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mhsw-espurna/code/espurna/settings_embedis.h

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/*
Part of the SETTINGS MODULE
Copyright (C) 2020 by Maxim Prokhorov <prokhorov dot max at outlook dot com>
Reimplementation of the Embedis storage format:
- https://github.com/thingSoC/embedis
*/
#pragma once
#include <Arduino.h>
#include <algorithm>
#include <memory>
#include <vector>
#include "libs/TypeChecks.h"
namespace settings {
namespace embedis {
// 'optional' type for byte range
struct ValueResult {
operator bool() {
return result;
}
bool result { false };
String value;
};
// Sum total is calculated from:
// - 4 bytes to store length of 2 values (stored as big-endian)
// - N bytes of values themselves
// We can't save empty keys but can save empty values as just 2 length bytes
inline size_t estimate(const String& key, const String& value) {
if (!key.length()) {
return 0;
}
return (4 + key.length() + value.length());
}
// Note: KeyValueStore is templated to avoid having to provide RawStorageBase via virtual inheritance.
template <typename RawStorageBase>
class KeyValueStore {
// -----------------------------------------------------------------------------------
// Notice: we can only use sfinae checks with the current compiler version
// TODO: provide actual benchmark comparison with 'lambda'-list-as-vtable (old Embedis style)
// and vtable approach (write(), read() and commit() as pure virtual)
// TODO: consider overrides for bulk operations like move (see ::del method)
template <typename T>
using storage_can_write_t = decltype(std::declval<T>().write(
std::declval<uint16_t>(), std::declval<uint8_t>()));
template <typename T>
using storage_can_write = is_detected<storage_can_write_t, T>;
template <typename T>
using storage_can_read_t = decltype(std::declval<T>().read(std::declval<uint16_t>()));
template <typename T>
using storage_can_read = is_detected<storage_can_read_t, T>;
template <typename T>
using storage_can_commit_t = decltype(std::declval<T>().commit());
template <typename T>
using storage_can_commit = is_detected<storage_can_commit_t, T>;
static_assert(
(storage_can_commit<RawStorageBase>{} &&
storage_can_read<RawStorageBase>{} &&
storage_can_write<RawStorageBase>{}),
"Storage class must implement read(index), write(index, byte) and commit()"
);
// -----------------------------------------------------------------------------------
protected:
// Tracking state of the parser inside of _raw_read()
enum class State {
Begin,
End,
LenByte1,
LenByte2,
Value
};
// Pointer to the region of data that we are using
//
// XXX: It does not matter right now, but we **will** overflow position when using sizes >= (2^16) - 1
// Note: Implementation is also in the header b/c c++ won't allow us
// to have a plain member (not a ptr or ref) of unknown size.
// Note: There was a considiration to implement this as 'stashing iterator' to be compatible with stl algorithms.
// In such implementation, we would store intermediate index and allow the user to receive a `value_proxy`,
// temporary returned by `value_proxy& operator*()' that is bound to Cursor instance.
// This **will** cause problems with 'reverse_iterator' or anything like it, as it expects reference to
// outlive the iterator object (specifically, result of `return *--tmp`, where `tmp` is created inside of a function block)
struct Cursor {
Cursor(RawStorageBase& storage, uint16_t position_, uint16_t begin_, uint16_t end_) :
position(position_),
begin(begin_),
end(end_),
_storage(storage)
{}
Cursor(RawStorageBase& storage, uint16_t begin_, uint16_t end_) :
Cursor(storage, 0, begin_, end_)
{}
explicit Cursor(RawStorageBase& storage) :
Cursor(storage, 0, 0, 0)
{}
static Cursor merge(RawStorageBase& storage, const Cursor& key, const Cursor& value) {
return Cursor(storage, key.begin, value.end);
}
static Cursor fromEnd(RawStorageBase& storage, uint16_t begin, uint16_t end) {
return Cursor(storage, end - begin, begin, end);
}
Cursor() = delete;
void reset(uint16_t begin_, uint16_t end_) {
position = 0;
begin = begin_;
end = end_;
}
uint8_t read() {
return _storage.read(begin + position);
}
void write(uint8_t value) {
_storage.write(begin + position, value);
}
void resetBeginning() {
position = 0;
}
void resetEnd() {
position = end - begin;
}
size_t size() {
return (end - begin);
}
bool inRange(uint16_t position_) {
return (position_ < (end - begin));
}
operator bool() {
return inRange(position);
}
uint8_t operator[](size_t position_) const {
return _storage.read(begin + position_);
}
bool operator ==(const Cursor& other) {
return (begin == other.begin) && (end == other.end);
}
bool operator !=(const Cursor& other) {
return !(*this == other);
}
Cursor& operator++() {
++position;
return *this;
}
Cursor operator++(int) {
Cursor other(*this);
++*this;
return other;
}
Cursor& operator--() {
--position;
return *this;
}
Cursor operator--(int) {
Cursor other(*this);
--*this;
return other;
}
uint16_t position;
uint16_t begin;
uint16_t end;
private:
RawStorageBase& _storage;
};
public:
// Store value location in a more reasonable forward-iterator-style manner
// Allows us to skip string creation when just searching for specific values
// XXX: be cautious that cursor positions **will** break when underlying storage changes
struct ReadResult {
friend class KeyValueStore<RawStorageBase>;
ReadResult(const Cursor& cursor_) :
length(0),
cursor(cursor_),
result(false)
{}
ReadResult(RawStorageBase& storage) :
length(0),
cursor(storage),
result(false)
{}
operator bool() {
return result;
}
String read() {
String out;
out.reserve(length);
if (!length) {
return out;
}
decltype(length) index = 0;
cursor.resetBeginning();
while (index < length) {
out += static_cast<char>(cursor.read());
++cursor;
++index;
}
return out;
}
uint16_t length;
private:
Cursor cursor;
bool result;
};
// Internal storage consists of sequences of <byte-range><length>
struct KeyValueResult {
operator bool() {
return (key) && (value) && (key.length > 0);
}
bool operator !() {
return !(static_cast<bool>(*this));
}
template <typename T = ReadResult>
KeyValueResult(T&& key_, T&& value_) :
key(std::forward<T>(key_)),
value(std::forward<T>(value_))
{}
KeyValueResult(RawStorageBase& storage) :
key(storage),
value(storage)
{}
ReadResult key;
ReadResult value;
};
// one and only possible constructor, simply move the class object into the
// member variable to avoid forcing the user of the API to keep 2 objects alive.
KeyValueStore(RawStorageBase&& storage, uint16_t begin, uint16_t end) :
_storage(std::move(storage)),
_cursor(_storage, begin, end),
_state(State::Begin)
{}
// Try to find the matching key. Datastructure that we use does not specify
// any value 'type' inside of it. We expect 'key' to be the first non-empty string,
// 'value' can be empty.
ValueResult get(const String& key) {
return _get(key, true);
}
bool has(const String& key) {
return static_cast<bool>(_get(key, false));
}
// We going be using this pattern all the time here, because we need 2 consecutive **valid** ranges
// TODO: expose _read_kv() and _cursor_reset_end() so we can have 'break' here?
// perhaps as a wrapper object, allow something like next() and seekBegin()
template <typename CallbackType>
void foreach(CallbackType callback) {
_cursor_reset_end();
do {
auto kv = _read_kv();
if (!kv) {
break;
}
callback(std::move(kv));
} while (_state != State::End);
}
// read every key into a vector
std::vector<String> keys() {
std::vector<String> out;
out.reserve(count());
foreach([&](KeyValueResult&& kv) {
out.push_back(kv.key.read());
});
return out;
}
// set or update key with value contents. ensure 'key' isn't empty, 'value' can be empty
bool set(const String& key, const String& value) {
// ref. 'estimate()' implementation in regards to the storage calculation
auto need = estimate(key, value);
if (!need) {
return false;
}
auto key_len = key.length();
auto value_len = value.length();
Cursor to_erase(_storage);
bool need_erase = false;
// we need the position at the 'end' of the free space
auto start_pos = _cursor_reset_end();
do {
auto kv = _read_kv();
if (!kv) {
break;
}
start_pos = kv.value.cursor.begin;
// in the very special case we can match the existing key, we either
if ((kv.key.length == key_len) && (kv.key.read() == key)) {
if (kv.value.length == value.length()) {
// - do nothing, as the value is already set
if (kv.value.read() == value) {
return true;
}
// - overwrite the space again, with the new kv of the same length
start_pos = kv.key.cursor.end;
break;
}
// - or, erase the existing kv and place new kv at the end
to_erase.reset(kv.value.cursor.begin, kv.key.cursor.end);
need_erase = true;
}
} while (_state != State::End);
if (need_erase) {
_raw_erase(start_pos, to_erase);
start_pos += to_erase.size();
}
// we should only insert when possition is still within possible size
if (start_pos && (start_pos >= need)) {
auto writer = Cursor::fromEnd(_storage, start_pos - need, start_pos);
// put the length of the value as 2 bytes and then write the data
(--writer).write(key_len & 0xff);
(--writer).write((key_len >> 8) & 0xff);
while (key_len--) {
(--writer).write(key[key_len]);
}
(--writer).write(value_len & 0xff);
(--writer).write((value_len >> 8) & 0xff);
while (value_len--) {
(--writer).write(value[value_len]);
}
// we also need to add an empty key *after* the value
// but, only when we still have some space left
if (writer.begin >= 2) {
_cursor_set_position(writer.begin - _cursor.begin);
auto next_kv = _read_kv();
if (!next_kv) {
auto empty = Cursor::fromEnd(_storage, writer.begin - 2, writer.begin);
(--empty).write(0);
(--empty).write(0);
}
}
_storage.commit();
return true;
}
return false;
}
// remove key from the storage. will check that 'key' argument isn't empty
bool del(const String& key) {
size_t key_len = key.length();
if (!key_len) {
return false;
}
// we should only compare strings of equal length.
// when matching, record { value ... key } range + 4 bytes for length
// continue searching for available keys and set start_pos and the 'end' of the free space
size_t start_pos = _cursor_reset_end();
auto to_erase = Cursor::fromEnd(_storage, _cursor.begin, _cursor.end);
foreach([&](KeyValueResult&& kv) {
start_pos = kv.value.cursor.begin;
if (!to_erase && (kv.key.length == key_len) && (kv.key.read() == key)) {
to_erase.reset(kv.value.cursor.begin, kv.key.cursor.end);
}
});
if (to_erase) {
_raw_erase(start_pos, to_erase);
return true;
}
return false;
}
// Simply count key-value pairs that we could parse
size_t count() {
size_t result = 0;
foreach([&result](KeyValueResult&&) {
++result;
});
return result;
}
// Do exactly the same thing as 'keys' does, but return the amount
// of bytes to the left of the last kv
size_t available() {
size_t result = _cursor.size();
foreach([&result](KeyValueResult&& kv) {
result -= kv.key.cursor.size();
result -= kv.value.cursor.size();
});
return result;
}
// How much bytes can be used is directly read from the cursor, based on begin and end values
size_t size() {
return _cursor.size();
}
protected:
// Try to find the matching key. Datastructure that we use does not specify
// any value 'type' inside of it. We expect 'key' to be the first non-empty string,
// 'value' can be empty.
// To implement has(), allow to skip reading the value
ValueResult _get(const String& key, bool read_value) {
ValueResult out;
auto len = key.length();
_cursor_reset_end();
do {
auto kv = _read_kv();
if (!kv) {
break;
}
// no point in comparing keys when length does not match
// (and we also don't want to allocate the string)
if (kv.key.length != len) {
continue;
}
auto key_result = kv.key.read();
if (key_result == key) {
if (read_value) {
out.value = kv.value.read();
}
out.result = true;
break;
}
} while (_state != State::End);
return out;
}
// Place cursor at the `end` and resets the parser to expect length byte
uint16_t _cursor_reset_end() {
_cursor.resetEnd();
_state = State::Begin;
return _cursor.end;
}
uint16_t _cursor_set_position(uint16_t position) {
_state = State::Begin;
_cursor.position = position;
return _cursor.position;
}
// implementation quirk is that `Cursor::operator=` won't work because of the `RawStorageBase&` member
// right now, just construct in place and assume that compiler will inline things
KeyValueResult _read_kv() {
auto key = _raw_read();
if (!key || !key.length) {
return {_storage};
}
auto value = _raw_read();
return {key, value};
};
void _raw_erase(size_t start_pos, Cursor& to_erase) {
// we either end up to the left or to the right of the boundary
size_t new_pos = (start_pos < to_erase.begin)
? (start_pos + to_erase.size())
: (to_erase.end);
if (start_pos < to_erase.begin) {
// shift storage to the right, overwriting over the now empty space
auto from = Cursor::fromEnd(_storage, start_pos, to_erase.begin);
auto to = Cursor::fromEnd(_storage, start_pos + to_erase.size(), to_erase.end);
while (--from && --to) {
to.write(from.read());
from.write(0xff);
}
} else {
// overwrite the now empty space with 0xff
to_erase.resetEnd();
while (--to_erase) {
to_erase.write(0xff);
}
}
// same as set(), add empty key as padding
auto empty = Cursor::fromEnd(_storage, new_pos - 2, new_pos);
(--empty).write(0);
(--empty).write(0);
_storage.commit();
}
// Returns Cursor to the region that holds the data
// Result object itself does not contain any data, we need to explicitly request it by calling read()
//
// Cursor object is always expected to point to something, e.g. minimum:
// 0x00 0x00
// len2 len1
// Position will be 0, end will be 2. Total length is 2, data length is 0
//
// Or, non-empty value:
// 0x01 0x00 0x01
// data len2 len1
// Position will be 0, end will be 3. Total length is 3, data length is 1
ReadResult _raw_read() {
uint16_t len = 0;
ReadResult out(_storage);
do {
// storage is written right-to-left, cursor is always decreasing
switch (_state) {
case State::Begin:
if (_cursor.position >= 2) {
--_cursor;
_state = State::LenByte1;
} else {
_state = State::End;
}
break;
// len is 16 bit uint (bigendian)
// special case is 0, which is valid and should be returned when encountered
// another special case is 0xffff, meaning we just hit an empty space
case State::LenByte1:
len = _cursor.read();
_state = State::LenByte2;
break;
case State::LenByte2:
{
uint8_t len2 = (--_cursor).read();
if ((0xff == len) && (0xff == len2)) {
_state = State::End;
} else {
len |= len2 << 8;
_state = State::Value;
}
break;
}
case State::Value: {
// ensure we don't go out-of-bounds
if (len && _cursor.position < len) {
_state = State::End;
break;
}
// and point at the beginning of the value
_cursor.position -= len;
auto value_start = (_cursor.begin + _cursor.position);
out.cursor.reset(value_start, value_start + len + 2);
out.length = len;
out.result = true;
_state = State::Begin;
goto return_result;
}
case State::End:
default:
break;
}
} while (_state != State::End);
return_result:
return out;
}
RawStorageBase _storage;
Cursor _cursor;
State _state { State::Begin };
};
} // namespace embedis
} // namespace settings