waybar/include/util/format.hpp

#pragma once

#include <fmt/format.h>

class pow_format {
  public:
    pow_format(long long val, std::string&& unit, bool binary = false):
      val_(val), unit_(unit), binary_(binary) { };

    long long val_;
    std::string unit_;
    bool binary_;
};


namespace fmt {
  template <>
    struct formatter<pow_format> {
      char spec = 0;
      int width = 0;

      template <typename ParseContext>
        constexpr auto parse(ParseContext& ctx) -> decltype (ctx.begin()) {
          auto it = ctx.begin(), end = ctx.end();
          if (it != end && *it == ':') ++it;
          if (it && (*it == '>' || *it == '<' || *it == '=')) {
            spec = *it;
            ++it;
          }
          if (it == end || *it == '}') return it;
          if ('0' <= *it && *it <= '9') {
            // We ignore it for now, but keep it for compatibility with
            // existing configs where the format for pow_format'ed numbers was
            // 'string' and specifications such as {:>9} were valid.
            // The rationale for ignoring it is that the only reason to specify
            // an alignment and a with is to get a fixed width bar, and ">" is
            // sufficient in this implementation.
#if FMT_VERSION < 80000
            width = parse_nonnegative_int(it, end, ctx);
#else
            width = detail::parse_nonnegative_int(it, end, -1);
#endif
          }
          return it;
        }

      template<class FormatContext>
        auto format(const pow_format& s, FormatContext &ctx) -> decltype (ctx.out()) {
          const char* units[] = { "", "k",  "M",  "G",  "T",  "P",  nullptr};

          auto base = s.binary_ ? 1024ull : 1000ll;
          auto fraction = (double) s.val_;

          int pow;
          for (pow = 0; units[pow+1] != nullptr && fraction / base >= 1; ++pow) {
            fraction /= base;
          }

          auto max_width = 4                  // coeff in {:.3g} format
                         + 1                  // prefix from units array
                         + s.binary_          // for the 'i' in GiB.
                         + s.unit_.length();

          const char * format;
          std::string string;
          switch (spec) {
            case '>':
              return format_to(ctx.out(), "{:>{}}", fmt::format("{}", s), max_width);
            case '<':
              return format_to(ctx.out(), "{:<{}}", fmt::format("{}", s), max_width);
            case '=':
              format = "{coefficient:<4.3g}{padding}{prefix}{unit}";
              break;
            case 0:
            default:
              format = "{coefficient:.3g}{prefix}{unit}";
              break;
          }
          return format_to(ctx.out(), format
              , fmt::arg("coefficient", fraction)
              , fmt::arg("prefix", std::string() + units[pow] + ((s.binary_ && pow) ? "i" : ""))
              , fmt::arg("unit", s.unit_)
              , fmt::arg("padding", pow ? "" : s.binary_ ? "  " : " ")
            );
        }
    };
}