From 35c58f084e7cd79b51997439cff68346f4c724c0 Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Fri, 9 Oct 2026 01:06:28 +0500
Subject: [PATCH] Fix hexfloat zero padding: zero padding must be placed after
 the sign and "0x" prefix (#4972)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/format.h | 21 +++++++++++++++++----
 test/format-test.cc  |  2 ++
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 53cfdd73..4e6f320f 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -3368,8 +3368,6 @@ FMT_CONSTEXPR20 void format_hexfloat(Float value, format_specs specs,
   // Remove zero tail.
   while (print_xdigits > 0 && xdigits[print_xdigits] == '0') --print_xdigits;
 
-  buf.push_back('0');
-  buf.push_back(specs.upper() ? 'X' : 'x');
   buf.push_back(xdigits[0]);
   if (specs.alt() || print_xdigits > 0 || print_xdigits < specs.precision)
     buf.push_back('.');
@@ -3745,9 +3743,24 @@ FMT_CONSTEXPR20 auto write(OutputIt out, T value, format_specs specs,
   memory_buffer buffer;
   if (specs.type() == presentation_type::hexfloat) {
     if (s != sign::none) buffer.push_back(detail::getsign<char>(s));
+
+    size_t prefix_length = 0;
+    buffer.push_back('0');
+    buffer.push_back(specs.upper() ? 'X' : 'x');
+    if (specs.align() == align::numeric) {
+      prefix_length = buffer.size();
+      const char* data = buffer.data();
+      out = copy<Char>(data, data + prefix_length, out);
+      buffer.clear();
+    }
+
     format_hexfloat(convert_float(value), specs, buffer);
-    return write_bytes<Char, align::right>(out, {buffer.data(), buffer.size()},
-                                           specs);
+    return write_padded<Char, align::right>(
+        out, specs, buffer.size(), buffer.size() + prefix_length,
+        [&buffer](reserve_iterator<OutputIt> it) {
+          const char* data = buffer.data();
+          return copy<Char>(data, data + buffer.size(), it);
+        });
   }
 
   if (specs.type() == presentation_type::exp) {
diff --git a/test/format-test.cc b/test/format-test.cc
index a7b47d31..208a6c1a 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1671,6 +1671,8 @@ TEST(format_test, format_double) {
   EXPECT_EQ(fmt::format("{:L}", 42.0), "42");
   EXPECT_EQ(fmt::format("{:24a}", 4.2f), "           0x1.0cccccp+2");
   EXPECT_EQ(fmt::format("{:24a}", 4.2), "    0x1.0cccccccccccdp+2");
+  EXPECT_EQ(fmt::format("{:012a}", 1.0), "0x0000001p+0");
+  EXPECT_EQ(fmt::format("{:*>12a}", 1.0), "******0x1p+0");
   EXPECT_EQ(fmt::format("{:<24a}", 4.2), "0x1.0cccccccccccdp+2    ");
   EXPECT_EQ(fmt::format("{0:e}", 392.65), "3.926500e+02");
   EXPECT_EQ(fmt::format("{0:E}", 392.65), "3.926500E+02");