From 6c025520aa8371eb40ac06e6d4569575d7e65802 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Wed, 23 Sep 2020 15:12:03 -0700
Subject: [PATCH] Test that max_k is correctly defined

---
 include/fmt/format-inl.h |  9 +++++----
 test/format-impl-test.cc | 13 +++++++++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 1c13902a..e3817886 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -1198,10 +1198,11 @@ inline fp operator*(fp x, fp y) { return {multiply(x.f, y.f), x.e + y.e + 64}; }
 // (binary) exponent satisfies `min_exponent <= c_k.e <= min_exponent + 28`.
 inline fp get_cached_power(int min_exponent, int& pow10_exponent) {
   const int shift = 32;
-  int index = static_cast<int>(((min_exponent + fp::significand_size - 1) *
-                                    (data::log10_2_significand >> shift) +
-                                ((int64_t(1) << shift) - 1))  // ceil
-                               >> 32  // arithmetic shift
+  const auto significand = static_cast<int64_t>(data::log10_2_significand);
+  int index = static_cast<int>(
+      ((min_exponent + fp::significand_size - 1) * (significand >> shift) +
+       ((int64_t(1) << shift) - 1))  // ceil
+      >> 32                          // arithmetic shift
   );
   // Decimal exponent of the first (smallest) cached power of 10.
   const int first_dec_exp = -348;
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index b0df24aa..dbb635bc 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -220,6 +220,19 @@ TEST(FPTest, GetCachedPower) {
   }
 }
 
+TEST(FPTest, DragonboxMaxK) {
+  using fmt::detail::dragonbox::floor_log10_pow2;
+  using float_info = fmt::detail::dragonbox::float_info<float>;
+  EXPECT_EQ(fmt::detail::const_check(float_info::max_k),
+            float_info::kappa - floor_log10_pow2(float_info::min_exponent -
+                                                 float_info::significand_bits));
+  using double_info = fmt::detail::dragonbox::float_info<double>;
+  EXPECT_EQ(
+      fmt::detail::const_check(double_info::max_k),
+      double_info::kappa - floor_log10_pow2(double_info::min_exponent -
+                                            double_info::significand_bits));
+}
+
 TEST(FPTest, GetRoundDirection) {
   using fmt::detail::get_round_direction;
   using fmt::detail::round_direction;