From 1e57c08686123d34b6161911ff786603f5af2d77 Mon Sep 17 00:00:00 2001
From: joaquintides <joaquin@tid.es>
Date: Mon, 28 Nov 2022 17:58:15 +0100
Subject: [PATCH] fourth attempt

---
 include/boost/unordered/detail/foa.hpp | 27 ++++++++------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

diff --git a/include/boost/unordered/detail/foa.hpp b/include/boost/unordered/detail/foa.hpp
index 01e3ba34..d05752c8 100644
--- a/include/boost/unordered/detail/foa.hpp
+++ b/include/boost/unordered/detail/foa.hpp
@@ -689,46 +689,35 @@ struct pow2_size_policy
     // of performance
 
     std::size_t used_bits_=n<=2?1:((std::size_t)(boost::core::bit_width(n-1)));
-    std::size_t used_hi_=used_bits_>=10?(5-(used_bits_%6))*2:0;
-    std::size_t used_hi_mask_=(std::size_t(1)<<used_hi_)-1;
-    std::size_t unused_bits_=total_bits-used_bits_;
+    std::size_t shift_right_=total_bits-used_bits_;
+    std::size_t shift_left_=shift_right_%2?0:shift_right_-8;
 
-    return (used_hi_mask_<<16)|(used_hi_<<8)|unused_bits_;
+    return (shift_left_<<16)|shift_right_;
   }
 
   static inline std::size_t size(std::size_t size_index_)
   {
-    return std::size_t(1)<<(total_bits-unused_bits(size_index_));
+    return std::size_t(1)<<(total_bits-shift_right(size_index_));
   }
     
   static constexpr std::size_t min_size(){return 2;}
 
   static inline std::size_t position(std::size_t hash,std::size_t size_index_)
   {
-    std::size_t used_hi_mask_=used_hi_mask(size_index_);
-    std::size_t used_hi_=used_hi(size_index_);
-    std::size_t unused_bits_=unused_bits(size_index_);
-
-    hash=boost::core::rotl(hash,(int)used_hi_);
-    return ((hash>>unused_bits_)&~used_hi_mask_)|(hash&used_hi_mask_);
+    return (hash<<shift_left(size_index_))>>shift_right(size_index_);
   }
 
 private:
   static constexpr std::size_t total_bits=sizeof(std::size_t)*CHAR_BIT;
 
-  static inline std::size_t used_hi_mask(std::size_t size_index_)
+  static inline std::size_t shift_left(std::size_t size_index_)
   {
     return size_index_>>16;
   }
 
-  static inline std::size_t used_hi(std::size_t size_index_)
+  static inline std::size_t shift_right(std::size_t size_index_)
   {
-    return (size_index_>>8)&0xffu;
-  }
-
-  static inline std::size_t unused_bits(std::size_t size_index_)
-  {
-    return size_index_&0xffu;
+    return size_index_&0xffffu;
   }
 };
 #endif