perf: formatters compile-time performance improved

Author: mpusz

example/kalman_filter/kalman.h

@@ -308,9 +308,7 @@ class MP_UNITS_STD_FMT::formatter<kalman::system_state<QPs...>, Char> {
     std::basic_string<Char> quantity_buffer;
     format_system_state(std::back_inserter(quantity_buffer), s, ctx, std::index_sequence_for<QPs...>{});
 
-    std::basic_string<Char> fill_align_width_format_str;
-    mp_units::detail::format_global_buffer(std::back_inserter(fill_align_width_format_str), specs);
-    return MP_UNITS_STD_FMT::vformat_to(ctx.out(), fill_align_width_format_str,
-                                        MP_UNITS_STD_FMT::make_format_args(quantity_buffer));
+    return mp_units::detail::write_padded<Char>(ctx.out(), std::basic_string_view<Char>{quantity_buffer}, specs.width,
+                                                specs.align, specs.fill);
   }
 };

src/core/include/mp-units/bits/format.h

@@ -67,28 +67,28 @@ template<std::forward_iterator It, typename Specs>
   return mp_units::detail::parse_dynamic_spec(it, end, specs.width, specs.width_ref, ctx);
 }
 
-template<typename Char, std::output_iterator<Char> It>
-constexpr It format_global_buffer(It out, const fill_align_width_format_specs<Char>& specs)
+/**
+ * @brief Writes a string to an output iterator with fill/align/width padding.
+ *
+ * Avoids instantiating format_to/vformat_to infrastructure for the common padding use-case
+ * inside formatter<Unit>, formatter<Dimension>, and formatter<quantity> specializations.
+ */
+template<typename Char, std::output_iterator<Char> Out>
+constexpr Out write_padded(Out out, std::basic_string_view<Char> s, int width, fmt_align align,
+                           const fill_t<Char>& fill)
 {
-  MP_UNITS_STD_FMT::format_to(out, "{{:");
-  if (specs.fill.size() != 1 || specs.fill[0] != ' ') {
-    MP_UNITS_STD_FMT::format_to(out, "{}", specs.fill.data());
-  }
-  switch (specs.align) {
-    case fmt_align::left:
-      MP_UNITS_STD_FMT::format_to(out, "<");
-      break;
-    case fmt_align::right:
-      MP_UNITS_STD_FMT::format_to(out, ">");
-      break;
-    case fmt_align::center:
-      MP_UNITS_STD_FMT::format_to(out, "^");
-      break;
-    default:
-      break;
-  }
-  if (specs.width >= 1) MP_UNITS_STD_FMT::format_to(out, "{}", specs.width);
-  return MP_UNITS_STD_FMT::format_to(out, "}}");
+  const int len = static_cast<int>(s.size());
+  const int pad = (width > len) ? width - len : 0;
+  const int lpad = (align == fmt_align::center) ? pad / 2 : (align == fmt_align::right) ? pad : 0;
+  const int rpad = pad - lpad;
+  auto write_fill = [&](int n) {
+    for (int i = 0; i < n; ++i)
+      for (std::size_t j = 0; j < fill.size(); ++j) *out++ = fill[j];
+  };
+  write_fill(lpad);
+  for (Char c : s) *out++ = c;
+  write_fill(rpad);
+  return out;
 }
 
 MP_UNITS_EXPORT_END

src/core/include/mp-units/framework/dimension.h

@@ -345,7 +345,6 @@ template<typename D, typename Char>
 class MP_UNITS_STD_FMT::formatter<D, Char> {
   struct format_specs : mp_units::detail::fill_align_width_format_specs<Char>, mp_units::dimension_symbol_formatting {};
   format_specs specs_{};
-  std::basic_string_view<Char> fill_align_width_format_str_;
 
   template<std::forward_iterator It>
   constexpr It parse_dimension_specs(It begin, It end)
@@ -374,7 +373,6 @@ class MP_UNITS_STD_FMT::formatter<D, Char> {
     auto end = ctx.end();
 
     auto it = parse_fill_align_width(ctx, begin, end, specs_);
-    fill_align_width_format_str_ = {begin, it};
     if (it == end) return it;
 
     return parse_dimension_specs(it, end);
@@ -391,11 +389,8 @@ class MP_UNITS_STD_FMT::formatter<D, Char> {
       return mp_units::dimension_symbol_to<Char>(ctx.out(), d, specs);
     std::basic_string<Char> unit_buffer;
     mp_units::dimension_symbol_to<Char>(std::back_inserter(unit_buffer), d, specs);
-
-    const std::basic_string<Char> global_format_buffer =
-      "{:" + std::basic_string<Char>{fill_align_width_format_str_} + "}";
-    return MP_UNITS_STD_FMT::vformat_to(ctx.out(), global_format_buffer,
-                                        MP_UNITS_STD_FMT::make_format_args(unit_buffer));
+    return mp_units::detail::write_padded<Char>(ctx.out(), std::basic_string_view<Char>{unit_buffer}, specs.width,
+                                                specs.align, specs.fill);
   }
 };
 

src/core/include/mp-units/framework/quantity.h

@@ -1080,18 +1080,25 @@ class MP_UNITS_STD_FMT::formatter<mp_units::quantity<Reference, Rep>, Char> {
     auto specs = specs_;
     mp_units::detail::handle_dynamic_spec<mp_units::detail::width_checker>(specs.width, specs.width_ref, ctx);
 
+    if (specs.width == 0 && modifiers_format_str_.empty()) {
+      // Common fast path: call pre-parsed sub-formatters directly — no vformat_to, no allocation
+      ctx.advance_to(rep_formatter_.format(q.numerical_value_ref_in(q.unit), ctx));
+      if constexpr (mp_units::space_before_unit_symbol<unit>) {
+        auto it = ctx.out();
+        *it++ = ' ';
+        ctx.advance_to(it);
+      }
+      return unit_formatter_.format(q.unit, ctx);
+    }
     if (specs.width == 0) {
-      // Avoid extra copying if width is not specified
+      // Custom modifiers, no width
       format_quantity(ctx.out(), q, ctx);
       return ctx.out();
     }
     std::basic_string<Char> quantity_buffer;
     format_quantity(std::back_inserter(quantity_buffer), q, ctx);
-
-    std::basic_string<Char> fill_align_width_format_str;
-    mp_units::detail::format_global_buffer(std::back_inserter(fill_align_width_format_str), specs);
-    return MP_UNITS_STD_FMT::vformat_to(ctx.out(), fill_align_width_format_str,
-                                        MP_UNITS_STD_FMT::make_format_args(quantity_buffer));
+    return mp_units::detail::write_padded<Char>(ctx.out(), std::basic_string_view<Char>{quantity_buffer}, specs.width,
+                                                specs.align, specs.fill);
   }
 };
 

src/core/include/mp-units/framework/unit.h

@@ -984,8 +984,6 @@ class MP_UNITS_STD_FMT::formatter<U, Char> {
   struct format_specs : mp_units::detail::fill_align_width_format_specs<Char>, mp_units::unit_symbol_formatting {};
   format_specs specs_{};
 
-  std::basic_string_view<Char> fill_align_width_format_str_;
-
   template<std::forward_iterator It>
   constexpr It parse_unit_specs(It begin, It end)
   {
@@ -1031,7 +1029,6 @@ class MP_UNITS_STD_FMT::formatter<U, Char> {
     auto end = ctx.end();
 
     auto it = parse_fill_align_width(ctx, begin, end, specs_);
-    fill_align_width_format_str_ = {begin, it};
     if (it == end) return it;
 
     return parse_unit_specs(it, end);
@@ -1048,11 +1045,8 @@ class MP_UNITS_STD_FMT::formatter<U, Char> {
       return mp_units::unit_symbol_to<Char>(ctx.out(), u, specs);
     std::basic_string<Char> unit_buffer;
     mp_units::unit_symbol_to<Char>(std::back_inserter(unit_buffer), u, specs);
-
-    const std::basic_string<Char> global_format_buffer =
-      "{:" + std::basic_string<Char>{fill_align_width_format_str_} + "}";
-    return MP_UNITS_STD_FMT::vformat_to(ctx.out(), global_format_buffer,
-                                        MP_UNITS_STD_FMT::make_format_args(unit_buffer));
+    return mp_units::detail::write_padded<Char>(ctx.out(), std::basic_string_view<Char>{unit_buffer}, specs.width,
+                                                specs.align, specs.fill);
   }
 };