; nothing more to skip afterward.
      PW_TRY_ASSIGN(size_t bytes_read,
                    varint::Read(reader_, &value, RemainingBytes()));
      position_ += bytes_read;
      break;
    }
    case WireType::kDelimited:
      bytes_to_skip = delimited_field_size_;
      break;

    case WireType::kFixed32:
      bytes_to_skip = sizeof(uint32_t);
      break;

    case WireType::kFixed64:
      bytes_to_skip = sizeof(uint64_t);
      break;
  }

  if (bytes_to_skip > 0) {
    // Check if the stream has the field available. If not, report it as a
    // DATA_LOSS since the proto is invalid (as opposed to OUT_OF_BOUNDS if we
    // just tried to seek beyond the end).
    if (reader_.ConservativeReadLimit() < bytes_to_skip) {
      status_ = Status::DataLoss();
      return status_;
    }

    if (RemainingBytes() < bytes_to_skip) {
      status_ = Status::DataLoss();
      return status_;
    }

    PW_TRY(Advance(position_ + bytes_to_skip));
  }

  field_consumed_ = true;
  return OkStatus();
}

Status StreamDecoder::ReadVarintField(std::span<std::byte> out,
                                      VarintType decode_type) {
  PW_CHECK(out.size() == sizeof(bool) || out.size() == sizeof(uint32_t) ||
               out.size() == sizeof(uint64_t),
           "Protobuf varints must only be used with bool, int32_t, uint32_t, "
           "int64_t, or uint64_t");
  PW_TRY(CheckOkToRead(WireType::kVarint));

  const StatusWithSize sws = ReadOneVarint(out, decode_type);
  if (sws.status() != Status::DataLoss())
    field_consumed_ = true;
  return sws.status();
}

StatusWithSize StreamDecoder::ReadOneVarint(std::span<std::byte> out,
                                            VarintType decode_type) {
  uint64_t value;
  StatusWithSize sws = varint::Read(reader_, &value, RemainingBytes());
  if (sws.IsOutOfRange()) {
    // Out of range indicates the end of the stream. As a value is expected
    // here, report it as a data loss and terminate the decode operation.
    status_ = Status::DataLoss();
    return StatusWithSize(status_, sws.size());
  }
  if (!sws.ok()) {
    return sws;
  }

  position_ += sws.size();

  if (out.size() == sizeof(uint64_t)) {
    if (decode_type == VarintType::kUnsigned) {
      std::memcpy(out.data(), &value, out.size());
    } else {
      const int64_t signed_value = decode_type == VarintType::kZigZag
                                       ? varint::ZigZagDecode(value)
                                       : static_cast<int64_t>(value);
      std::memcpy(out.data(), &signed_value, out.size());
    }
  } else if (out.size() == sizeof(uint32_t)) {
    if (decode_type == VarintType::kUnsigned) {
      if (value > std::numeric_limits<uint32_t>::max()) {
        return StatusWithSize(Status::FailedPrecondition(), sws.size());
      }
      std::memcpy(out.data(), &value, out.size());
    } else {
      const int64_t signed_value = decode_type == VarintType::kZigZag
                                       ? varint::ZigZagDecode(value)
                                       : static_cast<int64_t>(value);
      if (signed_value > std::numeric_limits<int32_t>::max() ||
          signed_value < std::numeric_limits<int32_t>::min()) {
        return StatusWithSize(Status::FailedPrecondition(), sws.size());
      }
      std::memcpy(out.data(), &signed_value, out.size());
    }
  } else if (out.size() == sizeof(bool)) {
    PW_CHECK(decode_type == VarintType::kUnsigned,
             "Protobuf bool can never be signed");
    std::memcpy(out.data(), &value, out.size());
  }

  return sws;
}

Status StreamDecoder::ReadFixedField(std::span<std::byte> out) {
  WireType expected_wire_type =
      out.size() == sizeof(uint32_t) ? WireType::kFixed32 : WireType::kFixed64;
  PW_TRY(CheckOkToRead(expected_wire_type));

  if (reader_.ConservativeReadLimit() < out.size()) {
    status_ = Status::DataLoss();
    return status_;
  }

  if (RemainingBytes() < out.size()) {
    status_ = Status::DataLoss();
    return status_;
  }

  PW_TRY(reader_.Read(out));
  position_ += out.size();
  field_consumed_ = true;

  if (std::endian::native != std::endian::little) {
    std::reverse(out.begin(), out.end());
  }

  return OkStatus();
}

StatusWithSize StreamDecoder::ReadDelimitedField(std::span<std::byte> out) {
  if (Status status = CheckOkToRead(WireType::kDelimited); !status.ok()) {
    return StatusWithSize(status, 0);
  }

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status_ = Status::DataLoss();
    return StatusWithSize(status_, 0);
  }

  if (out.size() < delimited_field_size_) {
    // Value can't fit into the provided buffer. Don't advance the cursor so
    // that the field can be re-read with a larger buffer or through the stream
    // API.
    return StatusWithSize::ResourceExhausted();
  }

  Result<ByteSpan> result = reader_.Read(out.first(delimited_field_size_));
  if (!result.ok()) {
    return StatusWithSize(result.status(), 0);
  }

  position_ += result.value().size();
  field_consumed_ = true;
  return StatusWithSize(result.value().size());
}

StatusWithSize StreamDecoder::ReadPackedFixedField(std::span<std::byte> out,
                                                   size_t elem_size) {
  if (Status status = CheckOkToRead(WireType::kDelimited); !status.ok()) {
    return StatusWithSize(status, 0);
  }

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status_ = Status::DataLoss();
    return StatusWithSize(status_, 0);
  }

  if (out.size() < delimited_field_size_) {
    // Value can't fit into the provided buffer. Don't advance the cursor so
    // that the field can be re-read with a larger buffer or through the stream
    // API.
    return StatusWithSize::ResourceExhausted();
  }

  Result<ByteSpan> result = reader_.Read(out.first(delimited_field_size_));
  if (!result.ok()) {
    return StatusWithSize(result.status(), 0);
  }

  position_ += result.value().size();
  field_consumed_ = true;

  // Decode little-endian serialized packed fields.
  if (std::endian::native != std::endian::little) {
    for (auto out_start = out.begin(); out_start != out.end();
         out_start += elem_size) {
      std::reverse(out_start, out_start + elem_size);
    }
  }

  return StatusWithSize(result.value().size() / elem_size);
}

StatusWithSize StreamDecoder::ReadPackedVarintField(std::span<std::byte> out,
                                                    size_t elem_size,
                                                    VarintType decode_type) {
  PW_CHECK(elem_size == sizeof(bool) || elem_size == sizeof(uint32_t) ||
               elem_size == sizeof(uint64_t),
           "Protobuf varints must only be used with bool, int32_t, uint32_t, "
           "int64_t, or uint64_t");

  if (Status status = CheckOkToRead(WireType::kDelimited); !status.ok()) {
    return StatusWithSize(status, 0);
  }

  if (reader_.ConservativeReadLimit() < delimited_field_size_) {
    status_ = Status::DataLoss();
    return StatusWithSize(status_, 0);
  }

  size_t bytes_read = 0;
  size_t number_out = 0;
  while (bytes_read < delimited_field_size_ && !out.empty()) {
    const StatusWithSize sws = ReadOneVarint(out.first(elem_size), decode_type);
    if (!sws.ok()) {
      return StatusWithSize(sws.status(), number_out);
    }

    bytes_read += sws.size();
    out = out.subspan(elem_size);
    ++number_out;
  }

  if (bytes_read < delimited_field_size_) {
    return StatusWithSize(Status::ResourceExhausted(), number_out);
  }

  field_consumed_ = true;
  return StatusWithSize(OkStatus(), number_out);
}

Status StreamDecoder::CheckOkToRead(WireType type) {
  PW_CHECK(!nested_reader_open_,
           "Cannot read from a decoder while a nested decoder is open");
  PW_CHECK(!field_consumed_,
           "Attempting to read from protobuf decoder without first calling "
           "Next()");

  // Attempting to read the wrong type is typically a programmer error;
  // however, it could also occur due to data corruption. As we don't want to
  // crash on bad data, return NOT_FOUND here to distinguish it from other
  // corruption cases.
  if (current_field_.wire_type() != type) {
    status_ = Status::NotFound();
  }

  return status_;
}

Status StreamDecoder::Read(std::span<std::byte> message,
                           std::span<const MessageField> table) {
  PW_TRY(status_);

  while (Next().ok()) {
    // Find the field in the table,
    // TODO(pwbug/650): Finding the field can be made more efficient.
    const auto field =
        std::find(table.begin(), table.end(), current_field_.field_number());
    if (field == table.end()) {
      // If the field is not found, skip to the next one.
      // TODO(pwbug/659): Provide a way to allow the caller to inspect unknown
      // fields, and serialize them back out later.
      continue;
    }

    // Calculate the span of bytes corresponding to the structure field to
    // output into.
    const auto out =
        message.subspan(field->field_offset(), field->field_size());
    PW_CHECK(out.begin() >= message.begin() && out.end() <= message.end());

    // If the field is using callbacks, interpret the output field accordingly
    // and allow the caller to provide custom handling.
    if (field->use_callback()) {
      const Callback<StreamEncoder, StreamDecoder>* callback =
          reinterpret_cast<const Callback<StreamEncoder, StreamDecoder>*>(
              out.data());
      PW_TRY(callback->Decode(*this));
      continue;
    }

    // Switch on the expected wire type of the field, not the actual, to ensure
    // the remote encoder doesn't influence our decoding unexpectedly.
    switch (field->wire_type()) {
      case WireType::kFixed64:
      case WireType::kFixed32: {
        // Fixed fields call ReadFixedField() for singular case, and either
        // ReadPackedFixedField() or ReadRepeatedFixedField() for repeated
        // fields.
        PW_CHECK(field->elem_size() == (field->wire_type() == WireType::kFixed32
                                            ? sizeof(uint32_t)
                                            : sizeof(uint64_t)),
                 "Mismatched message field type and size");
        if (field->is_fixed_size()) {
          PW_CHECK(field->is_repeated(), "Non-repeated fixed size field");
          PW_TRY(ReadPackedFixedField(out, field->elem_size()));
        } else if (field->is_repeated()) {
          // The struct member for this field is a vector of a type
          // corresponding to the field element size. Cast to the correct
          // vector type so we're not performing type aliasing (except for
          // unsigned vs signed which is explicitly allowed).
          if (field->elem_size() == sizeof(uint64_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint64_t>*>(out.data());
            PW_TRY(ReadRepeatedFixedField(*vector));
          } else if (field->elem_size() == sizeof(uint32_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint32_t>*>(out.data());
            PW_TRY(ReadRepeatedFixedField(*vector));
          }
        } else if (field->is_optional()) {
          // The struct member for this field is a std::optional of a type
          // corresponding to the field element size. Cast to the correct
          // optional type so we're not performing type aliasing (except for
          // unsigned vs signed which is explicitly allowed), and assign through
          // a temporary.
          if (field->elem_size() == sizeof(uint64_t)) {
            uint64_t value = 0;
            PW_TRY(
                ReadFixedField(std::as_writable_bytes(std::span(&value, 1))));
            auto* optional =
                reinterpret_cast<std::optional<uint64_t>*>(out.data());
            *optional = value;
          } else if (field->elem_size() == sizeof(uint32_t)) {
            uint32_t value = 0;
            PW_TRY(
                ReadFixedField(std::as_writable_bytes(std::span(&value, 1))));
            auto* optional =
                reinterpret_cast<std::optional<uint32_t>*>(out.data());
            *optional = value;
          }
        } else {
          PW_CHECK(out.size() == field->elem_size(),
                   "Mismatched message field type and size");
          PW_TRY(ReadFixedField(out));
        }
        break;
      }
      case WireType::kVarint: {
        // Varint fields call ReadVarintField() for singular case, and either
        // ReadPackedVarintField() or ReadRepeatedVarintField() for repeated
        // fields.
        PW_CHECK(field->elem_size() == sizeof(uint64_t) ||
                     field->elem_size() == sizeof(uint32_t) ||
                     field->elem_size() == sizeof(bool),
                 "Mismatched message field type and size");
        if (field->is_fixed_size()) {
          PW_CHECK(field->is_repeated(), "Non-repeated fixed size field");
          PW_TRY(ReadPackedVarintField(
              out, field->elem_size(), field->varint_type()));
        } else if (field->is_repeated()) {
          // The struct member for this field is a vector of a type
          // corresponding to the field element size. Cast to the correct
          // vector type so we're not performing type aliasing (except for
          // unsigned vs signed which is explicitly allowed).
          if (field->elem_size() == sizeof(uint64_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint64_t>*>(out.data());
            PW_TRY(ReadRepeatedVarintField(*vector, field->varint_type()));
          } else if (field->elem_size() == sizeof(uint32_t)) {
            auto* vector = reinterpret_cast<pw::Vector<uint32_t>*>(out.data());
            PW_TRY(ReadRepeatedVarintField(*vector, field->varint_type()));
          } else if (field->elem_size() == sizeof(bool)) {
            auto* vector = reinterpret_cast<pw::Vector<bool>*>(out.data());
            PW_TRY(ReadRepeatedVarintField(*vector, field->varint_type()));
          }
        } else if (field->is_optional()) {
          // The struct member for this field is a std::optional of a type
          // corresponding to the field element size. Cast to the correct
          // optional type so we're not performing type aliasing (except for
          // unsigned vs signed which is explicitly allowed), and assign through
          // a temporary.
          if (field->elem_size() == sizeof(uint64_t)) {
            uint64_t value = 0;
            PW_TRY(ReadVarintField(std::as_writable_bytes(std::span(&value, 1)),
                                   field->varint_type()));
            auto* optional =
                reinterpret_cast<std::optional<uint64_t>*>(out.data());
            *optional = value;
          } else if (field->elem_size() == sizeof(uint32_t)) {
            uint32_t value = 0;
            PW_TRY(ReadVarintField(std::as_writable_bytes(std::span(&value, 1)),
                                   field->varint_type()));
            auto* optional =
                reinterpret_cast<std::optional<uint32_t>*>(out.data());
            *optional = value;
          } else if (field->elem_size() == sizeof(bool)) {
            bool value = false;
            PW_TRY(ReadVarintField(std::as_writable_bytes(std::span(&value, 1)),
                                   field->varint_type()));
            auto* optional = reinterpret_cast<std::optional<bool>*>(out.data());
            *optional = value;
          }
        } else {
          PW_CHECK(out.size() == field->elem_size(),
                   "Mismatched message field type and size");
          PW_TRY(ReadVarintField(out, field->varint_type()));
        }
        break;
      }
      case WireType::kDelimited: {
        // Delimited fields are always a singular case because of the inability
        // to cast to a generic vector with an element of a certain size (we
        // always need a type).
        PW_CHECK(!field->is_repeated(),
                 "Repeated delimited messages always require a callback");
        if (field->nested_message_fields()) {
          // Nested Message. Struct member is an embedded struct for the
          // nested field. Obtain a nested decoder and recursively call Read()
          // using the fields table pointer from this field.
          auto nested_decoder = GetNestedDecoder();
          PW_TRY(nested_decoder.Read(out, *field->nested_message_fields()));
        } else if (field->is_fixed_size()) {
          // Fixed-length bytes field. Struct member is a std::array<std::byte>.
          // Call ReadDelimitedField() to populate it from the stream.
          PW_CHECK(field->elem_size() == sizeof(std::byte),
                   "Mismatched message field type and size");
          PW_TRY(ReadDelimitedField(out));
        } else {
          // bytes or string field with a maximum size. Struct member is a
          // pw::Vector<std::byte>. There's no vector equivalent of
          // ReadDelimitedField() to call, so just implement what that would
          // look like here (since it wouldn't be used anywhere else).
          PW_CHECK(field->elem_size() == sizeof(std::byte),
                   "Mismatched message field type and size");
          auto* vector = reinterpret_cast<pw::Vector<std::byte>*>(out.data());
          if (vector->capacity() < delimited_field_size_) {
            return Status::ResourceExhausted();
          }
          vector->resize(vector->capacity());
          const auto sws =
              ReadDelimitedField(std::span(vector->data(), vector->size()));
          vector->resize(sws.size());
          PW_TRY(sws);
        }
        break;
      }
    }
  }

  // Reaching the end of the encoded protobuf is not an error.
  if (status_ == Status::OutOfRange()) {
    return OkStatus();
  }

  return status_;
}

}  // namespace pw::protobuf
