From b9c0e4dd82874d784cd8bd70457c595658483d48 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Fri, 2 Aug 2024 11:57:02 -0700
Subject: [PATCH] Improve spec parsing

---
 include/fmt/base.h   | 30 +++++++-----------------------
 include/fmt/format.h | 13 ++++++++-----
 2 files changed, 15 insertions(+), 28 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 141ffe08..b40b3bfb 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -2398,14 +2398,11 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
   constexpr auto integral_set = sint_set | uint_set | bool_set | char_set;
   struct {
     const Char*& begin;
-    dynamic_format_specs<Char>& specs;
+    format_specs& specs;
     type arg_type;
 
     FMT_CONSTEXPR auto operator()(pres pres_type, int set) -> const Char* {
-      if (!in(arg_type, set)) {
-        if (arg_type == type::none_type) return begin;
-        report_error("invalid format specifier");
-      }
+      if (!in(arg_type, set)) report_error("invalid format specifier");
       specs.type = pres_type;
       return begin + 1;
     }
@@ -2421,34 +2418,23 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
       ++begin;
       break;
     case '+':
-    case '-':
+      FMT_FALLTHROUGH;
     case ' ':
-      if (arg_type == type::none_type) return begin;
+      specs.sign = c == ' ' ? sign::space : sign::plus;
+      FMT_FALLTHROUGH;
+    case '-':
       enter_state(state::sign, in(arg_type, sint_set | float_set));
-      switch (c) {
-      case '+':
-        specs.sign = sign::plus;
-        break;
-      case '-':
-        break;
-      case ' ':
-        specs.sign = sign::space;
-        break;
-      }
       ++begin;
       break;
     case '#':
-      if (arg_type == type::none_type) return begin;
       enter_state(state::hash, is_arithmetic_type(arg_type));
       specs.alt = true;
       ++begin;
       break;
     case '0':
       enter_state(state::zero);
-      if (!is_arithmetic_type(arg_type)) {
-        if (arg_type == type::none_type) return begin;
+      if (!is_arithmetic_type(arg_type))
         report_error("format specifier requires numeric argument");
-      }
       if (specs.align == align::none) {
         // Ignore 0 if align is specified for compatibility with std::format.
         specs.align = align::numeric;
@@ -2470,14 +2456,12 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
       begin = parse_dynamic_spec(begin, end, specs.width, specs.width_ref, ctx);
       break;
     case '.':
-      if (arg_type == type::none_type) return begin;
       enter_state(state::precision,
                   in(arg_type, float_set | string_set | cstring_set));
       begin = parse_precision(begin, end, specs.precision, specs.precision_ref,
                               ctx);
       break;
     case 'L':
-      if (arg_type == type::none_type) return begin;
       enter_state(state::locale, is_arithmetic_type(arg_type));
       specs.localized = true;
       ++begin;
diff --git a/include/fmt/format.h b/include/fmt/format.h
index b0976f47..33ed7034 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -2315,7 +2315,6 @@ FMT_CONSTEXPR auto write(OutputIt out, T value) -> OutputIt {
   return format_decimal<Char>(out, abs_value, num_digits);
 }
 
-// DEPRECATED!
 template <typename Char>
 FMT_CONSTEXPR auto parse_align(const Char* begin, const Char* end,
                                format_specs& specs) -> const Char* {
@@ -4068,12 +4067,16 @@ template <typename T, typename Char = char> struct nested_formatter {
 
   FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
       -> decltype(ctx.begin()) {
-    auto specs = detail::dynamic_format_specs<Char>();
-    auto it = parse_format_specs(ctx.begin(), ctx.end(), specs, ctx,
-                                 detail::type::none_type);
-    width_ = specs.width;
+    auto it = ctx.begin(), end = ctx.end();
+    if (it == end) return it;
+    auto specs = format_specs();
+    it = detail::parse_align(it, end, specs);
     fill_ = specs.fill;
     align_ = specs.align;
+    Char c = *it;
+    auto width_ref = detail::arg_ref<Char>();
+    if ((c >= '0' && c <= '9') || c == '{')
+      it = detail::parse_dynamic_spec(it, end, width_, width_ref, ctx);
     ctx.advance_to(it);
     return formatter_.parse(ctx);
   }