Faster http::string_to_field:

* Optimize hash calculation and equality for field strings

Experiments show the digest and equality functions taking the
most time. Optimizing L1 cache usage did not show significant
improvements.

CHANGELOG.md

@@ -1,3 +1,9 @@
+Version 209:
+
+* Faster http::string_to_field
+
+--------------------------------------------------------------------------------
+
 Version 208:
 
 * Add get_lowest_layer free function

include/boost/beast/http/impl/field.ipp

@@ -15,6 +15,7 @@
 #include <array>
 #include <unordered_map>
 #include <vector>
+#include <cstring>
 #include <boost/assert.hpp>
 
 namespace boost {
@@ -28,58 +29,66 @@ struct field_table
     using array_type =
         std::array<string_view, 353>;
 
-    struct hash
+    // Strings are converted to lowercase
+    static
+    std::uint32_t
+    digest(string_view s)
     {
-        std::size_t
-        operator()(string_view s) const
+        std::uint32_t r = 0;
+        std::size_t n = s.size();
+        unsigned char const* p =reinterpret_cast<
+            unsigned char const*>(s.data());
+        while(n >= 4)
         {
-            auto const n = s.size();
-            return
-                beast::detail::ascii_tolower(s[0]) *
-                beast::detail::ascii_tolower(s[n/2]) ^
-                beast::detail::ascii_tolower(s[n-1]);   // hist[] = 331, 10, max_load_factor = 0.15f
+            std::uint32_t v;
+            std::memcpy(&v, p, 4);
+            r = r * 5 + ( v | 0x20202020 );
+            p += 4;
+            n -= 4;
+        }
+        while( n > 0 )
+        {
+            r = r * 5 + ( *p | 0x20 );
+            ++p;
+            --n;
+        }
+        return r;
     }
-    };
 
-    struct iequal
-    {
-        // assumes inputs have equal length
+    // This comparison is case-insensitive, and the
+    // strings must contain only valid http field characters.
+    static
     bool
-        operator()(
-            string_view lhs,
-            string_view rhs) const
+    equals(string_view lhs, string_view rhs)
     {
+        using Int = std::uint32_t; // or std::size_t
+        auto n = lhs.size();
+        if(n != rhs.size())
+            return false;
         auto p1 = lhs.data();
         auto p2 = rhs.data();
-            auto pend = p1 + lhs.size();
-            char a, b;
-            while(p1 < pend)
+        auto constexpr S = sizeof(Int);
+        auto constexpr Mask = static_cast<Int>(
+            0xDFDFDFDFDFDFDFDF & ~Int{0});
+        for(; n >= S; p1 += S, p2 += S, n -= S)
         {
-                a = *p1++;
-                b = *p2++;
-                if(a != b)
-                    goto slow;
-            }
-            return true;
-
-            while(p1 < pend)
-            {
-            slow:
-                if( beast::detail::ascii_tolower(a) !=
-                    beast::detail::ascii_tolower(b))
+            Int v1, v2;
+            std::memcpy( &v1, p1, S );
+            std::memcpy( &v2, p2, S );
+            if((v1 ^ v2) & Mask)
                 return false;
-                a = *p1++;
-                b = *p2++;
         }
+        for(; n; ++p1, ++p2, --n)
+            if(( *p1 ^ *p2) & 0xDF)
+                return false;
         return true;
     }
-    };
-
-    using map_type = std::unordered_map<
-        string_view, field, hash, iequal>;
 
     array_type by_name_;
-    std::vector<map_type> by_size_;
+
+    enum { N = 5155 };
+    unsigned char map_[ N ][ 2 ] = {};
+
 /*
     From:
     
@@ -442,58 +451,43 @@ struct field_table
             "Xref"
         }})
     {
-        // find the longest field length
-        std::size_t high = 0;
-        for(auto const& s : by_name_)
-            if(high < s.size())
-                high = s.size();
-        // build by_size map
-        // skip field::unknown
-        by_size_.resize(high + 1);
-        for(auto& map : by_size_)
-            map.max_load_factor(.15f);
-        for(std::size_t i = 1;
-            i < by_name_.size(); ++i)
+        for(std::size_t i = 1, n = 256; i < n; ++i)
         {
-            auto const& s = by_name_[i];
-            by_size_[s.size()].emplace(
-                s, static_cast<field>(i));
+            auto sv = by_name_[ i ];
+            auto h = digest(sv);
+            auto j = h % N;
+            BOOST_ASSERT(map_[j][0] == 0);
+            map_[j][0] = static_cast<unsigned char>(i);
         }
 
-#if 0
-        // This snippet calculates the performance
-        // of the hash function and map settings
+        for(std::size_t i = 256, n = by_name_.size(); i < n; ++i)
         {
-            std::vector<std::size_t> hist;
-            for(auto const& map : by_size_)
-            {
-                for(std::size_t i = 0; i < map.bucket_count(); ++i)
-                {
-                    auto const n = map.bucket_size(i);
-                    if(n > 0)
-                    {
-                        if(hist.size() < n)
-                            hist.resize(n);
-                        ++hist[n-1];
+            auto sv = by_name_[i];
+            auto h = digest(sv);
+            auto j = h % N;
+            BOOST_ASSERT(map_[j][1] == 0);
+            map_[j][1] = static_cast<unsigned char>(i - 255);
         }
     }
-            }
-        }
-#endif
-    }
 
     field
     string_to_field(string_view s) const
     {
-        if(s.size() >= by_size_.size())
+        auto h = digest(s);
+        auto j = h % N;
+        int i = map_[j][0];
+        string_view s2 = by_name_[i];
+        if(i != 0 && equals(s, s2))
+            return static_cast<field>(i);
+        i = map_[j][1];
+        if(i == 0)
             return field::unknown;
-        auto const& map = by_size_[s.size()];
-        if(map.empty())
+        i += 255;
+        s2 = by_name_[i];
+
+        if(equals(s, s2))
+            return static_cast<field>(i);
         return field::unknown;
-        auto it = map.find(s);
-        if(it == map.end())
-            return field::unknown;
-        return it->second;
     }
 
     //