Improved speed of serializeXxx() when writing to a String

CHANGELOG.md

@@ -15,6 +15,7 @@ HEAD
   (This file is used for syntax highlighting in the Arduino IDE) 
 * Fixed `variant.is<nullptr_t>()`
 * Fixed value returned by `serializeJson()`, `serializeJsonPretty()`, and `serializeMsgPack()` when writing to a `String`
+* Improved speed of `serializeJson()`, `serializeJsonPretty()`, and `serializeMsgPack()` when writing to a `String`
 
 > ### BREAKING CHANGES
 > 

extras/tests/Helpers/WString.h

@@ -9,16 +9,10 @@
 // Reproduces Arduino's String class
 class String {
  public:
-  String& operator+=(char c) {
-    _str += c;
+  String& operator+=(const char* rhs) {
+    _str += rhs;
     return *this;
   }
-  String& operator+=(int);  // no used, just to add ambiguity
-
-  unsigned char reserve(size_t capacity) {
-    _str.reserve(capacity);
-    return 1;
-  }
 
   size_t length() const {
     return _str.size();
@@ -28,12 +22,21 @@ class String {
     return _str.c_str();
   }
 
+  bool operator==(const char* s) const {
+    return _str == s;
+  }
+
+  friend std::ostream& operator<<(std::ostream& lhs, const ::String& rhs) {
+    lhs << rhs._str;
+    return lhs;
+  }
+
  private:
   std::string _str;
 };
 
 class StringSumHelper;
 
-bool operator==(const std::string& lhs, const ::String& rhs) {
+inline bool operator==(const std::string& lhs, const ::String& rhs) {
   return lhs == rhs.c_str();
 }

extras/tests/Misc/StringWriter.cpp

@@ -3,6 +3,7 @@
 // MIT License
 
 #define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
+#define ARDUINOJSON_STRING_BUFFER_SIZE 5
 #include <ArduinoJson.h>
 #include <catch.hpp>
 #include "custom_string.hpp"
@@ -59,7 +60,39 @@ TEST_CASE("Writer<std::string>") {
 TEST_CASE("Writer<String>") {
   ::String output;
   Writer< ::String> sb(output);
+
   common_tests(sb, output);
+
+  SECTION("Writes characters to temporary buffer") {
+    // accumulate in buffer
+    sb.write('a');
+    sb.write('b');
+    sb.write('c');
+    REQUIRE(output == "");
+
+    // flush when full
+    sb.write('d');
+    REQUIRE(output == "abcd");
+
+    // flush on destruction
+    sb.write('e');
+    sb.~Writer();
+    REQUIRE(output == "abcde");
+  }
+
+  SECTION("Writes strings to temporary buffer") {
+    // accumulate in buffer
+    print(sb, "abc");
+    REQUIRE(output == "");
+
+    // flush when full, and continue to accumulate
+    print(sb, "de");
+    REQUIRE(output == "abcd");
+
+    // flush on destruction
+    sb.~Writer();
+    REQUIRE(output == "abcde");
+  }
 }
 
 TEST_CASE("Writer<custom_string>") {

src/ArduinoJson/Configuration.hpp

@@ -204,3 +204,7 @@
 #ifndef ARDUINOJSON_TAB
 #define ARDUINOJSON_TAB "  "
 #endif
+
+#ifndef ARDUINOJSON_STRING_BUFFER_SIZE
+#define ARDUINOJSON_STRING_BUFFER_SIZE 32
+#endif

src/ArduinoJson/Serialization/Writers/ArduinoStringWriter.hpp

@@ -10,26 +10,42 @@ namespace ARDUINOJSON_NAMESPACE {
 
 template <>
 class Writer< ::String, void> {
+  static const size_t bufferCapacity = ARDUINOJSON_STRING_BUFFER_SIZE;
+
  public:
-  explicit Writer(::String &str) : _str(&str) {}
+  explicit Writer(::String &str) : _destination(&str) {
+    _size = 0;
+  }
+
+  ~Writer() {
+    flush();
+  }
 
   size_t write(uint8_t c) {
-    _str->operator+=(static_cast<char>(c));
+    ARDUINOJSON_ASSERT(_size < bufferCapacity);
+    _buffer[_size++] = static_cast<char>(c);
+    if (_size + 1 >= bufferCapacity) flush();
     return 1;
   }
 
   size_t write(const uint8_t *s, size_t n) {
-    // CAUTION: Arduino String doesn't have append()
-    // and old version doesn't have size() either
-    _str->reserve(_str->length() + n);
     for (size_t i = 0; i < n; i++) {
-      _str->operator+=(static_cast<char>(*s++));
+      write(s[i]);
     }
     return n;
   }
 
  private:
-  ::String *_str;
+  void flush() {
+    ARDUINOJSON_ASSERT(_size < bufferCapacity);
+    _buffer[_size] = 0;
+    *_destination += _buffer;
+    _size = 0;
+  }
+
+  ::String *_destination;
+  char _buffer[bufferCapacity];
+  size_t _size;
 };
 
 }  // namespace ARDUINOJSON_NAMESPACE