Added string deduplication (closes #1303)

CHANGELOG.md

@@ -5,6 +5,7 @@ HEAD
 ----
 
 * Added comparisons (`>`, `>=`, `==`, `!=`, `<`, and `<=`) between `JsonVariant`s
+* Added string deduplication (issue #1303)
 
 v6.15.2 (2020-05-15)
 -------

README.md

@@ -31,6 +31,7 @@ ArduinoJson is a C++ JSON library for Arduino and IoT (Internet Of Things).
     * [Consumes roughly 10% less RAM than the "official" Arduino_JSON library](https://arduinojson.org/2019/11/19/arduinojson-vs-arduino_json/?utm_source=github&utm_medium=readme)
     * [Fixed memory allocation, no heap fragmentation](https://arduinojson.org/v6/api/jsondocument/?utm_source=github&utm_medium=readme)
     * [Optionally works without heap memory (zero malloc)](https://arduinojson.org/v6/api/staticjsondocument/?utm_source=github&utm_medium=readme)
+    * Deduplicates strings
 * Versatile
     * [Supports custom allocators (to use external RAM chip, for example)](https://arduinojson.org/v6/how-to/use-external-ram-on-esp32/?utm_source=github&utm_medium=readme)
     * Supports [Arduino's `String`](https://arduinojson.org/v6/api/config/enable_arduino_string/) and [STL's `std::string`](https://arduinojson.org/v6/api/config/enable_std_string/?utm_source=github&utm_medium=readme)

extras/tests/Helpers/WString.h

@@ -9,6 +9,9 @@
 // Reproduces Arduino's String class
 class String {
  public:
+  String() {}
+  explicit String(const char* s) : _str(s) {}
+
   String& operator+=(const char* rhs) {
     _str += rhs;
     return *this;

extras/tests/JsonDeserializer/filter.cpp

@@ -239,7 +239,7 @@ TEST_CASE("Filtering") {
       10,
       DeserializationError::Ok,
       "[{\"example\":1},{\"example\":3}]",
-      JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 16
+      JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 8
     },
     {
       "[',2,3]",

extras/tests/JsonDeserializer/string.cpp

@@ -74,16 +74,16 @@ TEST_CASE("Invalid JSON string") {
   }
 }
 
-TEST_CASE("Not enough room to duplicate the string") {
-  DynamicJsonDocument doc(JSON_OBJECT_SIZE(0));
+TEST_CASE("Not enough room to save the key") {
+  DynamicJsonDocument doc(JSON_OBJECT_SIZE(1) + 8);
 
   SECTION("Quoted string") {
-    REQUIRE(deserializeJson(doc, "{\"example\":1}") ==
+    REQUIRE(deserializeJson(doc, "{\"accuracy\":1}") ==
             DeserializationError::NoMemory);
   }
 
   SECTION("Non-quoted string") {
-    REQUIRE(deserializeJson(doc, "{example:1}") ==
+    REQUIRE(deserializeJson(doc, "{accuracy:1}") ==
             DeserializationError::NoMemory);
   }
 }

extras/tests/JsonDocument/StaticJsonDocument.cpp

@@ -212,7 +212,7 @@ TEST_CASE("StaticJsonDocument") {
 
   SECTION("garbageCollect()") {
     StaticJsonDocument<256> doc;
-    doc[std::string("example")] = std::string("example");
+    doc[std::string("example")] = std::string("jukebox");
     doc.remove("example");
     REQUIRE(doc.memoryUsage() == JSON_OBJECT_SIZE(1) + 16);
 

extras/tests/MemoryPool/CMakeLists.txt

@@ -4,8 +4,8 @@
 
 add_executable(MemoryPoolTests 
 	allocVariant.cpp
-	allocString.cpp
 	clear.cpp
+	saveString.cpp
 	size.cpp
 	StringCopier.cpp
 )

extras/tests/MemoryPool/StringCopier.cpp

@@ -38,8 +38,47 @@ TEST_CASE("StringCopier") {
 
     str.startString(&pool);
     str.append('h');
-    str.commit(&pool);
+    str.save(&pool);
 
     REQUIRE(1 == pool.size());
   }
 }
+
+static const char* addStringToPool(MemoryPool* pool, const char* s) {
+  StringCopier str;
+  str.startString(pool);
+  str.append(s);
+  str.append('\0');
+  return str.save(pool);
+}
+
+TEST_CASE("StringCopier::save() deduplicates strings") {
+  char buffer[4096];
+  MemoryPool pool(buffer, 4096);
+
+  SECTION("Basic") {
+    const char* s1 = addStringToPool(&pool, "hello");
+    const char* s2 = addStringToPool(&pool, "world");
+    const char* s3 = addStringToPool(&pool, "hello");
+
+    REQUIRE(s1 == s3);
+    REQUIRE(s2 != s3);
+    REQUIRE(pool.size() == 12);
+  }
+
+  SECTION("Requires terminator") {
+    const char* s1 = addStringToPool(&pool, "hello world");
+    const char* s2 = addStringToPool(&pool, "hello");
+
+    REQUIRE(s2 != s1);
+    REQUIRE(pool.size() == 12 + 6);
+  }
+
+  SECTION("Don't overrun") {
+    const char* s1 = addStringToPool(&pool, "hello world");
+    const char* s2 = addStringToPool(&pool, "wor");
+
+    REQUIRE(s2 != s1);
+    REQUIRE(pool.size() == 12 + 4);
+  }
+}

extras/tests/MemoryPool/allocString.cpp

@@ -1,67 +0,0 @@
-// ArduinoJson - arduinojson.org
-// Copyright Benoit Blanchon 2014-2020
-// MIT License
-
-#include <ArduinoJson/Memory/MemoryPool.hpp>
-#include <catch.hpp>
-
-using namespace ARDUINOJSON_NAMESPACE;
-
-TEST_CASE("MemoryPool::allocFrozenString()") {
-  const size_t poolCapacity = 64;
-  const size_t longestString = poolCapacity;
-  char buffer[poolCapacity];
-  MemoryPool pool(buffer, poolCapacity);
-
-  SECTION("Returns different addresses") {
-    char *a = pool.allocFrozenString(1);
-    char *b = pool.allocFrozenString(1);
-    REQUIRE(a != b);
-  }
-
-  SECTION("Returns NULL when full") {
-    void *p1 = pool.allocFrozenString(longestString);
-    REQUIRE(p1 != 0);
-
-    void *p2 = pool.allocFrozenString(1);
-    REQUIRE(p2 == 0);
-  }
-
-  SECTION("Returns NULL when pool is too small") {
-    void *p = pool.allocFrozenString(longestString + 1);
-    REQUIRE(0 == p);
-  }
-
-  SECTION("Returns NULL when buffer is NULL") {
-    MemoryPool pool2(0, poolCapacity);
-    REQUIRE(0 == pool2.allocFrozenString(2));
-  }
-
-  SECTION("Returns NULL when capacity is 0") {
-    MemoryPool pool2(buffer, 0);
-    REQUIRE(0 == pool2.allocFrozenString(2));
-  }
-
-  SECTION("Returns same address after clear()") {
-    void *a = pool.allocFrozenString(1);
-    pool.clear();
-    void *b = pool.allocFrozenString(1);
-
-    REQUIRE(a == b);
-  }
-
-  SECTION("Can use full capacity when fresh") {
-    void *a = pool.allocFrozenString(longestString);
-
-    REQUIRE(a != 0);
-  }
-
-  SECTION("Can use full capacity after clear") {
-    pool.allocFrozenString(longestString);
-    pool.clear();
-
-    void *a = pool.allocFrozenString(longestString);
-
-    REQUIRE(a != 0);
-  }
-}

extras/tests/MemoryPool/clear.cpp

@@ -3,6 +3,7 @@
 // MIT License
 
 #include <ArduinoJson/Memory/MemoryPool.hpp>
+#include <ArduinoJson/Strings/StringAdapters.hpp>
 #include <catch.hpp>
 
 using namespace ARDUINOJSON_NAMESPACE;
@@ -21,8 +22,8 @@ TEST_CASE("MemoryPool::clear()") {
   }
 
   SECTION("Discards allocated strings") {
-    pool.allocFrozenString(10);
-    REQUIRE(pool.size() > 0);
+    pool.saveString(adaptString(const_cast<char *>("123456789")));
+    REQUIRE(pool.size() == 10);
 
     pool.clear();
 

extras/tests/MemoryPool/saveString.cpp

@@ -0,0 +1,81 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#include <ArduinoJson/Memory/MemoryPool.hpp>
+#include <ArduinoJson/Strings/StringAdapters.hpp>
+#include <catch.hpp>
+
+using namespace ARDUINOJSON_NAMESPACE;
+
+static const char *saveString(MemoryPool &pool, const char *s) {
+  return pool.saveString(adaptString(const_cast<char *>(s)));
+}
+
+TEST_CASE("MemoryPool::saveString()") {
+  char buffer[32];
+  MemoryPool pool(buffer, 32);
+
+  SECTION("Duplicates different strings") {
+    const char *a = saveString(pool, "hello");
+    const char *b = saveString(pool, "world");
+    REQUIRE(a != b);
+    REQUIRE(pool.size() == 6 + 6);
+  }
+
+  SECTION("Deduplicates identical strings") {
+    const char *a = saveString(pool, "hello");
+    const char *b = saveString(pool, "hello");
+    REQUIRE(a == b);
+    REQUIRE(pool.size() == 6);
+  }
+
+  SECTION("Returns NULL when full") {
+    REQUIRE(pool.capacity() == 32);
+
+    const void *p1 = saveString(pool, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+    REQUIRE(p1 != 0);
+    REQUIRE(pool.size() == 32);
+
+    const void *p2 = saveString(pool, "b");
+    REQUIRE(p2 == 0);
+  }
+
+  SECTION("Returns NULL when pool is too small") {
+    const void *p = saveString(pool, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+    REQUIRE(0 == p);
+  }
+
+  SECTION("Returns NULL when buffer is NULL") {
+    MemoryPool pool2(0, 32);
+    REQUIRE(0 == saveString(pool2, "a"));
+  }
+
+  SECTION("Returns NULL when capacity is 0") {
+    MemoryPool pool2(buffer, 0);
+    REQUIRE(0 == saveString(pool2, "a"));
+  }
+
+  SECTION("Returns same address after clear()") {
+    const void *a = saveString(pool, "hello");
+    pool.clear();
+    const void *b = saveString(pool, "world");
+
+    REQUIRE(a == b);
+  }
+
+  SECTION("Can use full capacity when fresh") {
+    const void *a = saveString(pool, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+
+    REQUIRE(a != 0);
+  }
+
+  SECTION("Can use full capacity after clear") {
+    saveString(pool, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+    pool.clear();
+
+    const void *a = saveString(pool, "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
+
+    REQUIRE(a != 0);
+  }
+}

extras/tests/MemoryPool/size.cpp

@@ -22,24 +22,6 @@ TEST_CASE("MemoryPool::size()") {
     REQUIRE(0 == pool.size());
   }
 
-  SECTION("Decreases after freezeString()") {
-    StringSlot a = pool.allocExpandableString();
-    pool.freezeString(a, 1);
-    REQUIRE(pool.size() == 1);
-
-    StringSlot b = pool.allocExpandableString();
-    pool.freezeString(b, 1);
-    REQUIRE(pool.size() == 2);
-  }
-
-  SECTION("Increases after allocFrozenString()") {
-    pool.allocFrozenString(1);
-    REQUIRE(pool.size() == 1);
-
-    pool.allocFrozenString(2);
-    REQUIRE(pool.size() == 3);
-  }
-
   SECTION("Doesn't grow when memory pool is full") {
     const size_t variantCount = sizeof(buffer) / sizeof(VariantSlot);
 

extras/tests/Misc/StringAdapters.cpp

@@ -6,6 +6,7 @@
 #include "progmem_emulation.hpp"
 #include "weird_strcmp.hpp"
 
+#include <ArduinoJson/Strings/ArduinoStringAdapter.hpp>
 #include <ArduinoJson/Strings/ConstRamStringAdapter.hpp>
 #include <ArduinoJson/Strings/FlashStringAdapter.hpp>
 #include <ArduinoJson/Strings/SizedRamStringAdapter.hpp>
@@ -114,6 +115,21 @@ TEST_CASE("std::string") {
   CHECK(adapter.size() == 5);
 }
 
+TEST_CASE("Arduino String") {
+  ::String str("bravo");
+  ArduinoStringAdapter adapter = adaptString(str);
+
+  CHECK(adapter.compare(NULL) > 0);
+  CHECK(adapter.compare("alpha") > 0);
+  CHECK(adapter.compare("bravo") == 0);
+  CHECK(adapter.compare("charlie") < 0);
+
+  CHECK(adapter.equals("bravo"));
+  CHECK_FALSE(adapter.equals("charlie"));
+
+  CHECK(adapter.size() == 5);
+}
+
 TEST_CASE("custom_string") {
   custom_string str("bravo");
   StlStringAdapter<custom_string> adapter = adaptString(str);

extras/tests/MixedConfiguration/CMakeLists.txt

@@ -18,6 +18,8 @@ add_executable(MixedConfigurationTests
 	enable_nan_0.cpp
 	enable_nan_1.cpp
 	enable_progmem_1.cpp
+	enable_string_deduplication_0.cpp
+	enable_string_deduplication_1.cpp
 	use_double_0.cpp
 	use_double_1.cpp
 	use_long_long_0.cpp

extras/tests/MixedConfiguration/enable_string_deduplication_0.cpp

@@ -0,0 +1,125 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#include "progmem_emulation.hpp"
+
+#define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
+#define ARDUINOJSON_ENABLE_PROGMEM 1
+#define ARDUINOJSON_ENABLE_STRING_DEDUPLICATION 0
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+
+TEST_CASE("ARDUINOJSON_ENABLE_STRING_DEDUPLICATION = 0") {
+  StaticJsonDocument<1024> doc;
+
+  SECTION("deserializeJson()") {
+    SECTION("Deduplicate values") {
+      deserializeJson(doc, "[\"example\",\"example\"]");
+
+      CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 16);
+      CHECK(doc[0].as<char*>() != doc[1].as<char*>());
+    }
+
+    SECTION("Deduplicate keys") {
+      deserializeJson(doc, "[{\"example\":1},{\"example\":2}]");
+
+      CHECK(doc.memoryUsage() ==
+            2 * JSON_OBJECT_SIZE(1) + JSON_ARRAY_SIZE(2) + 16);
+
+      const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+      const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+
+      CHECK(key1 != key2);
+    }
+  }
+
+  SECTION("JsonDocument") {
+    SECTION("values") {
+      SECTION("std::string") {
+        doc.add(std::string("example"));
+        doc.add(std::string("example"));
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 16);
+        CHECK(doc[0].as<char*>() != doc[1].as<char*>());
+      }
+
+      SECTION("char*") {
+        char value[] = "example";
+        doc.add(value);
+        doc.add(value);
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 16);
+        CHECK(doc[0].as<char*>() != doc[1].as<char*>());
+      }
+
+      SECTION("Arduino String") {
+        doc.add(String("example"));
+        doc.add(String("example"));
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 16);
+        CHECK(doc[0].as<char*>() != doc[1].as<char*>());
+      }
+
+      SECTION("Flash string") {
+        doc.add(F("example"));
+        doc.add(F("example"));
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 16);
+        CHECK(doc[0].as<char*>() != doc[1].as<char*>());
+      }
+    }
+
+    SECTION("keys") {
+      SECTION("std::string") {
+        doc[0][std::string("example")] = 1;
+        doc[1][std::string("example")] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 16);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 != key2);
+      }
+
+      SECTION("char*") {
+        char key[] = "example";
+        doc[0][key] = 1;
+        doc[1][key] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 16);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 != key2);
+      }
+
+      SECTION("Arduino String") {
+        doc[0][String("example")] = 1;
+        doc[1][String("example")] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 16);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 != key2);
+      }
+
+      SECTION("Flash string") {
+        doc[0][F("example")] = 1;
+        doc[1][F("example")] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 16);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 != key2);
+      }
+    }
+  }
+}

extras/tests/MixedConfiguration/enable_string_deduplication_1.cpp

@@ -0,0 +1,124 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#include "progmem_emulation.hpp"
+
+#define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
+#define ARDUINOJSON_ENABLE_PROGMEM 1
+#define ARDUINOJSON_ENABLE_STRING_DEDUPLICATION 1
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+
+TEST_CASE("ARDUINOJSON_ENABLE_STRING_DEDUPLICATION = 1") {
+  StaticJsonDocument<1024> doc;
+
+  SECTION("deserializeJson()") {
+    SECTION("Deduplicate values") {
+      deserializeJson(doc, "[\"example\",\"example\"]");
+
+      CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 8);
+      CHECK(doc[0].as<char*>() == doc[1].as<char*>());
+    }
+
+    SECTION("Deduplicate keys") {
+      deserializeJson(doc, "[{\"example\":1},{\"example\":2}]");
+
+      CHECK(doc.memoryUsage() ==
+            2 * JSON_OBJECT_SIZE(1) + JSON_ARRAY_SIZE(2) + 8);
+
+      const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+      const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+      CHECK(key1 == key2);
+    }
+  }
+
+  SECTION("JsonDocument") {
+    SECTION("values") {
+      SECTION("std::string") {
+        doc.add(std::string("example"));
+        doc.add(std::string("example"));
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 8);
+        CHECK(doc[0].as<char*>() == doc[1].as<char*>());
+      }
+
+      SECTION("char*") {
+        char value[] = "example";
+        doc.add(value);
+        doc.add(value);
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 8);
+        CHECK(doc[0].as<char*>() == doc[1].as<char*>());
+      }
+
+      SECTION("Arduino String") {
+        doc.add(String("example"));
+        doc.add(String("example"));
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 8);
+        CHECK(doc[0].as<char*>() == doc[1].as<char*>());
+      }
+
+      SECTION("Flash string") {
+        doc.add(F("example"));
+        doc.add(F("example"));
+
+        CHECK(doc.memoryUsage() == JSON_ARRAY_SIZE(2) + 8);
+        CHECK(doc[0].as<char*>() == doc[1].as<char*>());
+      }
+    }
+
+    SECTION("keys") {
+      SECTION("std::string") {
+        doc[0][std::string("example")] = 1;
+        doc[1][std::string("example")] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 8);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 == key2);
+      }
+
+      SECTION("char*") {
+        char key[] = "example";
+        doc[0][key] = 1;
+        doc[1][key] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 8);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 == key2);
+      }
+
+      SECTION("Arduino String") {
+        doc[0][String("example")] = 1;
+        doc[1][String("example")] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 8);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 == key2);
+      }
+
+      SECTION("Flash string") {
+        doc[0][F("example")] = 1;
+        doc[1][F("example")] = 2;
+
+        CHECK(doc.memoryUsage() ==
+              JSON_ARRAY_SIZE(2) + 2 * JSON_OBJECT_SIZE(1) + 8);
+
+        const char* key1 = doc[0].as<JsonObject>().begin()->key().c_str();
+        const char* key2 = doc[1].as<JsonObject>().begin()->key().c_str();
+        CHECK(key1 == key2);
+      }
+    }
+  }
+}

src/ArduinoJson/Configuration.hpp

@@ -215,6 +215,10 @@
 #define ARDUINOJSON_TAB "  "
 #endif
 
+#ifndef ARDUINOJSON_ENABLE_STRING_DEDUPLICATION
+#define ARDUINOJSON_ENABLE_STRING_DEDUPLICATION 1
+#endif
+
 #ifndef ARDUINOJSON_STRING_BUFFER_SIZE
 #define ARDUINOJSON_STRING_BUFFER_SIZE 32
 #endif

src/ArduinoJson/Json/JsonDeserializer.hpp

@@ -212,8 +212,6 @@ class JsonDeserializer {
 
     // Read each key value pair
     for (;;) {
-      _stringStorage.startString(_pool);
-
       // Parse key
       err = parseKey();
       if (err)
@@ -233,7 +231,9 @@ class JsonDeserializer {
       if (memberFilter.allow()) {
         VariantData *variant = object.getMember(adaptString(key));
         if (!variant) {
-          _stringStorage.commit(_pool);
+          // Save key in memory pool.
+          // This MUST be done before adding the slot.
+          key = _stringStorage.save(_pool);
 
           // Allocate slot in object
           VariantSlot *slot = object.addSlot(_pool);
@@ -325,6 +325,7 @@ class JsonDeserializer {
   }
 
   DeserializationError parseKey() {
+    _stringStorage.startString(_pool);
     if (isQuote(current())) {
       return parseQuotedString();
     } else {
@@ -337,8 +338,8 @@ class JsonDeserializer {
     DeserializationError err = parseQuotedString();
     if (err)
       return err;
-    _stringStorage.commit(_pool);
-    variant.setOwnedString(make_not_null(_stringStorage.c_str()));
+    const char *value = _stringStorage.save(_pool);
+    variant.setOwnedString(make_not_null(value));
     return DeserializationError::Ok;
   }
 

src/ArduinoJson/Memory/MemoryPool.hpp

@@ -51,40 +51,43 @@ class MemoryPool {
     return allocRight<VariantSlot>();
   }
 
-  char* allocFrozenString(size_t n) {
-    if (!canAlloc(n))
-      return 0;
-    char* s = _left;
-    _left += n;
-    checkInvariants();
-    return s;
-  }
-
   template <typename TAdaptedString>
-  char* saveString(const TAdaptedString& str) {
+  const char* saveString(const TAdaptedString& str) {
     if (str.isNull())
       return 0;
+
+#if ARDUINOJSON_ENABLE_STRING_DEDUPLICATION
+    const char* existingCopy = findString(str.begin());
+    if (existingCopy)
+      return existingCopy;
+#endif
+
     size_t n = str.size();
-    char* dup = allocFrozenString(n + 1);
-    if (dup) {
-      str.copyTo(dup, n);
-      dup[n] = 0;  // force null-terminator
+
+    char* newCopy = allocString(n + 1);
+    if (newCopy) {
+      str.copyTo(newCopy, n);
+      newCopy[n] = 0;  // force null-terminator
     }
-    return dup;
+    return newCopy;
   }
 
-  StringSlot allocExpandableString() {
-    StringSlot s;
-    s.value = _left;
-    s.size = size_t(_right - _left);
-    checkInvariants();
-    return s;
+  void getFreeZone(char** zoneStart, size_t* zoneSize) const {
+    *zoneStart = _left;
+    *zoneSize = size_t(_right - _left);
   }
 
-  void freezeString(StringSlot& s, size_t newSize) {
-    _left = (s.value + newSize);
-    s.size = newSize;
+  const char* saveStringFromFreeZone(size_t len) {
+#if ARDUINOJSON_ENABLE_STRING_DEDUPLICATION
+    const char* dup = findString(_left);
+    if (dup)
+      return dup;
+#endif
+
+    const char* str = _left;
+    _left += len;
     checkInvariants();
+    return str;
   }
 
   void clear() {
@@ -100,18 +103,6 @@ class MemoryPool {
     return _begin <= p && p < _end;
   }
 
-  template <typename T>
-  T* allocRight() {
-    return reinterpret_cast<T*>(allocRight(sizeof(T)));
-  }
-
-  void* allocRight(size_t bytes) {
-    if (!canAlloc(bytes))
-      return 0;
-    _right -= bytes;
-    return _right;
-  }
-
   // Workaround for missing placement new
   void* operator new(size_t, void* p) {
     return p;
@@ -163,6 +154,46 @@ class MemoryPool {
     ARDUINOJSON_ASSERT(isAligned(_right));
   }
 
+#if ARDUINOJSON_ENABLE_STRING_DEDUPLICATION
+  template <typename TIterator>
+  const char* findString(TIterator str) {
+    for (char* next = _begin; next < _left; ++next) {
+      char* begin = next;
+
+      // try to match
+      for (TIterator it = str; *it == *next; ++it) {
+        if (*next++ == 0)
+          return begin;
+      }
+
+      // jump to next terminator
+      while (*next) ++next;
+    }
+    return 0;
+  }
+#endif
+
+  char* allocString(size_t n) {
+    if (!canAlloc(n))
+      return 0;
+    char* s = _left;
+    _left += n;
+    checkInvariants();
+    return s;
+  }
+
+  template <typename T>
+  T* allocRight() {
+    return reinterpret_cast<T*>(allocRight(sizeof(T)));
+  }
+
+  void* allocRight(size_t bytes) {
+    if (!canAlloc(bytes))
+      return 0;
+    _right -= bytes;
+    return _right;
+  }
+
   char *_begin, *_left, *_right, *_end;
 };
 

src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp

@@ -248,8 +248,8 @@ class MsgPackDeserializer {
     _stringStorage.append('\0');
     if (!_stringStorage.isValid())
       return DeserializationError::NoMemory;
-    _stringStorage.commit(_pool);
-    result = _stringStorage.c_str();
+
+    result = _stringStorage.save(_pool);
     return DeserializationError::Ok;
   }
 

src/ArduinoJson/Namespace.hpp

@@ -16,16 +16,17 @@
 #define ARDUINOJSON_CONCAT8(A, B, C, D, E, F, G, H)    \
   ARDUINOJSON_CONCAT2(ARDUINOJSON_CONCAT4(A, B, C, D), \
                       ARDUINOJSON_CONCAT4(E, F, G, H))
-#define ARDUINOJSON_CONCAT12(A, B, C, D, E, F, G, H, I, J, K, L) \
-  ARDUINOJSON_CONCAT8(A, B, C, D, E, F, G,                       \
-                      ARDUINOJSON_CONCAT4(H, I, J, ARDUINOJSON_CONCAT2(K, L)))
+#define ARDUINOJSON_CONCAT13(A, B, C, D, E, F, G, H, I, J, K, L, M)   \
+  ARDUINOJSON_CONCAT8(A, B, C, D, E, ARDUINOJSON_CONCAT4(F, G, H, I), \
+                      ARDUINOJSON_CONCAT2(J, K), ARDUINOJSON_CONCAT2(L, M))
 
 #define ARDUINOJSON_NAMESPACE                                            \
-  ARDUINOJSON_CONCAT12(                                                  \
+  ARDUINOJSON_CONCAT13(                                                  \
       ArduinoJson, ARDUINOJSON_VERSION_MAJOR, ARDUINOJSON_VERSION_MINOR, \
       ARDUINOJSON_VERSION_REVISION, _, ARDUINOJSON_USE_LONG_LONG,        \
       ARDUINOJSON_USE_DOUBLE, ARDUINOJSON_DECODE_UNICODE,                \
       ARDUINOJSON_ENABLE_NAN, ARDUINOJSON_ENABLE_INFINITY,               \
-      ARDUINOJSON_ENABLE_PROGMEM, ARDUINOJSON_ENABLE_COMMENTS)
+      ARDUINOJSON_ENABLE_PROGMEM, ARDUINOJSON_ENABLE_COMMENTS,           \
+      ARDUINOJSON_ENABLE_STRING_DEDUPLICATION)
 
 #endif

src/ArduinoJson/StringStorage/StringCopier.hpp

@@ -11,13 +11,13 @@ namespace ARDUINOJSON_NAMESPACE {
 class StringCopier {
  public:
   void startString(MemoryPool* pool) {
-    _slot = pool->allocExpandableString();
+    pool->getFreeZone(&_ptr, &_capacity);
     _size = 0;
   }
 
-  void commit(MemoryPool* pool) {
-    ARDUINOJSON_ASSERT(_slot.value);
-    pool->freezeString(_slot, _size);
+  const char* save(MemoryPool* pool) {
+    ARDUINOJSON_ASSERT(_ptr);
+    return pool->saveStringFromFreeZone(_size);
   }
 
   void append(const char* s) {
@@ -29,27 +29,28 @@ class StringCopier {
   }
 
   void append(char c) {
-    if (!_slot.value)
+    if (!_ptr)
       return;
 
-    if (_size >= _slot.size) {
-      _slot.value = 0;
+    if (_size >= _capacity) {
+      _ptr = 0;
       return;
     }
 
-    _slot.value[_size++] = c;
+    _ptr[_size++] = c;
   }
 
   bool isValid() {
-    return _slot.value != 0;
+    return _ptr != 0;
   }
 
   const char* c_str() {
-    return _slot.value;
+    return _ptr;
   }
 
  private:
+  char* _ptr;
   size_t _size;
-  StringSlot _slot;
+  size_t _capacity;
 };
 }  // namespace ARDUINOJSON_NAMESPACE

src/ArduinoJson/StringStorage/StringMover.hpp

@@ -16,7 +16,9 @@ class StringMover {
     _startPtr = _writePtr;
   }
 
-  void commit(MemoryPool*) const {}
+  const char* save(MemoryPool*) const {
+    return _startPtr;
+  }
 
   void append(char c) {
     *_writePtr++ = c;

src/ArduinoJson/Strings/ArduinoStringAdapter.hpp

@@ -39,6 +39,10 @@ class ArduinoStringAdapter {
     return _str->length();
   }
 
+  const char* begin() const {
+    return _str->c_str();
+  }
+
   typedef storage_policies::store_by_copy storage_policy;
 
  private:

src/ArduinoJson/Strings/ConstRamStringAdapter.hpp

@@ -39,6 +39,10 @@ class ConstRamStringAdapter {
     return _str;
   }
 
+  const char* begin() const {
+    return _str;
+  }
+
   typedef storage_policies::store_by_address storage_policy;
 
  protected:

src/ArduinoJson/Strings/FlashStringAdapter.hpp

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/Polyfills/pgmspace.hpp>
+#include <ArduinoJson/Strings/FlashStringIterator.hpp>
 #include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
@@ -42,6 +43,10 @@ class FlashStringAdapter {
     return strlen_P(reinterpret_cast<const char*>(_str));
   }
 
+  FlashStringIterator begin() const {
+    return FlashStringIterator(_str);
+  }
+
   typedef storage_policies::store_by_copy storage_policy;
 
  private:

src/ArduinoJson/Strings/FlashStringIterator.hpp

@@ -0,0 +1,44 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#pragma once
+
+namespace ARDUINOJSON_NAMESPACE {
+
+class FlashStringIterator {
+ public:
+  explicit FlashStringIterator(const __FlashStringHelper* ptr)
+      : _ptr(reinterpret_cast<const char*>(ptr)) {}
+
+  explicit FlashStringIterator(const char* ptr) : _ptr(ptr) {}
+
+  FlashStringIterator operator+(ptrdiff_t d) const {
+    return FlashStringIterator(_ptr + d);
+  }
+
+  ptrdiff_t operator-(FlashStringIterator other) const {
+    return _ptr - other._ptr;
+  }
+
+  FlashStringIterator operator++(int) {
+    return FlashStringIterator(_ptr++);
+  }
+
+  FlashStringIterator operator++() {
+    return FlashStringIterator(++_ptr);
+  }
+
+  bool operator!=(FlashStringIterator other) const {
+    return _ptr != other._ptr;
+  }
+
+  char operator*() const {
+    return char(pgm_read_byte(_ptr));
+  }
+
+ private:
+  const char* _ptr;
+};
+
+}  // namespace ARDUINOJSON_NAMESPACE

src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/Namespace.hpp>
+#include <ArduinoJson/Strings/FlashStringIterator.hpp>
 #include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
@@ -41,6 +42,10 @@ class SizedFlashStringAdapter {
     return _size;
   }
 
+  FlashStringIterator begin() const {
+    return FlashStringIterator(_str);
+  }
+
   typedef storage_policies::store_by_copy storage_policy;
 
  private:

src/ArduinoJson/Strings/SizedRamStringAdapter.hpp

@@ -36,6 +36,10 @@ class SizedRamStringAdapter {
     return _size;
   }
 
+  const char* begin() const {
+    return _str;
+  }
+
   typedef storage_policies::store_by_copy storage_policy;
 
  private:

src/ArduinoJson/Strings/StlStringAdapter.hpp

@@ -41,6 +41,10 @@ class StlStringAdapter {
     return _str->size();
   }
 
+  const char* begin() const {
+    return _str->c_str();
+  }
+
   typedef storage_policies::store_by_copy storage_policy;
 
  private:

src/ArduinoJson/Variant/VariantData.hpp

@@ -192,7 +192,7 @@ class VariantData {
 
   template <typename T>
   bool setOwnedRaw(SerializedValue<T> value, MemoryPool *pool) {
-    char *dup = pool->saveString(adaptString(value.data(), value.size()));
+    const char *dup = pool->saveString(adaptString(value.data(), value.size()));
     if (dup) {
       setType(VALUE_IS_OWNED_RAW);
       _content.asRaw.data = dup;