preferences_rtc.h

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#pragma once
 
#include <cstddef>
#include <cstdint>
#include <cstring>
 
namespace esphome {
 
// Shared storage format for word-addressable preference backends.
//
// Several platforms persist preferences as a buffer of 32-bit words followed by a
// single checksum word, seeded with the preference's `type` (its hashed key). This
// format is used for RTC user memory (ESP8266, ESP32) and for the ESP8266
// flash-emulation buffer. The helpers here are platform independent; each backend
// supplies its own word read/write primitives and offset allocation.
 
inline size_t rtc_pref_bytes_to_words(size_t bytes) { return (bytes + 3) / 4; }
 
template<class It> uint32_t rtc_pref_calculate_checksum(It first, It last, uint32_t type) {
  uint32_t checksum = type;
  while (first != last) {
    // UINT32_C keeps the multiply wrapping at 32 bits regardless of the width of
    // unsigned long, so 64-bit host builds compute the same value as the devices.
    checksum ^= (*first++ * UINT32_C(2654435769)) >> 1;
  }
  return checksum;
}
 
inline void rtc_pref_encode(uint32_t *buffer, uint32_t type, uint8_t length_words, const uint8_t *data, size_t len) {
  memset(buffer, 0, (static_cast<size_t>(length_words) + 1) * sizeof(uint32_t));
  memcpy(buffer, data, len);
  buffer[length_words] = rtc_pref_calculate_checksum(buffer, buffer + length_words, type);
}
 
inline bool rtc_pref_decode(const uint32_t *buffer, uint32_t type, uint8_t length_words, uint8_t *data, size_t len) {
  if (buffer[length_words] != rtc_pref_calculate_checksum(buffer, buffer + length_words, type)) {
    return false;
  }
  memcpy(data, buffer, len);
  return true;
}
 
}  // namespace esphome