custom formatter for numbers in 'pow' units format

include/util/format.hpp

@@ -1,9 +1,84 @@
 #pragma once
 
-#include <sstream>
+#include <fmt/format.h>
 
-namespace waybar::util {
+class pow_format {
+  public:
+    pow_format(long long val, std::string&& unit, bool binary = false):
+      val_(val), unit_(unit), binary_(binary) { };
 
-std::string pow_format(unsigned long long value, const std::string &unit, bool binary = false);
+    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 == '=') {
+            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.
+            width = parse_nonnegative_int(it, end, ctx);
+          }
+          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_ ? "  " : " ")
+            );
+        }
+    };
 }
+