pw_ring_buffer/prefixed_entry_ring_buffer.cc - pigweed/pigweed - Git at Google

 // 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_ring_buffer/prefixed_entry_ring_buffer.h"

 #include <algorithm>
 #include <cstring>

 #include "pw_assert/assert.h"
 #include "pw_varint/varint.h"

 namespace pw {
 namespace ring_buffer {

 using std::byte;
 using Reader = PrefixedEntryRingBufferMulti::Reader;

 void PrefixedEntryRingBufferMulti::Clear() {
   write_idx_ = 0;
   for (Reader& reader : readers_) {
     reader.read_idx = 0;
     reader.entry_count = 0;
   }
 }

 Status PrefixedEntryRingBufferMulti::SetBuffer(std::span<byte> buffer) {
   if ((buffer.data() == nullptr) ||  //
       (buffer.size_bytes() == 0) ||  //
       (buffer.size_bytes() > kMaxBufferBytes)) {
     return Status::InvalidArgument();
   }

   buffer_ = buffer.data();
   buffer_bytes_ = buffer.size_bytes();

   Clear();
   return OkStatus();
 }

 Status PrefixedEntryRingBufferMulti::AttachReader(Reader& reader) {
   if (reader.buffer != nullptr) {
     return Status::InvalidArgument();
   }
   reader.buffer = this;

   // Note that a newly attached reader sees the buffer as empty,
   // and is not privy to entries pushed before being attached.
   reader.read_idx = write_idx_;
   reader.entry_count = 0;
   readers_.push_back(reader);
   return OkStatus();
 }

 Status PrefixedEntryRingBufferMulti::DetachReader(Reader& reader) {
   if (reader.buffer != this) {
     return Status::InvalidArgument();
   }
   reader.buffer = nullptr;
   reader.read_idx = 0;
   reader.entry_count = 0;
   readers_.remove(reader);
   return OkStatus();
 }

 Status PrefixedEntryRingBufferMulti::InternalPushBack(
     std::span<const byte> data,
     uint32_t user_preamble_data,
     bool drop_elements_if_needed) {
   if (buffer_ == nullptr) {
     return Status::FailedPrecondition();
   }
   if (data.size_bytes() == 0) {
     return Status::InvalidArgument();
   }

   // Prepare a single buffer that can hold both the user preamble and entry
   // length.
   byte preamble_buf[varint::kMaxVarint32SizeBytes * 2];
   size_t user_preamble_bytes = 0;
   if (user_preamble_) {
     user_preamble_bytes =
         varint::Encode<uint32_t>(user_preamble_data, preamble_buf);
   }
   size_t length_bytes = varint::Encode<uint32_t>(
       data.size_bytes(), std::span(preamble_buf).subspan(user_preamble_bytes));
   size_t total_write_bytes =
       user_preamble_bytes + length_bytes + data.size_bytes();
   if (buffer_bytes_ < total_write_bytes) {
     return Status::OutOfRange();
   }

   if (drop_elements_if_needed) {
     // PushBack() case: evict items as needed.
     // Drop old entries until we have space for the new entry.
     while (RawAvailableBytes() < total_write_bytes) {
       InternalPopFrontAll();
     }
   } else if (RawAvailableBytes() < total_write_bytes) {
     // TryPushBack() case: don't evict items.
     return Status::ResourceExhausted();
   }

   // Write the new entry into the ring buffer.
   RawWrite(std::span(preamble_buf, user_preamble_bytes + length_bytes));
   RawWrite(data);

   // Update all readers of the new count.
   for (Reader& reader : readers_) {
     reader.entry_count++;
   }
   return OkStatus();
 }

 auto GetOutput(std::span<byte> data_out, size_t* write_index) {
   return [data_out, write_index](std::span<const byte> src) -> Status {
     size_t copy_size = std::min(data_out.size_bytes(), src.size_bytes());

     memcpy(data_out.data() + *write_index, src.data(), copy_size);
     *write_index += copy_size;

     return (copy_size == src.size_bytes()) ? OkStatus()
                                            : Status::ResourceExhausted();
   };
 }

 Status PrefixedEntryRingBufferMulti::InternalPeekFront(Reader& reader,
                                                        std::span<byte> data,
                                                        size_t* bytes_read_out) {
   *bytes_read_out = 0;
   return InternalRead(reader, GetOutput(data, bytes_read_out), false);
 }

 Status PrefixedEntryRingBufferMulti::InternalPeekFront(Reader& reader,
                                                        ReadOutput output) {
   return InternalRead(reader, output, false);
 }

 Status PrefixedEntryRingBufferMulti::InternalPeekFrontWithPreamble(
     Reader& reader, std::span<byte> data, size_t* bytes_read_out) {
   *bytes_read_out = 0;
   return InternalRead(reader, GetOutput(data, bytes_read_out), true);
 }

 Status PrefixedEntryRingBufferMulti::InternalPeekFrontWithPreamble(
     Reader& reader, ReadOutput output) {
   return InternalRead(reader, output, true);
 }

 // TODO(pwbug/339): Consider whether this internal templating is required, or if
 // we can simply promote GetOutput to a static function and remove the template.
 // T should be similar to Status (*read_output)(std::span<const byte>)
 template <typename T>
 Status PrefixedEntryRingBufferMulti::InternalRead(
     Reader& reader,
     T read_output,
     bool include_preamble_in_output,
     uint32_t* user_preamble_out) {
   if (buffer_ == nullptr) {
     return Status::FailedPrecondition();
   }
   if (reader.entry_count == 0) {
     return Status::OutOfRange();
   }

   // Figure out where to start reading (wrapped); accounting for preamble.
   EntryInfo info = FrontEntryInfo(reader);
   size_t read_bytes = info.data_bytes;
   size_t data_read_idx = reader.read_idx;
   if (user_preamble_out) {
     *user_preamble_out = info.user_preamble;
   }
   if (include_preamble_in_output) {
     read_bytes += info.preamble_bytes;
   } else {
     data_read_idx = IncrementIndex(data_read_idx, info.preamble_bytes);
   }

   // Read bytes, stopping at the end of the buffer if this entry wraps.
   size_t bytes_until_wrap = buffer_bytes_ - data_read_idx;
   size_t bytes_to_copy = std::min(read_bytes, bytes_until_wrap);
   Status status =
       read_output(std::span(buffer_ + data_read_idx, bytes_to_copy));

   // If the entry wrapped, read the remaining bytes.
   if (status.ok() && (bytes_to_copy < read_bytes)) {
     status = read_output(std::span(buffer_, read_bytes - bytes_to_copy));
   }
   return status;
 }

 void PrefixedEntryRingBufferMulti::InternalPopFrontAll() {
   // Forcefully pop all readers. Find the slowest reader, which must have
   // the highest entry count, then pop all readers that have the same count.
   //
   // It is expected that InternalPopFrontAll is called only when there is
   // something to pop from at least one reader. If no readers exist, or all
   // readers are caught up, this function will assert.
   size_t entry_count = GetSlowestReader().entry_count;
   PW_DASSERT(entry_count != 0);
   // Otherwise, pop the readers that have the largest value.
   for (Reader& reader : readers_) {
     if (reader.entry_count == entry_count) {
       reader.PopFront();
     }
   }
 }

 Reader& PrefixedEntryRingBufferMulti::GetSlowestReader() {
   // Readers are guaranteed to be before the writer pointer (the class enforces
   // this on every read/write operation that forces the write pointer ahead of
   // an existing reader). To determine the slowest reader, we consider three
   // scenarios:
   //
   // In all below cases, WH is the write-head, and R# are readers, with R1
   // representing the slowest reader.
   // [[R1 R2 R3 WH]] => Right-hand writer, slowest reader is left-most reader.
   // [[WH R1 R2 R3]] => Left-hand writer, slowest reader is left-most reader.
   // [[R3 WH R1 R2]] => Middle-writer, slowest reader is left-most reader after
   // writer.
   //
   // Formally, choose the left-most reader after the writer (ex.2,3), but if
   // that doesn't exist, choose the left-most reader before the writer (ex.1).
   PW_DASSERT(readers_.size() > 0);
   Reader* slowest_reader_after_writer = nullptr;
   Reader* slowest_reader_before_writer = nullptr;
   for (Reader& reader : readers_) {
     if (reader.read_idx < write_idx_) {
       if (!slowest_reader_before_writer ||
           reader.read_idx < slowest_reader_before_writer->read_idx) {
         slowest_reader_before_writer = &reader;
       }
     } else {
       if (!slowest_reader_after_writer ||
           reader.read_idx < slowest_reader_after_writer->read_idx) {
         slowest_reader_after_writer = &reader;
       }
     }
   }
   return *(slowest_reader_after_writer ? slowest_reader_after_writer
                                        : slowest_reader_before_writer);
 }

 Status PrefixedEntryRingBufferMulti::Dering() {
   if (buffer_ == nullptr || readers_.size() == 0) {
     return Status::FailedPrecondition();
   }

   // Check if by luck we're already deringed.
   Reader* slowest_reader = &GetSlowestReader();
   if (slowest_reader->read_idx == 0) {
     return OkStatus();
   }

   auto buffer_span = std::span(buffer_, buffer_bytes_);
   std::rotate(buffer_span.begin(),
               buffer_span.begin() + slowest_reader->read_idx,
               buffer_span.end());

   // If the new index is past the end of the buffer,
   // alias it back (wrap) to the start of the buffer.
   if (write_idx_ < slowest_reader->read_idx) {
     write_idx_ += buffer_bytes_;
   }
   write_idx_ -= slowest_reader->read_idx;

   for (Reader& reader : readers_) {
     if (&reader == slowest_reader) {
       continue;
     }
     if (reader.read_idx < slowest_reader->read_idx) {
       reader.read_idx += buffer_bytes_;
     }
     reader.read_idx -= slowest_reader->read_idx;
   }

   slowest_reader->read_idx = 0;
   return OkStatus();
 }

 Status PrefixedEntryRingBufferMulti::InternalPopFront(Reader& reader) {
   if (buffer_ == nullptr) {
     return Status::FailedPrecondition();
   }
   if (reader.entry_count == 0) {
     return Status::OutOfRange();
   }

   // Advance the read pointer past the front entry to the next one.
   EntryInfo info = FrontEntryInfo(reader);
   size_t entry_bytes = info.preamble_bytes + info.data_bytes;
   size_t prev_read_idx = reader.read_idx;
   reader.read_idx = IncrementIndex(prev_read_idx, entry_bytes);
   reader.entry_count--;
   return OkStatus();
 }

 size_t PrefixedEntryRingBufferMulti::InternalFrontEntryDataSizeBytes(
     Reader& reader) {
   if (reader.entry_count == 0) {
     return 0;
   }
   return FrontEntryInfo(reader).data_bytes;
 }

 size_t PrefixedEntryRingBufferMulti::InternalFrontEntryTotalSizeBytes(
     Reader& reader) {
   if (reader.entry_count == 0) {
     return 0;
   }
   EntryInfo info = FrontEntryInfo(reader);
   return info.preamble_bytes + info.data_bytes;
 }

 PrefixedEntryRingBufferMulti::EntryInfo
 PrefixedEntryRingBufferMulti::FrontEntryInfo(Reader& reader) {
   // Entry headers consists of: (optional prefix byte, varint size, data...)

   // If a preamble exists, extract the varint and it's bytes in bytes.
   size_t user_preamble_bytes = 0;
   uint64_t user_preamble_data = 0;
   byte varint_buf[varint::kMaxVarint32SizeBytes];
   if (user_preamble_) {
     RawRead(varint_buf, reader.read_idx, varint::kMaxVarint32SizeBytes);
     user_preamble_bytes = varint::Decode(varint_buf, &user_preamble_data);
     PW_DASSERT(user_preamble_bytes != 0u);
   }

   // Read the entry header; extract the varint and it's bytes in bytes.
   RawRead(varint_buf,
           IncrementIndex(reader.read_idx, user_preamble_bytes),
           varint::kMaxVarint32SizeBytes);
   uint64_t entry_bytes;
   size_t length_bytes = varint::Decode(varint_buf, &entry_bytes);
   PW_DASSERT(length_bytes != 0u);

   EntryInfo info = {};
   info.preamble_bytes = user_preamble_bytes + length_bytes;
   info.user_preamble = static_cast<uint32_t>(user_preamble_data);
   info.data_bytes = entry_bytes;
   return info;
 }

 // Comparisons ordered for more probable early exits, assuming the reader is
 // not far behind the writer compared to the size of the ring.
 size_t PrefixedEntryRingBufferMulti::RawAvailableBytes() {
   // Compute slowest reader.
   // TODO: Alternatively, the slowest reader could be actively mantained on
   // every read operation, but reads are more likely than writes.
   if (readers_.size() == 0) {
     return buffer_bytes_;
   }

   size_t read_idx = GetSlowestReader().read_idx;
   // Case: Not wrapped.
   if (read_idx < write_idx_) {
     return buffer_bytes_ - (write_idx_ - read_idx);
   }
   // Case: Wrapped
   if (read_idx > write_idx_) {
     return read_idx - write_idx_;
   }
   // Case: Matched read and write heads; empty or full.
   for (Reader& reader : readers_) {
     if (reader.read_idx == read_idx && reader.entry_count != 0) {
       return 0;
     }
   }
   return buffer_bytes_;
 }

 void PrefixedEntryRingBufferMulti::RawWrite(std::span<const std::byte> source) {
   // Write until the end of the source or the backing buffer.
   size_t bytes_until_wrap = buffer_bytes_ - write_idx_;
   size_t bytes_to_copy = std::min(source.size(), bytes_until_wrap);
   memcpy(buffer_ + write_idx_, source.data(), bytes_to_copy);

   // If there wasn't space in the backing buffer, wrap to the front.
   if (bytes_to_copy < source.size()) {
     memcpy(
         buffer_, source.data() + bytes_to_copy, source.size() - bytes_to_copy);
   }
   write_idx_ = IncrementIndex(write_idx_, source.size());
 }

 void PrefixedEntryRingBufferMulti::RawRead(byte* destination,
                                            size_t source_idx,
                                            size_t length_bytes) {
   // Read the pre-wrap bytes.
   size_t bytes_until_wrap = buffer_bytes_ - source_idx;
   size_t bytes_to_copy = std::min(length_bytes, bytes_until_wrap);
   memcpy(destination, buffer_ + source_idx, bytes_to_copy);

   // Read the post-wrap bytes, if needed.
   if (bytes_to_copy < length_bytes) {
     memcpy(destination + bytes_to_copy, buffer_, length_bytes - bytes_to_copy);
   }
 }

 size_t PrefixedEntryRingBufferMulti::IncrementIndex(size_t index,
                                                     size_t count) {
   // Note: This doesn't use modulus (%) since the branch is cheaper, and we
   // guarantee that count will never be greater than buffer_bytes_.
   index += count;
   if (index > buffer_bytes_) {
     index -= buffer_bytes_;
   }
   return index;
 }

 Status PrefixedEntryRingBufferMulti::Reader::PeekFrontWithPreamble(
     std::span<byte> data,
     uint32_t& user_preamble_out,
     size_t& entry_bytes_read_out) {
   entry_bytes_read_out = 0;
   return buffer->InternalRead(
       *this, GetOutput(data, &entry_bytes_read_out), false, &user_preamble_out);
 }

 }  // namespace ring_buffer
 }  // namespace pw
	// 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_ring_buffer/prefixed_entry_ring_buffer.h"

	#include <algorithm>
	#include <cstring>

	#include "pw_assert/assert.h"
	#include "pw_varint/varint.h"

	namespace pw {
	namespace ring_buffer {

	using std::byte;
	using Reader = PrefixedEntryRingBufferMulti::Reader;

	void PrefixedEntryRingBufferMulti::Clear() {
	write_idx_ = 0;
	for (Reader& reader : readers_) {
	reader.read_idx = 0;
	reader.entry_count = 0;
	}
	}

	Status PrefixedEntryRingBufferMulti::SetBuffer(std::span<byte> buffer) {
	if ((buffer.data() == nullptr) \|\| //
	(buffer.size_bytes() == 0) \|\| //
	(buffer.size_bytes() > kMaxBufferBytes)) {
	return Status::InvalidArgument();
	}

	buffer_ = buffer.data();
	buffer_bytes_ = buffer.size_bytes();

	Clear();
	return OkStatus();
	}

	Status PrefixedEntryRingBufferMulti::AttachReader(Reader& reader) {
	if (reader.buffer != nullptr) {
	return Status::InvalidArgument();
	}
	reader.buffer = this;

	// Note that a newly attached reader sees the buffer as empty,
	// and is not privy to entries pushed before being attached.
	reader.read_idx = write_idx_;
	reader.entry_count = 0;
	readers_.push_back(reader);
	return OkStatus();
	}

	Status PrefixedEntryRingBufferMulti::DetachReader(Reader& reader) {
	if (reader.buffer != this) {
	return Status::InvalidArgument();
	}
	reader.buffer = nullptr;
	reader.read_idx = 0;
	reader.entry_count = 0;
	readers_.remove(reader);
	return OkStatus();
	}

	Status PrefixedEntryRingBufferMulti::InternalPushBack(
	std::span<const byte> data,
	uint32_t user_preamble_data,
	bool drop_elements_if_needed) {
	if (buffer_ == nullptr) {
	return Status::FailedPrecondition();
	}
	if (data.size_bytes() == 0) {
	return Status::InvalidArgument();
	}

	// Prepare a single buffer that can hold both the user preamble and entry
	// length.
	byte preamble_buf[varint::kMaxVarint32SizeBytes * 2];
	size_t user_preamble_bytes = 0;
	if (user_preamble_) {
	user_preamble_bytes =
	varint::Encode<uint32_t>(user_preamble_data, preamble_buf);
	}
	size_t length_bytes = varint::Encode<uint32_t>(
	data.size_bytes(), std::span(preamble_buf).subspan(user_preamble_bytes));
	size_t total_write_bytes =
	user_preamble_bytes + length_bytes + data.size_bytes();
	if (buffer_bytes_ < total_write_bytes) {
	return Status::OutOfRange();
	}

	if (drop_elements_if_needed) {
	// PushBack() case: evict items as needed.
	// Drop old entries until we have space for the new entry.
	while (RawAvailableBytes() < total_write_bytes) {
	InternalPopFrontAll();
	}
	} else if (RawAvailableBytes() < total_write_bytes) {
	// TryPushBack() case: don't evict items.
	return Status::ResourceExhausted();
	}

	// Write the new entry into the ring buffer.
	RawWrite(std::span(preamble_buf, user_preamble_bytes + length_bytes));
	RawWrite(data);

	// Update all readers of the new count.
	for (Reader& reader : readers_) {
	reader.entry_count++;
	}
	return OkStatus();
	}

	auto GetOutput(std::span<byte> data_out, size_t* write_index) {
	return [data_out, write_index](std::span<const byte> src) -> Status {
	size_t copy_size = std::min(data_out.size_bytes(), src.size_bytes());

	memcpy(data_out.data() + *write_index, src.data(), copy_size);
	*write_index += copy_size;

	return (copy_size == src.size_bytes()) ? OkStatus()
	: Status::ResourceExhausted();
	};
	}

	Status PrefixedEntryRingBufferMulti::InternalPeekFront(Reader& reader,
	std::span<byte> data,
	size_t* bytes_read_out) {
	*bytes_read_out = 0;
	return InternalRead(reader, GetOutput(data, bytes_read_out), false);
	}

	Status PrefixedEntryRingBufferMulti::InternalPeekFront(Reader& reader,
	ReadOutput output) {
	return InternalRead(reader, output, false);
	}

	Status PrefixedEntryRingBufferMulti::InternalPeekFrontWithPreamble(
	Reader& reader, std::span<byte> data, size_t* bytes_read_out) {
	*bytes_read_out = 0;
	return InternalRead(reader, GetOutput(data, bytes_read_out), true);
	}

	Status PrefixedEntryRingBufferMulti::InternalPeekFrontWithPreamble(
	Reader& reader, ReadOutput output) {
	return InternalRead(reader, output, true);
	}

	// TODO(pwbug/339): Consider whether this internal templating is required, or if
	// we can simply promote GetOutput to a static function and remove the template.
	// T should be similar to Status (*read_output)(std::span<const byte>)
	template <typename T>
	Status PrefixedEntryRingBufferMulti::InternalRead(
	Reader& reader,
	T read_output,
	bool include_preamble_in_output,
	uint32_t* user_preamble_out) {
	if (buffer_ == nullptr) {
	return Status::FailedPrecondition();
	}
	if (reader.entry_count == 0) {
	return Status::OutOfRange();
	}

	// Figure out where to start reading (wrapped); accounting for preamble.
	EntryInfo info = FrontEntryInfo(reader);
	size_t read_bytes = info.data_bytes;
	size_t data_read_idx = reader.read_idx;
	if (user_preamble_out) {
	*user_preamble_out = info.user_preamble;
	}
	if (include_preamble_in_output) {
	read_bytes += info.preamble_bytes;
	} else {
	data_read_idx = IncrementIndex(data_read_idx, info.preamble_bytes);
	}

	// Read bytes, stopping at the end of the buffer if this entry wraps.
	size_t bytes_until_wrap = buffer_bytes_ - data_read_idx;
	size_t bytes_to_copy = std::min(read_bytes, bytes_until_wrap);
	Status status =
	read_output(std::span(buffer_ + data_read_idx, bytes_to_copy));

	// If the entry wrapped, read the remaining bytes.
	if (status.ok() && (bytes_to_copy < read_bytes)) {
	status = read_output(std::span(buffer_, read_bytes - bytes_to_copy));
	}
	return status;
	}

	void PrefixedEntryRingBufferMulti::InternalPopFrontAll() {
	// Forcefully pop all readers. Find the slowest reader, which must have
	// the highest entry count, then pop all readers that have the same count.
	//
	// It is expected that InternalPopFrontAll is called only when there is
	// something to pop from at least one reader. If no readers exist, or all
	// readers are caught up, this function will assert.
	size_t entry_count = GetSlowestReader().entry_count;
	PW_DASSERT(entry_count != 0);
	// Otherwise, pop the readers that have the largest value.
	for (Reader& reader : readers_) {
	if (reader.entry_count == entry_count) {
	reader.PopFront();
	}
	}
	}

	Reader& PrefixedEntryRingBufferMulti::GetSlowestReader() {
	// Readers are guaranteed to be before the writer pointer (the class enforces
	// this on every read/write operation that forces the write pointer ahead of
	// an existing reader). To determine the slowest reader, we consider three
	// scenarios:
	//
	// In all below cases, WH is the write-head, and R# are readers, with R1
	// representing the slowest reader.
	// [[R1 R2 R3 WH]] => Right-hand writer, slowest reader is left-most reader.
	// [[WH R1 R2 R3]] => Left-hand writer, slowest reader is left-most reader.
	// [[R3 WH R1 R2]] => Middle-writer, slowest reader is left-most reader after
	// writer.
	//
	// Formally, choose the left-most reader after the writer (ex.2,3), but if
	// that doesn't exist, choose the left-most reader before the writer (ex.1).
	PW_DASSERT(readers_.size() > 0);
	Reader* slowest_reader_after_writer = nullptr;
	Reader* slowest_reader_before_writer = nullptr;
	for (Reader& reader : readers_) {
	if (reader.read_idx < write_idx_) {
	if (!slowest_reader_before_writer \|\|
	reader.read_idx < slowest_reader_before_writer->read_idx) {
	slowest_reader_before_writer = &reader;
	}
	} else {
	if (!slowest_reader_after_writer \|\|
	reader.read_idx < slowest_reader_after_writer->read_idx) {
	slowest_reader_after_writer = &reader;
	}
	}
	}
	return *(slowest_reader_after_writer ? slowest_reader_after_writer
	: slowest_reader_before_writer);
	}

	Status PrefixedEntryRingBufferMulti::Dering() {
	if (buffer_ == nullptr \|\| readers_.size() == 0) {
	return Status::FailedPrecondition();
	}

	// Check if by luck we're already deringed.
	Reader* slowest_reader = &GetSlowestReader();
	if (slowest_reader->read_idx == 0) {
	return OkStatus();
	}

	auto buffer_span = std::span(buffer_, buffer_bytes_);
	std::rotate(buffer_span.begin(),
	buffer_span.begin() + slowest_reader->read_idx,
	buffer_span.end());

	// If the new index is past the end of the buffer,
	// alias it back (wrap) to the start of the buffer.
	if (write_idx_ < slowest_reader->read_idx) {
	write_idx_ += buffer_bytes_;
	}
	write_idx_ -= slowest_reader->read_idx;

	for (Reader& reader : readers_) {
	if (&reader == slowest_reader) {
	continue;
	}
	if (reader.read_idx < slowest_reader->read_idx) {
	reader.read_idx += buffer_bytes_;
	}
	reader.read_idx -= slowest_reader->read_idx;
	}

	slowest_reader->read_idx = 0;
	return OkStatus();
	}

	Status PrefixedEntryRingBufferMulti::InternalPopFront(Reader& reader) {
	if (buffer_ == nullptr) {
	return Status::FailedPrecondition();
	}
	if (reader.entry_count == 0) {
	return Status::OutOfRange();
	}

	// Advance the read pointer past the front entry to the next one.
	EntryInfo info = FrontEntryInfo(reader);
	size_t entry_bytes = info.preamble_bytes + info.data_bytes;
	size_t prev_read_idx = reader.read_idx;
	reader.read_idx = IncrementIndex(prev_read_idx, entry_bytes);
	reader.entry_count--;
	return OkStatus();
	}

	size_t PrefixedEntryRingBufferMulti::InternalFrontEntryDataSizeBytes(
	Reader& reader) {
	if (reader.entry_count == 0) {
	return 0;
	}
	return FrontEntryInfo(reader).data_bytes;
	}

	size_t PrefixedEntryRingBufferMulti::InternalFrontEntryTotalSizeBytes(
	Reader& reader) {
	if (reader.entry_count == 0) {
	return 0;
	}
	EntryInfo info = FrontEntryInfo(reader);
	return info.preamble_bytes + info.data_bytes;
	}

	PrefixedEntryRingBufferMulti::EntryInfo
	PrefixedEntryRingBufferMulti::FrontEntryInfo(Reader& reader) {
	// Entry headers consists of: (optional prefix byte, varint size, data...)

	// If a preamble exists, extract the varint and it's bytes in bytes.
	size_t user_preamble_bytes = 0;
	uint64_t user_preamble_data = 0;
	byte varint_buf[varint::kMaxVarint32SizeBytes];
	if (user_preamble_) {
	RawRead(varint_buf, reader.read_idx, varint::kMaxVarint32SizeBytes);
	user_preamble_bytes = varint::Decode(varint_buf, &user_preamble_data);
	PW_DASSERT(user_preamble_bytes != 0u);
	}

	// Read the entry header; extract the varint and it's bytes in bytes.
	RawRead(varint_buf,
	IncrementIndex(reader.read_idx, user_preamble_bytes),
	varint::kMaxVarint32SizeBytes);
	uint64_t entry_bytes;
	size_t length_bytes = varint::Decode(varint_buf, &entry_bytes);
	PW_DASSERT(length_bytes != 0u);

	EntryInfo info = {};
	info.preamble_bytes = user_preamble_bytes + length_bytes;
	info.user_preamble = static_cast<uint32_t>(user_preamble_data);
	info.data_bytes = entry_bytes;
	return info;
	}

	// Comparisons ordered for more probable early exits, assuming the reader is
	// not far behind the writer compared to the size of the ring.
	size_t PrefixedEntryRingBufferMulti::RawAvailableBytes() {
	// Compute slowest reader.
	// TODO: Alternatively, the slowest reader could be actively mantained on
	// every read operation, but reads are more likely than writes.
	if (readers_.size() == 0) {
	return buffer_bytes_;
	}

	size_t read_idx = GetSlowestReader().read_idx;
	// Case: Not wrapped.
	if (read_idx < write_idx_) {
	return buffer_bytes_ - (write_idx_ - read_idx);
	}
	// Case: Wrapped
	if (read_idx > write_idx_) {
	return read_idx - write_idx_;
	}
	// Case: Matched read and write heads; empty or full.
	for (Reader& reader : readers_) {
	if (reader.read_idx == read_idx && reader.entry_count != 0) {
	return 0;
	}
	}
	return buffer_bytes_;
	}

	void PrefixedEntryRingBufferMulti::RawWrite(std::span<const std::byte> source) {
	// Write until the end of the source or the backing buffer.
	size_t bytes_until_wrap = buffer_bytes_ - write_idx_;
	size_t bytes_to_copy = std::min(source.size(), bytes_until_wrap);
	memcpy(buffer_ + write_idx_, source.data(), bytes_to_copy);

	// If there wasn't space in the backing buffer, wrap to the front.
	if (bytes_to_copy < source.size()) {
	memcpy(
	buffer_, source.data() + bytes_to_copy, source.size() - bytes_to_copy);
	}
	write_idx_ = IncrementIndex(write_idx_, source.size());
	}

	void PrefixedEntryRingBufferMulti::RawRead(byte* destination,
	size_t source_idx,
	size_t length_bytes) {
	// Read the pre-wrap bytes.
	size_t bytes_until_wrap = buffer_bytes_ - source_idx;
	size_t bytes_to_copy = std::min(length_bytes, bytes_until_wrap);
	memcpy(destination, buffer_ + source_idx, bytes_to_copy);

	// Read the post-wrap bytes, if needed.
	if (bytes_to_copy < length_bytes) {
	memcpy(destination + bytes_to_copy, buffer_, length_bytes - bytes_to_copy);
	}
	}

	size_t PrefixedEntryRingBufferMulti::IncrementIndex(size_t index,
	size_t count) {
	// Note: This doesn't use modulus (%) since the branch is cheaper, and we
	// guarantee that count will never be greater than buffer_bytes_.
	index += count;
	if (index > buffer_bytes_) {
	index -= buffer_bytes_;
	}
	return index;
	}

	Status PrefixedEntryRingBufferMulti::Reader::PeekFrontWithPreamble(
	std::span<byte> data,
	uint32_t& user_preamble_out,
	size_t& entry_bytes_read_out) {
	entry_bytes_read_out = 0;
	return buffer->InternalRead(
	*this, GetOutput(data, &entry_bytes_read_out), false, &user_preamble_out);
	}

	} // namespace ring_buffer
	} // namespace pw