Introduce byteswap helpers (#2661)

* Backport std::byteswap() in helpers.h

* Introduce convert_big_endian() function

* Use convert_big_endian() in i2c byte swap functions

esphome/components/i2c/i2c.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "i2c_bus.h"
+#include "esphome/core/helpers.h"
 #include "esphome/core/optional.h"
 #include <array>
 #include <vector>
@@ -32,16 +33,8 @@ class I2CRegister {
 
 // like ntohs/htons but without including networking headers.
 // ("i2c" byte order is big-endian)
-inline uint16_t i2ctohs(uint16_t i2cshort) {
+inline uint16_t i2ctohs(uint16_t i2cshort) { return convert_big_endian(i2cshort); }
-  union {
+inline uint16_t htoi2cs(uint16_t hostshort) { return convert_big_endian(hostshort); }
-    uint16_t x;
-    uint8_t y[2];
-  } conv;
-  conv.x = i2cshort;
-  return ((uint16_t) conv.y[0] << 8) | ((uint16_t) conv.y[1] << 0);
-}
-
-inline uint16_t htoi2cs(uint16_t hostshort) { return i2ctohs(hostshort); }
 
 class I2CDevice {
  public:

esphome/core/helpers.h

@@ -306,6 +306,31 @@ template<typename T> T *new_buffer(size_t length) {
 
 // ---------------------------------------------------------------------------------------------------------------------
 
+/// @name STL backports
+///@{
+
+// std::byteswap is from C++23 and technically should be a template, but this will do for now.
+constexpr uint8_t byteswap(uint8_t n) { return n; }
+constexpr uint16_t byteswap(uint16_t n) { return __builtin_bswap16(n); }
+constexpr uint32_t byteswap(uint32_t n) { return __builtin_bswap32(n); }
+constexpr uint64_t byteswap(uint64_t n) { return __builtin_bswap64(n); }
+
+///@}
+
+/// @name Bit manipulation
+///@{
+
+/// Convert a value between host byte order and big endian (most significant byte first) order.
+template<typename T, enable_if_t<std::is_unsigned<T>::value, int> = 0> constexpr T convert_big_endian(T val) {
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+  return byteswap(val);
+#else
+  return val;
+#endif
+}
+
+///@}
+
 /// @name Parsing & formatting
 ///@{