Restore istream performance #764

src/json.hpp

@@ -1410,6 +1410,11 @@ using input_adapter_t = std::shared_ptr<input_adapter_protocol>;
 class input_stream_adapter : public input_adapter_protocol
 {
   public:
+    ~input_stream_adapter() override
+    {
+        // clear stream flags; we use underlying streambuf I/O, do not maintain ifstream flags
+        is.clear();
+    }
     explicit input_stream_adapter(std::istream& i)
         : is(i)
     {
@@ -1441,17 +1446,15 @@ class input_stream_adapter : public input_adapter_protocol
         }
     }
 
-    ~input_stream_adapter() override {}
-
     int get_character() override
     {
-	int c = is.get();
+        int c = is.rdbuf()->sbumpc(); // Avoided for performance: int c = is.get();
         return c  == std::char_traits<char>::eof() ? c : ( c & 0xFF );
     }
 
     void unget_character() override
     {
-	is.unget();
+        is.rdbuf()->sungetc(); // Avoided for performance: is.unget();
     }
   private:
 
@@ -1489,7 +1492,7 @@ class input_buffer_adapter : public input_adapter_protocol
 
     void unget_character() noexcept override
     {
-	if (JSON_LIKELY(cursor > 0))
+        if (JSON_LIKELY(cursor > start))
         {
             --cursor;
         }
@@ -2652,17 +2655,16 @@ scan_number_done:
     void reset() noexcept
     {
         yylen = 0;
-        start_pos = chars_read - 1;
-	token_string = static_cast<char>( current );
+        token_string.clear();
+        token_string.push_back(static_cast<char>(current));
     }
 
     /// get a character from the input
     int get()
     {
         ++chars_read;
-
         int c = current = ia->get_character();
-	token_string += static_cast<char>( c );
+        token_string.push_back(static_cast<char>(c));
         return c;
     }
 
@@ -2670,9 +2672,12 @@ scan_number_done:
     void unget()
     {
         --chars_read;
-
-	if (token_string.size() > 0)
-	    token_string.resize( token_string.size() - 1 );
+        if (JSON_LIKELY(current != std::char_traits<char>::eof()))
+        {
+            ia->unget_character();
+        }
+        if (! token_string.empty())
+            token_string.pop_back();
     }
         
     /// add a character to yytext
@@ -2736,7 +2741,9 @@ scan_number_done:
         std::string result;
         for (auto c : token_string)
         {
-	    if ( c == std::char_traits<char>::eof() ) {
+            if (c == '\0' or c == std::char_traits<char>::eof())
+            {
+                // ignore EOF
                 continue;
             }
             else if ('\x00' <= c and c <= '\x1f')
@@ -2840,10 +2847,8 @@ scan_number_done:
 
     /// the number of characters read
     std::size_t chars_read = 0;
-    /// the start position of the current token
-    std::size_t start_pos = 0;
     /// raw input token string (for error messages)
-    std::string token_string = "";
+    std::vector<char> token_string = std::vector<char>();
 
     /// buffer for variable-length tokens (numbers, strings)
     std::vector<char> yytext = std::vector<char>(1024, '\0');