Optimize storage of tiny strings (up to 3 characters)

src/ArduinoJson/Memory/StringBuffer.hpp

@@ -34,12 +34,16 @@ class StringBuffer {
 
   JsonString str() const {
     ARDUINOJSON_ASSERT(node_ != nullptr);
-
     return JsonString(node_->data, node_->length);
   }
 
   void save(VariantData* data) {
-    data->setOwnedString(commitStringNode());
+    ARDUINOJSON_ASSERT(node_ != nullptr);
+    const char* s = node_->data;
+    if (isTinyString(s, size_))
+      data->setTinyString(s, static_cast<uint8_t>(size_));
+    else
+      data->setOwnedString(commitStringNode());
   }
 
   void saveRaw(VariantData* data) {

src/ArduinoJson/Memory/StringBuilder.hpp

@@ -28,8 +28,15 @@ class StringBuilder {
   void save(VariantData* variant) {
     ARDUINOJSON_ASSERT(variant != nullptr);
     ARDUINOJSON_ASSERT(node_ != nullptr);
-    node_->data[size_] = 0;
-    StringNode* node = resources_->getString(adaptString(node_->data, size_));
+
+    char* p = node_->data;
+    if (isTinyString(p, size_)) {
+      variant->setTinyString(p, static_cast<uint8_t>(size_));
+      return;
+    }
+
+    p[size_] = 0;
+    StringNode* node = resources_->getString(adaptString(p, size_));
     if (!node) {
       node = resources_->resizeString(node_, size_);
       ARDUINOJSON_ASSERT(node != nullptr);  // realloc to smaller can't fail

src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp

@@ -403,7 +403,7 @@ class MsgPackDeserializer {
 
       JsonString key = stringBuffer_.str();
       TFilter memberFilter = filter[key.c_str()];
-      VariantData* member;
+      VariantData* member = 0;
 
       if (memberFilter.allow()) {
         ARDUINOJSON_ASSERT(object != 0);
@@ -413,8 +413,6 @@ class MsgPackDeserializer {
           return DeserializationError::NoMemory;
 
         stringBuffer_.save(keyVariant);
-      } else {
-        member = 0;
       }
 
       err = parseVariant(member, memberFilter, nestingLimit.decrement());

src/ArduinoJson/Variant/VariantContent.hpp

@@ -24,6 +24,7 @@ enum class VariantTypeBits : uint8_t {
 
 enum class VariantType : uint8_t {
   Null = 0,             // 0000 0000
+  TinyString = 0x02,    // 0000 0010
   RawString = 0x03,     // 0000 0011
   LinkedString = 0x04,  // 0000 0100
   OwnedString = 0x05,   // 0000 0101
@@ -46,6 +47,8 @@ inline bool operator&(VariantType type, VariantTypeBits bit) {
   return (uint8_t(type) & uint8_t(bit)) != 0;
 }
 
+const size_t tinyStringMaxLength = 3;
+
 union VariantContent {
   VariantContent() {}
 
@@ -61,6 +64,7 @@ union VariantContent {
   CollectionData asCollection;
   const char* asLinkedString;
   struct StringNode* asOwnedString;
+  char asTinyString[tinyStringMaxLength + 1];
 };
 
 #if ARDUINOJSON_USE_EXTENSIONS

src/ArduinoJson/Variant/VariantData.hpp

@@ -17,6 +17,16 @@ ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 template <typename T>
 T parseNumber(const char* s);
 
+template <typename T>
+static bool isTinyString(const T& s, size_t n) {
+  if (n > tinyStringMaxLength)
+    return false;
+  bool containsNul = false;
+  for (uint8_t i = 0; i < uint8_t(n); i++)
+    containsNul |= !s[i];
+  return !containsNul;
+}
+
 class VariantData {
   VariantContent content_;  // must be first to allow cast from array to variant
   VariantType type_;
@@ -63,6 +73,9 @@ class VariantData {
       case VariantType::Object:
         return visit.visit(content_.asObject);
 
+      case VariantType::TinyString:
+        return visit.visit(JsonString(content_.asTinyString));
+
       case VariantType::LinkedString:
         return visit.visit(JsonString(content_.asLinkedString, true));
 
@@ -199,6 +212,9 @@ class VariantData {
       case VariantType::Int64:
         return static_cast<T>(extension->asInt64);
 #endif
+      case VariantType::TinyString:
+        str = content_.asTinyString;
+        break;
       case VariantType::LinkedString:
         str = content_.asLinkedString;
         break;
@@ -241,6 +257,9 @@ class VariantData {
       case VariantType::Int64:
         return convertNumber<T>(extension->asInt64);
 #endif
+      case VariantType::TinyString:
+        str = content_.asTinyString;
+        break;
       case VariantType::LinkedString:
         str = content_.asLinkedString;
         break;
@@ -281,6 +300,8 @@ class VariantData {
 
   JsonString asString() const {
     switch (type_) {
+      case VariantType::TinyString:
+        return JsonString(content_.asTinyString);
       case VariantType::LinkedString:
         return JsonString(content_.asLinkedString, true);
       case VariantType::OwnedString:
@@ -395,7 +416,8 @@ class VariantData {
 
   bool isString() const {
     return type_ == VariantType::LinkedString ||
-           type_ == VariantType::OwnedString;
+           type_ == VariantType::OwnedString ||
+           type_ == VariantType::TinyString;
   }
 
   size_t nesting(const ResourceManager* resources) const {
@@ -504,6 +526,15 @@ class VariantData {
     content_.asLinkedString = s;
   }
 
+  void setTinyString(const char* s, uint8_t n) {
+    ARDUINOJSON_ASSERT(type_ == VariantType::Null);  // must call clear() first
+    ARDUINOJSON_ASSERT(s);
+    type_ = VariantType::TinyString;
+    for (uint8_t i = 0; i < n; i++)
+      content_.asTinyString[i] = s[i];
+    content_.asTinyString[n] = 0;
+  }
+
   void setOwnedString(StringNode* s) {
     ARDUINOJSON_ASSERT(type_ == VariantType::Null);  // must call clear() first
     ARDUINOJSON_ASSERT(s);

src/ArduinoJson/Variant/VariantImpl.hpp

@@ -31,6 +31,11 @@ inline bool VariantData::setString(TAdaptedString value,
     return true;
   }
 
+  if (isTinyString(value, value.size())) {
+    setTinyString(value.data(), uint8_t(value.size()));
+    return true;
+  }
+
   auto dup = resources->saveString(value);
   if (dup) {
     setOwnedString(dup);