From 06311ed1ced891f0b360a25b352cefafa0d98c46 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Sat, 3 Feb 2024 09:31:36 -0800
Subject: [PATCH] Fix fixed rounding around zero in Dragon

---
 include/fmt/format.h | 7 +++++--
 test/format-test.cc  | 3 +++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index ef63f43bf06a..b7169280b375 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -3104,8 +3104,11 @@ FMT_CONSTEXPR20 inline void format_dragon(basic_fp<uint128_t> value,
   // Generate the given number of digits.
   exp10 -= num_digits - 1;
   if (num_digits <= 0) {
-    denominator *= 10;
-    auto digit = add_compare(numerator, numerator, denominator) > 0 ? '1' : '0';
+    auto digit = '0';
+    if (num_digits == 0) {
+      denominator *= 10;
+      digit = add_compare(numerator, numerator, denominator) > 0 ? '1' : '0';
+    }
     buf.push_back(digit);
     return;
   }
diff --git a/test/format-test.cc b/test/format-test.cc
index 8406d86a1e19..67d0631fc2fa 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -953,6 +953,9 @@ TEST(format_test, precision) {
   EXPECT_EQ(fmt::format("{0:.3}", 1.1), "1.1");
   EXPECT_EQ(fmt::format("{:.0e}", 1.0L), "1e+00");
   EXPECT_EQ(fmt::format("{:9.1e}", 0.0), "  0.0e+00");
+  EXPECT_EQ(fmt::format("{:.7f}", 0.0000000000000071054273576010018587L),
+            "0.0000000");
+
   EXPECT_EQ(
       fmt::format("{:.494}", 4.9406564584124654E-324),
       "4.9406564584124654417656879286822137236505980261432476442558568250067550"