Improve performance of JSON rendering

.github/workflows/osrm-backend.yml

@@ -715,6 +715,7 @@ jobs:
         pushd ${OSRM_BUILD_DIR}
         make --jobs=${JOBS} benchmarks
         ./src/benchmarks/alias-bench 
+        ./src/benchmarks/json-render-bench
         ./src/benchmarks/match-bench ../test/data/ch/monaco.osrm
         ./src/benchmarks/packedvector-bench
         ./src/benchmarks/rtree-bench ../test/data/monaco.osrm.ramIndex ../test/data/monaco.osrm.fileIndex ../test/data/monaco.osrm.nbg_nodes

include/util/json_renderer.hpp

@@ -27,18 +27,32 @@ namespace
 constexpr int MAX_FLOAT_STRING_LENGTH = 256;
 }
 
-struct Renderer
+template <typename Out> struct Renderer
 {
-    explicit Renderer(std::ostream &_out) : out(_out) {}
+    explicit Renderer(Out &_out) : out(_out) {}
 
-    void operator()(const String &string) const
+    void operator()(const String &string)
     {
-        out << "\"";
-        out << escape_JSON(string.value);
-        out << "\"";
+        write('"');
+        // here we assume that vast majority of strings don't need to be escaped,
+        // so we check it first and escape only if needed
+        auto size = SizeOfEscapedJSONString(string.value);
+        if (size == string.value.size())
+        {
+            write(string.value);
+        }
+        else
+        {
+            std::string escaped;
+            escaped.reserve(size);
+            EscapeJSONString(string.value, escaped);
+
+            write(escaped);
+        }
+        write('"');
     }
 
-    void operator()(const Number &number) const
+    void operator()(const Number &number)
     {
         char buffer[MAX_FLOAT_STRING_LENGTH] = {'\0'};
         ieee754::dtoa_milo(number.value, buffer);
@@ -64,131 +78,95 @@ struct Renderer
             }
             ++pos;
         }
-        out << buffer;
+        write(buffer);
     }
 
-    void operator()(const Object &object) const
+    void operator()(const Object &object)
     {
-        out << "{";
+        write('{');
         for (auto it = object.values.begin(), end = object.values.end(); it != end;)
         {
-            out << "\"" << it->first << "\":";
+            write('\"');
+            write(it->first);
+            write("\":");
             mapbox::util::apply_visitor(Renderer(out), it->second);
             if (++it != end)
             {
-                out << ",";
+                write(',');
             }
         }
-        out << "}";
+        write('}');
     }
 
-    void operator()(const Array &array) const
+    void operator()(const Array &array)
     {
-        out << "[";
+        write('[');
         for (auto it = array.values.cbegin(), end = array.values.cend(); it != end;)
         {
             mapbox::util::apply_visitor(Renderer(out), *it);
             if (++it != end)
             {
-                out << ",";
+                write(',');
             }
         }
-        out << "]";
+        write(']');
     }
 
-    void operator()(const True &) const { out << "true"; }
+    void operator()(const True &) { write("true"); }
 
-    void operator()(const False &) const { out << "false"; }
+    void operator()(const False &) { write("false"); }
 
-    void operator()(const Null &) const { out << "null"; }
+    void operator()(const Null &) { write("null"); }
 
   private:
-    std::ostream &out;
+    void write(const std::string &str);
+    void write(const char *str);
+    void write(char ch);
+
+  private:
+    Out &out;
 };
 
-struct ArrayRenderer
+template <> void Renderer<std::vector<char>>::write(const std::string &str)
 {
-    explicit ArrayRenderer(std::vector<char> &_out) : out(_out) {}
-
-    void operator()(const String &string) const
-    {
-        out.push_back('\"');
-        const auto string_to_insert = escape_JSON(string.value);
-        out.insert(std::end(out), std::begin(string_to_insert), std::end(string_to_insert));
-        out.push_back('\"');
-    }
+    out.insert(out.end(), str.begin(), str.end());
+}
 
-    void operator()(const Number &number) const
-    {
-        const std::string number_string = cast::to_string_with_precision(number.value);
-        out.insert(out.end(), number_string.begin(), number_string.end());
-    }
+template <> void Renderer<std::vector<char>>::write(const char *str)
+{
+    out.insert(out.end(), str, str + strlen(str));
+}
 
-    void operator()(const Object &object) const
-    {
-        out.push_back('{');
-        for (auto it = object.values.begin(), end = object.values.end(); it != end;)
-        {
-            out.push_back('\"');
-            out.insert(out.end(), it->first.begin(), it->first.end());
-            out.push_back('\"');
-            out.push_back(':');
+template <> void Renderer<std::vector<char>>::write(char ch) { out.push_back(ch); }
 
-            mapbox::util::apply_visitor(ArrayRenderer(out), it->second);
-            if (++it != end)
-            {
-                out.push_back(',');
-            }
-        }
-        out.push_back('}');
-    }
+template <> void Renderer<std::ostream>::write(const std::string &str) { out << str; }
 
-    void operator()(const Array &array) const
-    {
-        out.push_back('[');
-        for (auto it = array.values.cbegin(), end = array.values.cend(); it != end;)
-        {
-            mapbox::util::apply_visitor(ArrayRenderer(out), *it);
-            if (++it != end)
-            {
-                out.push_back(',');
-            }
-        }
-        out.push_back(']');
-    }
+template <> void Renderer<std::ostream>::write(const char *str) { out << str; }
 
-    void operator()(const True &) const
-    {
-        const std::string temp("true");
-        out.insert(out.end(), temp.begin(), temp.end());
-    }
+template <> void Renderer<std::ostream>::write(char ch) { out << ch; }
 
-    void operator()(const False &) const
-    {
-        const std::string temp("false");
-        out.insert(out.end(), temp.begin(), temp.end());
-    }
+template <> void Renderer<std::string>::write(const std::string &str) { out += str; }
 
-    void operator()(const Null &) const
-    {
-        const std::string temp("null");
-        out.insert(out.end(), temp.begin(), temp.end());
-    }
+template <> void Renderer<std::string>::write(const char *str) { out += str; }
 
-  private:
-    std::vector<char> &out;
-};
+template <> void Renderer<std::string>::write(char ch) { out += ch; }
 
 inline void render(std::ostream &out, const Object &object)
 {
-    Value value = object;
-    mapbox::util::apply_visitor(Renderer(out), value);
+    Renderer renderer(out);
+    renderer(object);
+}
+
+inline void render(std::string &out, const Object &object)
+{
+    Renderer renderer(out);
+    renderer(object);
 }
 
 inline void render(std::vector<char> &out, const Object &object)
 {
-    Value value = object;
-    mapbox::util::apply_visitor(ArrayRenderer(out), value);
+    Renderer renderer(out);
+    renderer(object);
 }
 
 } // namespace json

include/util/string_util.hpp

@@ -58,11 +58,32 @@ template <int length, int precision> char *printInt(char *buffer, int value)
     return buffer;
 }
 
-inline std::string escape_JSON(const std::string &input)
+inline size_t SizeOfEscapedJSONString(const std::string &string)
+{
+    size_t size = 0;
+    for (const char letter : string)
+    {
+        switch (letter)
+        {
+        case '\\':
+        case '"':
+        case '/':
+        case '\b':
+        case '\f':
+        case '\n':
+        case '\r':
+        case '\t':
+            size += 2;
+            break;
+        default:
+            size += 1;
+        }
+    }
+    return size;
+}
+
+inline void EscapeJSONString(const std::string &input, std::string &output)
 {
-    // escape and skip reallocations if possible
-    std::string output;
-    output.reserve(input.size() + 4); // +4 assumes two backslashes on avg
     for (const char letter : input)
     {
         switch (letter)
@@ -96,7 +117,6 @@ inline std::string escape_JSON(const std::string &input)
             break;
         }
     }
-    return output;
 }
 
 inline std::size_t URIDecode(const std::string &input, std::string &output)

src/benchmarks/CMakeLists.txt

@@ -30,6 +30,18 @@ target_link_libraries(match-bench
 	${TBB_LIBRARIES}
 	${MAYBE_SHAPEFILE})
 
+add_executable(json-render-bench
+	EXCLUDE_FROM_ALL
+	json_render.cpp
+	$<TARGET_OBJECTS:UTIL>)
+
+target_link_libraries(json-render-bench
+	osrm
+	${BOOST_BASE_LIBRARIES}
+	${CMAKE_THREAD_LIBS_INIT}
+	${TBB_LIBRARIES}
+	${MAYBE_SHAPEFILE})
+
 add_executable(alias-bench
 	EXCLUDE_FROM_ALL
     ${AliasBenchmarkSources}
@@ -41,6 +53,8 @@ target_link_libraries(alias-bench
 	${TBB_LIBRARIES}
 	${MAYBE_SHAPEFILE})
 
+
+
 add_executable(packedvector-bench
 	EXCLUDE_FROM_ALL
     ${PackedVectorBenchmarkSources}
@@ -58,4 +72,5 @@ add_custom_target(benchmarks
 	rtree-bench
 	packedvector-bench
 	match-bench
+	json-render-bench
     alias-bench)