cpp-ipc/include/libimp/fmt.h

/**
 * @file libimp/fmt.h
 * @author mutouyun (orz@orzz.org)
 * @brief String formatting.
 * @date 2022-11-26
 * 
 * @remarks The current performance is not high, 
 * because I use std::sprintf directly for formatting for convenience.
 */
#pragma once

#include <string>
#include <utility>
#include <type_traits>
#include <chrono>   // std::chrono::time_point
#include <cstddef>
#include <ctime>    // std::tm, std::localtime

#include "libimp/def.h"
#include "libimp/fmt_cpo.h"
#include "libimp/span.h"
#include "libimp/detect_plat.h"
#include "libimp/export.h"

LIBIMP_NAMESPACE_BEG_

/**
 * @brief The format string reference wrapper.
 */
template <typename T>
struct fmt_ref {
  span<char const> fstr;
  T param;
};

/**
 * @brief Conversion specifiers.
 * 
 * @remarks Just like printf, the format string is of the form
 * [flags][field_width][.precision][conversion_character]
 * 
 * @see http://personal.ee.surrey.ac.uk/Personal/R.Bowden/C/printf.html
 */
template <std::size_t N>
auto spec(char const (&fstr)[N]) noexcept {
  return [&fstr](auto &&arg) noexcept {
    using arg_t = decltype(arg);
    return fmt_ref<arg_t> {{fstr}, static_cast<arg_t>(arg)};
  };
}

/**
 * @brief String formatting function.
 * 
 * @param args arguments that support the fmt output
 * @return an empty string if the fmt output fails 
 */
template <typename... A>
LIBIMP_NODISCARD std::string fmt(A &&...args) {
  std::string joined;
  fmt_context ctx(joined);
  if (fmt_to(ctx, std::forward<A>(args)...)) {
    return ctx.finish() ? joined : "";
  }
  return {};
}

/// @brief String types.
LIBIMP_EXPORT bool to_string(fmt_context &ctx, char const *       a) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, std::string const &a) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, char const *       a, span<char const> fstr) noexcept;
       inline bool to_string(fmt_context &ctx, std::string const &a, span<char const> fstr) noexcept { return to_string(ctx, a.c_str(), fstr); }

/// @brief Character to string conversion.
LIBIMP_EXPORT bool to_string(fmt_context &ctx, char     a) noexcept;
#if defined(LIBIMP_CPP_20)
       inline bool to_string(fmt_context &ctx, char8_t  a) noexcept { return to_string(ctx, (char)a); }
#endif // defined(LIBIMP_CPP_20)
LIBIMP_EXPORT bool to_string(fmt_context &ctx, wchar_t  a) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, char16_t a) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, char32_t a) noexcept;

/// @brief Conversion of numeric types to strings.
LIBIMP_EXPORT bool to_string(fmt_context &ctx,   signed short     a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, unsigned short     a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx,   signed int       a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, unsigned int       a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx,   signed long      a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, unsigned long      a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx,   signed long long a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, unsigned long long a, span<char const> fstr = {}) noexcept;
       inline bool to_string(fmt_context &ctx,   signed char      a, span<char const> fstr = {}) noexcept { return to_string(ctx,      (int)a, fstr); }
       inline bool to_string(fmt_context &ctx, unsigned char      a, span<char const> fstr = {}) noexcept { return to_string(ctx, (unsigned)a, fstr); }

/// @brief Conversion of floating point type to strings.
LIBIMP_EXPORT bool to_string(fmt_context &ctx,      double a, span<char const> fstr = {}) noexcept;
LIBIMP_EXPORT bool to_string(fmt_context &ctx, long double a, span<char const> fstr = {}) noexcept;
       inline bool to_string(fmt_context &ctx, float       a, span<char const> fstr = {}) noexcept { return to_string(ctx, (double)a, fstr); }

/// @brief Pointer.
LIBIMP_EXPORT bool to_string(fmt_context &ctx, std::nullptr_t) noexcept;
template <typename T,
          typename = std::enable_if_t<std::is_same<T, void>::value>>
LIBIMP_EXPORT bool to_string(fmt_context &ctx, T const volatile *a) noexcept;

/// @brief Date and time.
LIBIMP_EXPORT bool to_string(fmt_context &ctx, std::tm const &a, span<char const> fstr = {}) noexcept;

namespace detail {

/**
 * @brief Convert std::time_t to std::string.
 * @return an empty string if the conversion fails
 */
inline bool time_to_string(fmt_context &ctx, std::time_t tt, span<char const> fstr) noexcept {
#if defined(LIBIMP_CC_MSVC)
  /// @see https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
  std::tm tm {};
  if (::localtime_s(&tm, &tt) != 0) {
    return {};
  }
  return to_string(ctx, tm, fstr);
#else
  return to_string(ctx, *std::localtime(&tt), fstr);
#endif
}

} // namespace detail

template <class Clock, class Duration>
bool to_string(fmt_context &ctx, std::chrono::time_point<Clock, Duration> const &a, span<char const> fstr = {}) noexcept {
  return detail::time_to_string(ctx, std::chrono::system_clock::to_time_t(a), fstr);
}

/**
 * @brief Predefined fmt_to method
 */
namespace detail {

template <typename T>
auto tag_invoke(fmt_to_t, fmt_context &ctx, T &&arg) noexcept
  -> decltype(::LIBIMP::to_string(ctx, std::forward<T>(arg))) {
  return ::LIBIMP::to_string(ctx, std::forward<T>(arg));
}

template <typename T>
auto tag_invoke(fmt_to_t, fmt_context &ctx, fmt_ref<T> arg) noexcept
  -> decltype(::LIBIMP::to_string(ctx, static_cast<T>(arg.param), arg.fstr)) {
  return ::LIBIMP::to_string(ctx, static_cast<T>(arg.param), arg.fstr);
}

template <typename T>
bool tag_invoke(decltype(::LIBIMP::fmt_to), fmt_context &ctx, span<T> s) {
  if (s.empty()) {
    return false;
  }
  if (!fmt_to(ctx, s[0])) {
    return false;
  }
  for (std::size_t i = 1; i < s.size(); ++i) {
    if (!fmt_to(ctx, ' ', s[i])) return false;
  }
  return true;
}

} // namespace detail
LIBIMP_NAMESPACE_END_