pw_kvs/public/pw_kvs/key_value_store.h - 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.
 #pragma once

 #include <array>
 #include <cstddef>
 #include <cstdint>
 #include <string_view>

 #include "pw_kvs/checksum.h"
 #include "pw_kvs/flash_memory.h"
 #include "pw_span/span.h"
 #include "pw_status/status.h"
 #include "pw_status/status_with_size.h"

 namespace pw::kvs {
 namespace internal {

 template <typename T, typename = decltype(span(std::declval<T>()))>
 constexpr bool ConvertsToSpan(int) {
   return true;
 }

 // If the expression span(T) fails, then the type can't be converted to a span.
 template <typename T>
 constexpr bool ConvertsToSpan(...) {
   return false;
 }

 }  // namespace internal

 // Traits class to detect if the type is a span. This is used to ensure that the
 // correct overload of the Put function is selected.
 template <typename T>
 using ConvertsToSpan =
     std::bool_constant<internal::ConvertsToSpan<std::remove_reference_t<T>>(0)>;

 // Internal-only persistent storage header format.
 class EntryHeader;

 struct EntryHeaderFormat {
   // Magic is a unique constant identifier for entries.
   //
   // Upon reading from an address in flash, the magic number facilitiates
   // quickly differentiating between:
   //
   // - Reading erased data - typically 0xFF - from flash.
   // - Reading corrupted data
   // - Reading a valid entry
   //
   // When selecting a magic for your particular KVS, pick a random 32 bit
   // integer rather than a human readable 4 bytes. This decreases the
   // probability of a collision with a real string when scanning in the case of
   // corruption. To generate such a number:
   /*
        $ python3 -c 'import random; print(hex(random.randint(0,2**32)))'
        0xaf741757
   */
   uint32_t magic;
   ChecksumAlgorithm* checksum;
 };

 // TODO: Select the appropriate defaults, add descriptions.
 struct Options {
   bool partial_gc_on_write = true;
   bool verify_on_read = true;
   bool verify_on_write = true;
 };

 class KeyValueStore {
  private:
   struct KeyDescriptor;

  public:
   // TODO: Make these configurable
   static constexpr size_t kMaxKeyLength = 64;
   static constexpr size_t kMaxEntries = 64;
   static constexpr size_t kMaxUsableSectors = 64;
   static constexpr size_t kWorkingBufferSizeBytes = (4 * 1024);

   // +1 for null-terminator.
   using KeyBuffer = std::array<char, kMaxKeyLength + 1>;

   // In the future, will be able to provide additional EntryHeaderFormats for
   // backwards compatibility.
   KeyValueStore(FlashPartition* partition,
                 const EntryHeaderFormat& format,
                 const Options& options = {});

   Status Init();

   bool initialized() const { return initialized_; }

   StatusWithSize Get(std::string_view key, span<std::byte> value) const;

   // This overload of Get accepts a pointer to a trivially copyable object.
   // const T& is used instead of T* to prevent arrays from satisfying this
   // overload. To call Get with an array, pass as_writable_bytes(span(array)),
   // or pass a pointer to the array instead of the array itself.
   template <typename Pointer,
             typename = std::enable_if_t<std::is_pointer_v<Pointer>>>
   Status Get(const std::string_view& key, const Pointer& pointer) const {
     using T = std::remove_reference_t<std::remove_pointer_t<Pointer>>;

     static_assert(std::is_trivially_copyable<T>(), "Values must be copyable");
     static_assert(!std::is_pointer<T>(), "Values cannot be pointers");

     return FixedSizeGet(key, reinterpret_cast<std::byte*>(pointer), sizeof(T));
   }

   Status Put(std::string_view key, span<const std::byte> value);

   template <typename T,
             typename = std::enable_if_t<std::is_trivially_copyable_v<T> &&
                                         !std::is_pointer_v<T> &&
                                         !ConvertsToSpan<T>::value>>
   Status Put(const std::string_view& key, const T& value) {
     return Put(key, as_bytes(span(&value, 1)));
   }

   Status Delete(std::string_view key);

   StatusWithSize ValueSize(std::string_view key) const;

   void LogDebugInfo();

   // Classes and functions to support STL-style iteration.
   class iterator;

   class Item {
    public:
     // Guaranteed to be null-terminated
     std::string_view key() const { return key_buffer_.data(); }

     Status Get(span<std::byte> value_buffer) const {
       return kvs_.Get(key(), value_buffer).status();
     }

     template <typename Pointer,
               typename = std::enable_if_t<std::is_pointer_v<Pointer>>>
     Status Get(const Pointer& pointer) const {
       return kvs_.Get(key(), pointer);
     }

     StatusWithSize ValueSize() const { return kvs_.ValueSize(key()); }

    private:
     friend class iterator;

     constexpr Item(const KeyValueStore& kvs) : kvs_(kvs), key_buffer_{} {}

     const KeyValueStore& kvs_;
     KeyBuffer key_buffer_;
   };

   class iterator {
    public:
     iterator& operator++();

     iterator& operator++(int) { return operator++(); }

     // Reads the entry's key from flash.
     const Item& operator*();

     const Item* operator->() {
       operator*();  // Read the key into the Item object.
       return &item_;
     }

     constexpr bool operator==(const iterator& rhs) const {
       return index_ == rhs.index_;
     }

     constexpr bool operator!=(const iterator& rhs) const {
       return index_ != rhs.index_;
     }

    private:
     friend class KeyValueStore;

     constexpr iterator(const KeyValueStore& kvs, size_t index)
         : item_(kvs), index_(index) {}

     const KeyDescriptor& descriptor() const {
       return item_.kvs_.key_descriptor_list_[index_];
     }

     Item item_;
     size_t index_;
   };

   using const_iterator = iterator;  // Standard alias for iterable types.

   iterator begin() const;
   iterator end() const { return iterator(*this, key_descriptor_list_size_); }

   // Returns the number of valid entries in the KeyValueStore.
   size_t size() const;

   static constexpr size_t max_size() { return kMaxKeyLength; }

   size_t empty() const { return size() == 0u; }

  private:
   using Address = FlashPartition::Address;

   struct KeyDescriptor {
     enum State { kValid, kDeleted };

     KeyDescriptor() = default;

     KeyDescriptor(std::string_view key,
                   uint32_t version,
                   Address address,
                   State state = kValid)
         : key_hash(HashKey(key)),
           key_version(version),
           address(address),
           state(state) {}

     bool deleted() const { return state == kDeleted; }

     uint32_t key_hash;
     uint32_t key_version;
     Address address;  // In partition address.

     // TODO: This information should be packed into the above fields to save RAM
     State state;
   };

   struct SectorDescriptor {
     uint16_t tail_free_bytes;
     uint16_t valid_bytes;  // sum of sizes of valid entries

     bool HasSpace(size_t required_space) const {
       return (tail_free_bytes >= required_space);
     }

     void RemoveFreeBytes(size_t size) {
       // TODO: add safety check for tail_free_bytes > size.
       tail_free_bytes -= size;
     }

     void RemoveValidBytes(size_t size) {
       // TODO: add safety check for valid_bytes > size.
       valid_bytes -= size;
     }
   };

   static uint32_t HashKey(std::string_view string);

   Status FixedSizeGet(std::string_view key,
                       std::byte* value,
                       size_t size_bytes) const;

   Status CheckOperation(std::string_view key) const;

   static constexpr bool InvalidKey(std::string_view key) {
     return key.empty() || (key.size() > kMaxKeyLength);
   }

   Status FindKeyDescriptor(std::string_view key,
                            const KeyDescriptor** result) const;

   // Non-const version of FindKeyDescriptor.
   Status FindKeyDescriptor(std::string_view key, KeyDescriptor** result) {
     return static_cast<const KeyValueStore&>(*this).FindKeyDescriptor(
         key, const_cast<const KeyDescriptor**>(result));
   }

   Status ReadEntryHeader(Address address, EntryHeader* header) const;
   Status ReadEntryKey(Address address, size_t key_length, char* key) const;

   StatusWithSize ReadEntryValue(const KeyDescriptor& key_descriptor,
                                 const EntryHeader& header,
                                 span<std::byte> value) const;

   Status LoadEntry(Address entry_address, Address* next_entry_address);
   Status AppendNewOrOverwriteStaleExistingDescriptor(
       const KeyDescriptor& key_descriptor);
   Status AppendEmptyDescriptor(KeyDescriptor** new_descriptor);

   Status ValidateEntryChecksumInFlash(const EntryHeader& header,
                                       std::string_view key,
                                       const KeyDescriptor& entry) const;

   Status WriteEntryForExistingKey(KeyDescriptor* key_descriptor,
                                   KeyDescriptor::State new_state,
                                   std::string_view key,
                                   span<const std::byte> value);

   Status WriteEntryForNewKey(std::string_view key, span<const std::byte> value);

   Status RelocateEntry(KeyDescriptor& key_descriptor);

   Status FindSectorWithSpace(SectorDescriptor** found_sector,
                              size_t size,
                              const SectorDescriptor* sector_to_skip = nullptr,
                              bool bypass_empty_sector_rule = false);

   Status FindOrRecoverSectorWithSpace(SectorDescriptor** sector, size_t size);

   Status GarbageCollectOneSector(SectorDescriptor** sector);

   SectorDescriptor* FindSectorToGarbageCollect();

   bool HeaderLooksLikeUnwrittenData(const EntryHeader& header) const;

   KeyDescriptor* FindDescriptor(uint32_t hash);

   Status AppendEntry(SectorDescriptor* sector,
                      KeyDescriptor* key_descriptor,
                      std::string_view key,
                      span<const std::byte> value,
                      KeyDescriptor::State new_state = KeyDescriptor::kValid);

   bool AddressInSector(const SectorDescriptor& sector, Address address) const {
     const Address sector_base = SectorBaseAddress(&sector);
     const Address sector_end = sector_base + partition_.sector_size_bytes();

     return ((address >= sector_base) && (address < sector_end));
   }

   bool SectorEmpty(const SectorDescriptor& sector) const {
     return (sector.tail_free_bytes == partition_.sector_size_bytes());
   }

   size_t RecoverableBytes(const SectorDescriptor& sector) {
     return partition_.sector_size_bytes() - sector.valid_bytes -
            sector.tail_free_bytes;
   }

   size_t SectorIndex(const SectorDescriptor* sector) const {
     // TODO: perhaps add assert that the index is valid.
     return (sector - sector_map_.data());
   }

   Address SectorBaseAddress(const SectorDescriptor* sector) const {
     return SectorIndex(sector) * partition_.sector_size_bytes();
   }

   SectorDescriptor& SectorFromAddress(Address address) {
     const size_t index = address / partition_.sector_size_bytes();
     // TODO: Add boundary checking once asserts are supported.
     // DCHECK_LT(index, sector_map_size_);
     return sector_map_[index];
   }

   Address NextWritableAddress(SectorDescriptor* sector) const {
     return SectorBaseAddress(sector) + partition_.sector_size_bytes() -
            sector->tail_free_bytes;
   }

   bool KeyListFull() const { return key_descriptor_list_size_ == kMaxEntries; }

   span<KeyDescriptor> key_descriptors() {
     return span(key_descriptor_list_.data(), key_descriptor_list_size_);
   }

   span<const KeyDescriptor> key_descriptors() const {
     return span(key_descriptor_list_.data(), key_descriptor_list_size_);
   }

   span<SectorDescriptor> sectors() {
     return {sector_map_.data(), sector_map_size_};
   }

   FlashPartition& partition_;
   EntryHeaderFormat entry_header_format_;
   Options options_;

   // Map is unordered; finding a key requires scanning and
   // verifying a match by reading the actual entry.
   std::array<KeyDescriptor, kMaxEntries> key_descriptor_list_;
   size_t key_descriptor_list_size_;  // Number of valid entries in
                                      // key_descriptor_list_

   // This is dense, so sector_id == indexof(SectorDescriptor) in sector_map
   // TODO: This may need to be a span that points to an externally allocated
   // array. This could be handled by a templated KVS derived class.
   std::array<SectorDescriptor, kMaxUsableSectors> sector_map_;
   const size_t sector_map_size_;

   // The last sector that was selected as the "new empty sector" to write to.
   // This last new sector is used as the starting point for the next "find a new
   // empty sector to write to" operation. By using the last new sector as the
   // start point we will cycle which empty sector is selected next, spreading
   // the wear across all the empty sectors, rather than putting more wear on the
   // lower number sectors.
   //
   // Use SectorDescriptor* for the persistent storage rather than sector index
   // because SectorDescriptor* is the standard way to identify a sector.
   SectorDescriptor* last_new_sector_;

   bool initialized_ = false;

   // Working buffer is a general purpose buffer for operations (such as init or
   // relcate) to use for working space to remove the need to allocate temporary
   // space.
   std::array<char, kWorkingBufferSizeBytes> working_buffer_;
 };

 }  // namespace pw::kvs
	// 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 <array>
	#include <cstddef>
	#include <cstdint>
	#include <string_view>

	#include "pw_kvs/checksum.h"
	#include "pw_kvs/flash_memory.h"
	#include "pw_span/span.h"
	#include "pw_status/status.h"
	#include "pw_status/status_with_size.h"

	namespace pw::kvs {
	namespace internal {

	template <typename T, typename = decltype(span(std::declval<T>()))>
	constexpr bool ConvertsToSpan(int) {
	return true;
	}

	// If the expression span(T) fails, then the type can't be converted to a span.
	template <typename T>
	constexpr bool ConvertsToSpan(...) {
	return false;
	}

	} // namespace internal

	// Traits class to detect if the type is a span. This is used to ensure that the
	// correct overload of the Put function is selected.
	template <typename T>
	using ConvertsToSpan =
	std::bool_constant<internal::ConvertsToSpan<std::remove_reference_t<T>>(0)>;

	// Internal-only persistent storage header format.
	class EntryHeader;

	struct EntryHeaderFormat {
	// Magic is a unique constant identifier for entries.
	//
	// Upon reading from an address in flash, the magic number facilitiates
	// quickly differentiating between:
	//
	// - Reading erased data - typically 0xFF - from flash.
	// - Reading corrupted data
	// - Reading a valid entry
	//
	// When selecting a magic for your particular KVS, pick a random 32 bit
	// integer rather than a human readable 4 bytes. This decreases the
	// probability of a collision with a real string when scanning in the case of
	// corruption. To generate such a number:
	/*
	$ python3 -c 'import random; print(hex(random.randint(0,2**32)))'
	0xaf741757
	*/
	uint32_t magic;
	ChecksumAlgorithm* checksum;
	};

	// TODO: Select the appropriate defaults, add descriptions.
	struct Options {
	bool partial_gc_on_write = true;
	bool verify_on_read = true;
	bool verify_on_write = true;
	};

	class KeyValueStore {
	private:
	struct KeyDescriptor;

	public:
	// TODO: Make these configurable
	static constexpr size_t kMaxKeyLength = 64;
	static constexpr size_t kMaxEntries = 64;
	static constexpr size_t kMaxUsableSectors = 64;
	static constexpr size_t kWorkingBufferSizeBytes = (4 * 1024);

	// +1 for null-terminator.
	using KeyBuffer = std::array<char, kMaxKeyLength + 1>;

	// In the future, will be able to provide additional EntryHeaderFormats for
	// backwards compatibility.
	KeyValueStore(FlashPartition* partition,
	const EntryHeaderFormat& format,
	const Options& options = {});

	Status Init();

	bool initialized() const { return initialized_; }

	StatusWithSize Get(std::string_view key, span<std::byte> value) const;

	// This overload of Get accepts a pointer to a trivially copyable object.
	// const T& is used instead of T* to prevent arrays from satisfying this
	// overload. To call Get with an array, pass as_writable_bytes(span(array)),
	// or pass a pointer to the array instead of the array itself.
	template <typename Pointer,
	typename = std::enable_if_t<std::is_pointer_v<Pointer>>>
	Status Get(const std::string_view& key, const Pointer& pointer) const {
	using T = std::remove_reference_t<std::remove_pointer_t<Pointer>>;

	static_assert(std::is_trivially_copyable<T>(), "Values must be copyable");
	static_assert(!std::is_pointer<T>(), "Values cannot be pointers");

	return FixedSizeGet(key, reinterpret_cast<std::byte*>(pointer), sizeof(T));
	}

	Status Put(std::string_view key, span<const std::byte> value);

	template <typename T,
	typename = std::enable_if_t<std::is_trivially_copyable_v<T> &&
	!std::is_pointer_v<T> &&
	!ConvertsToSpan<T>::value>>
	Status Put(const std::string_view& key, const T& value) {
	return Put(key, as_bytes(span(&value, 1)));
	}

	Status Delete(std::string_view key);

	StatusWithSize ValueSize(std::string_view key) const;

	void LogDebugInfo();

	// Classes and functions to support STL-style iteration.
	class iterator;

	class Item {
	public:
	// Guaranteed to be null-terminated
	std::string_view key() const { return key_buffer_.data(); }

	Status Get(span<std::byte> value_buffer) const {
	return kvs_.Get(key(), value_buffer).status();
	}

	template <typename Pointer,
	typename = std::enable_if_t<std::is_pointer_v<Pointer>>>
	Status Get(const Pointer& pointer) const {
	return kvs_.Get(key(), pointer);
	}

	StatusWithSize ValueSize() const { return kvs_.ValueSize(key()); }

	private:
	friend class iterator;

	constexpr Item(const KeyValueStore& kvs) : kvs_(kvs), key_buffer_{} {}

	const KeyValueStore& kvs_;
	KeyBuffer key_buffer_;
	};

	class iterator {
	public:
	iterator& operator++();

	iterator& operator++(int) { return operator++(); }

	// Reads the entry's key from flash.
	const Item& operator*();

	const Item* operator->() {
	operator*(); // Read the key into the Item object.
	return &item_;
	}

	constexpr bool operator==(const iterator& rhs) const {
	return index_ == rhs.index_;
	}

	constexpr bool operator!=(const iterator& rhs) const {
	return index_ != rhs.index_;
	}

	private:
	friend class KeyValueStore;

	constexpr iterator(const KeyValueStore& kvs, size_t index)
	: item_(kvs), index_(index) {}

	const KeyDescriptor& descriptor() const {
	return item_.kvs_.key_descriptor_list_[index_];
	}

	Item item_;
	size_t index_;
	};

	using const_iterator = iterator; // Standard alias for iterable types.

	iterator begin() const;
	iterator end() const { return iterator(*this, key_descriptor_list_size_); }

	// Returns the number of valid entries in the KeyValueStore.
	size_t size() const;

	static constexpr size_t max_size() { return kMaxKeyLength; }

	size_t empty() const { return size() == 0u; }

	private:
	using Address = FlashPartition::Address;

	struct KeyDescriptor {
	enum State { kValid, kDeleted };

	KeyDescriptor() = default;

	KeyDescriptor(std::string_view key,
	uint32_t version,
	Address address,
	State state = kValid)
	: key_hash(HashKey(key)),
	key_version(version),
	address(address),
	state(state) {}

	bool deleted() const { return state == kDeleted; }

	uint32_t key_hash;
	uint32_t key_version;
	Address address; // In partition address.

	// TODO: This information should be packed into the above fields to save RAM
	State state;
	};

	struct SectorDescriptor {
	uint16_t tail_free_bytes;
	uint16_t valid_bytes; // sum of sizes of valid entries

	bool HasSpace(size_t required_space) const {
	return (tail_free_bytes >= required_space);
	}

	void RemoveFreeBytes(size_t size) {
	// TODO: add safety check for tail_free_bytes > size.
	tail_free_bytes -= size;
	}

	void RemoveValidBytes(size_t size) {
	// TODO: add safety check for valid_bytes > size.
	valid_bytes -= size;
	}
	};

	static uint32_t HashKey(std::string_view string);

	Status FixedSizeGet(std::string_view key,
	std::byte* value,
	size_t size_bytes) const;

	Status CheckOperation(std::string_view key) const;

	static constexpr bool InvalidKey(std::string_view key) {
	return key.empty() \|\| (key.size() > kMaxKeyLength);
	}

	Status FindKeyDescriptor(std::string_view key,
	const KeyDescriptor** result) const;

	// Non-const version of FindKeyDescriptor.
	Status FindKeyDescriptor(std::string_view key, KeyDescriptor** result) {
	return static_cast<const KeyValueStore&>(*this).FindKeyDescriptor(
	key, const_cast<const KeyDescriptor**>(result));
	}

	Status ReadEntryHeader(Address address, EntryHeader* header) const;
	Status ReadEntryKey(Address address, size_t key_length, char* key) const;

	StatusWithSize ReadEntryValue(const KeyDescriptor& key_descriptor,
	const EntryHeader& header,
	span<std::byte> value) const;

	Status LoadEntry(Address entry_address, Address* next_entry_address);
	Status AppendNewOrOverwriteStaleExistingDescriptor(
	const KeyDescriptor& key_descriptor);
	Status AppendEmptyDescriptor(KeyDescriptor** new_descriptor);

	Status ValidateEntryChecksumInFlash(const EntryHeader& header,
	std::string_view key,
	const KeyDescriptor& entry) const;

	Status WriteEntryForExistingKey(KeyDescriptor* key_descriptor,
	KeyDescriptor::State new_state,
	std::string_view key,
	span<const std::byte> value);

	Status WriteEntryForNewKey(std::string_view key, span<const std::byte> value);

	Status RelocateEntry(KeyDescriptor& key_descriptor);

	Status FindSectorWithSpace(SectorDescriptor** found_sector,
	size_t size,
	const SectorDescriptor* sector_to_skip = nullptr,
	bool bypass_empty_sector_rule = false);

	Status FindOrRecoverSectorWithSpace(SectorDescriptor** sector, size_t size);

	Status GarbageCollectOneSector(SectorDescriptor** sector);

	SectorDescriptor* FindSectorToGarbageCollect();

	bool HeaderLooksLikeUnwrittenData(const EntryHeader& header) const;

	KeyDescriptor* FindDescriptor(uint32_t hash);

	Status AppendEntry(SectorDescriptor* sector,
	KeyDescriptor* key_descriptor,
	std::string_view key,
	span<const std::byte> value,
	KeyDescriptor::State new_state = KeyDescriptor::kValid);

	bool AddressInSector(const SectorDescriptor& sector, Address address) const {
	const Address sector_base = SectorBaseAddress(&sector);
	const Address sector_end = sector_base + partition_.sector_size_bytes();

	return ((address >= sector_base) && (address < sector_end));
	}

	bool SectorEmpty(const SectorDescriptor& sector) const {
	return (sector.tail_free_bytes == partition_.sector_size_bytes());
	}

	size_t RecoverableBytes(const SectorDescriptor& sector) {
	return partition_.sector_size_bytes() - sector.valid_bytes -
	sector.tail_free_bytes;
	}

	size_t SectorIndex(const SectorDescriptor* sector) const {
	// TODO: perhaps add assert that the index is valid.
	return (sector - sector_map_.data());
	}

	Address SectorBaseAddress(const SectorDescriptor* sector) const {
	return SectorIndex(sector) * partition_.sector_size_bytes();
	}

	SectorDescriptor& SectorFromAddress(Address address) {
	const size_t index = address / partition_.sector_size_bytes();
	// TODO: Add boundary checking once asserts are supported.
	// DCHECK_LT(index, sector_map_size_);
	return sector_map_[index];
	}

	Address NextWritableAddress(SectorDescriptor* sector) const {
	return SectorBaseAddress(sector) + partition_.sector_size_bytes() -
	sector->tail_free_bytes;
	}

	bool KeyListFull() const { return key_descriptor_list_size_ == kMaxEntries; }

	span<KeyDescriptor> key_descriptors() {
	return span(key_descriptor_list_.data(), key_descriptor_list_size_);
	}

	span<const KeyDescriptor> key_descriptors() const {
	return span(key_descriptor_list_.data(), key_descriptor_list_size_);
	}

	span<SectorDescriptor> sectors() {
	return {sector_map_.data(), sector_map_size_};
	}

	FlashPartition& partition_;
	EntryHeaderFormat entry_header_format_;
	Options options_;

	// Map is unordered; finding a key requires scanning and
	// verifying a match by reading the actual entry.
	std::array<KeyDescriptor, kMaxEntries> key_descriptor_list_;
	size_t key_descriptor_list_size_; // Number of valid entries in
	// key_descriptor_list_

	// This is dense, so sector_id == indexof(SectorDescriptor) in sector_map
	// TODO: This may need to be a span that points to an externally allocated
	// array. This could be handled by a templated KVS derived class.
	std::array<SectorDescriptor, kMaxUsableSectors> sector_map_;
	const size_t sector_map_size_;

	// The last sector that was selected as the "new empty sector" to write to.
	// This last new sector is used as the starting point for the next "find a new
	// empty sector to write to" operation. By using the last new sector as the
	// start point we will cycle which empty sector is selected next, spreading
	// the wear across all the empty sectors, rather than putting more wear on the
	// lower number sectors.
	//
	// Use SectorDescriptor* for the persistent storage rather than sector index
	// because SectorDescriptor* is the standard way to identify a sector.
	SectorDescriptor* last_new_sector_;

	bool initialized_ = false;

	// Working buffer is a general purpose buffer for operations (such as init or
	// relcate) to use for working space to remove the need to allocate temporary
	// space.
	std::array<char, kWorkingBufferSizeBytes> working_buffer_;
	};

	} // namespace pw::kvs