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// 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
// 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.
// Utilities for building std::byte arrays from strings or integer values at
// compile time.
#pragma once
#include <array>
#include <cstddef>
#include <iterator>
#include "pw_polyfill/language_feature_macros.h"
namespace pw::bytes {
namespace internal {
template <typename T>
constexpr bool UseBytesDirectly = std::is_integral_v<T> || std::is_enum_v<T>;
// Internal implementation functions. CopyBytes copies bytes from an array of
// byte-sized elements or the underlying bytes of an integer (as little-endian).
// std::memcpy cannot be used since it is not constexpr.
template <typename B, typename T, typename... Args>
PW_CONSTEVAL void CopyBytes(B* array, T value, Args... args) {
static_assert(sizeof(B) == sizeof(std::byte));
if constexpr (UseBytesDirectly<T>) {
if constexpr (sizeof(T) == 1u) {
*array++ = static_cast<B>(value);
} else {
for (size_t i = 0; i < sizeof(T); ++i) {
*array++ = static_cast<B>(value & 0xFF);
value >>= 8;
} else {
static_assert(sizeof(value[0]) == sizeof(B));
for (auto b : value) {
*array++ = static_cast<B>(b);
if constexpr (sizeof...(args) > 0u) {
CopyBytes(array, args...);
// Evaluates to the size in bytes of an integer or byte array.
template <typename T>
PW_CONSTEVAL size_t SizeOfBytes(const T& arg) {
if constexpr (UseBytesDirectly<T>) {
return sizeof(arg);
} else {
static_assert(sizeof(arg[0]) == sizeof(std::byte));
return std::size(arg);
template <typename B, typename T, size_t... kIndex>
PW_CONSTEVAL auto String(const T& array, std::index_sequence<kIndex...>) {
return std::array{static_cast<B>(array[kIndex])...};
template <typename T, typename U>
PW_CONSTEVAL bool CanBeRepresentedAsByteType(const U& value) {
return static_cast<U>(static_cast<T>(value)) == value;
} // namespace internal
// Concatenates arrays or integers as a byte array at compile time. Integer
// values are copied little-endian. Spans are copied byte-for-byte.
template <typename B = std::byte, typename... Args>
PW_CONSTEVAL auto Concat(Args... args) {
std::array<B, (internal::SizeOfBytes(args) + ...)> bytes{};
internal::CopyBytes(bytes.begin(), args...);
return bytes;
// Converts a string literal to an array of bytes, without the trailing '\0'.
template <typename B = std::byte,
size_t kSize,
typename Indices = std::make_index_sequence<kSize - 1>>
PW_CONSTEVAL auto String(const char (&str)[kSize]) {
return internal::String<B>(str, Indices{});
// String overload for the empty string "".
template <typename B = std::byte>
PW_CONSTEVAL auto String(const char (&)[1]) {
return std::array<B, 0>{};
// Creates an array of bytes from values passed as template parameters. The
// values are guaranteed to be representable in the destination byte type.
template <typename B, auto... values>
PW_CONSTEVAL auto Array() {
static_assert((internal::CanBeRepresentedAsByteType<B>(values) && ...));
return std::array<B, sizeof...(values)>{static_cast<B>(values)...};
// Array() defaults to using std::byte.
template <auto... values>
PW_CONSTEVAL auto Array() {
return Array<std::byte, values...>();
// Creates an initialized array of bytes. Initializes the array to a value or
// the return values from a function that accepts the index as a parameter.
template <typename B, size_t kSize, typename T>
constexpr auto Initialized(const T& value_or_function) {
std::array<B, kSize> array{};
for (size_t i = 0; i < kSize; ++i) {
if constexpr (std::is_integral_v<T>) {
array[i] = static_cast<B>(value_or_function);
} else {
array[i] = static_cast<B>(value_or_function(i));
return array;
// Initialized(value_or_function) defaults to using std::byte.
template <size_t kSize, typename T>
constexpr auto Initialized(const T& value_or_function) {
return Initialized<std::byte, kSize>(value_or_function);
// Creates an array of bytes from a series of function arguments. Unlike
// Array(), MakeArray() cannot check if the values fit in the destination type.
// MakeArray() should only be used when Array() is not suitable.
template <typename B = std::byte, typename... Args>
constexpr auto MakeArray(const Args&... args) {
return std::array<B, sizeof...(args)>{static_cast<B>(args)...};
} // namespace pw::bytes