From dc6ce63e01d06e705f7eb065b2b28a6fee27164f Mon Sep 17 00:00:00 2001
From: Matt Borland <matt@mattborland.com>
Date: Fri, 30 Jun 2023 17:27:52 +0200
Subject: [PATCH] Fix 256-bit high bit calculation

---
 include/boost/charconv/detail/compute_float80.hpp | 7 ++++---
 include/boost/charconv/detail/emulated256.hpp     | 4 ++--
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/boost/charconv/detail/compute_float80.hpp b/include/boost/charconv/detail/compute_float80.hpp
index 6c67ad8..1daf794 100644
--- a/include/boost/charconv/detail/compute_float80.hpp
+++ b/include/boost/charconv/detail/compute_float80.hpp
@@ -177,8 +177,9 @@ inline uint128 mask_mantissa(uint256 z) noexcept;
 template <typename T>
 inline uint128 mask_mantissa(uint256 z) noexcept
 {
-    static constexpr uint128 mask {0x1FFFFFFFFFFFF, UINT64_MAX};
-    return (z >> 113) & mask;
+    //static constexpr uint128 mask {0x1FFFFFFFFFFFF, UINT64_MAX};
+    //return static_cast<uint128>((z >> 113) & mask);
+    return z.high;
 }
 #elif BOOST_CHARCONV_LDBL_BITS == 128
 
@@ -337,7 +338,7 @@ inline ResultType compute_float80(std::int64_t q, Unsigned_Integer w, bool negat
     #endif
 
     // Step 8: Value of the most significant bit of z
-    const auto u = significant_bit<ResultType>(z) == m ? 0 : 1;
+    const auto u = high_bit(z) == 255 ? 0 : 1;
     #ifdef BOOST_CHARCONV_DEBUG_FLOAT128
     std::cerr << "u: " << u << std::endl;
     #endif
diff --git a/include/boost/charconv/detail/emulated256.hpp b/include/boost/charconv/detail/emulated256.hpp
index 50bfbe8..7a7c299 100644
--- a/include/boost/charconv/detail/emulated256.hpp
+++ b/include/boost/charconv/detail/emulated256.hpp
@@ -242,11 +242,11 @@ int high_bit(uint256 v) noexcept
 {
     if (v.high != 0)
     {
-        return high_bit(v.high);
+        return 255 - high_bit(v.high);
     }
     else if (v.low != 0)
     {
-        return high_bit(v.low);
+        return 127 - high_bit(v.low);
     }
 
     return 0;