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Store static strings in a dedicated pool
Because a slot id is smaller than a pointer, this change will ultimately allow reducing the slot size.
1 parent 411424b commit bd2dccd

36 files changed

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lines changed

‎CHANGELOG.md‎

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@@ -1,6 +1,43 @@
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ArduinoJson: change log
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=======================
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HEAD
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----
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* Optimize storage of static strings
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> ### BREAKING CHANGES
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>
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> Static string cannot contain NUL characters anymore (they could since 7.3.0).
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> This is an extremely rare case, so you probably won't be affected.
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>
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> For example, the following code produces different output in 7.3 and 7.4:
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>
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> ```cpp
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> JsonDocument doc;
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> doc["a\0b"] = "c\0d";
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> serializeJson(doc, Serial);
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> // With Arduino 7.3 -> {"a\u0000b":"c\u0000d"}
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> // With Arduino 7.4 -> {"a":"c"}
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> ```
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>
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> `JsonString` contructor now only accepts two arguments, not three.
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> If your code uses `JsonString` to store a string as a pointer, you must remove the size argument.
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>
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> For example, if you have something like this:
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>
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> ```cpp
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> doc["key"] = JsonString(str.c_str(), str.size(), true);
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> ```
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>
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> You must replace with either:
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>
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> ```cpp
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> doc["key"] = JsonString(str.c_str(), true); // store as pointer, cannot contain NUL characters
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> doc["key"] = JsonString(str.c_str(), str.size()); // store by copy, NUL characters allowed
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> doc["key"] = str; // same as previous line for supported string classes (`String`, `std::string`, etc.)
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> ```
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v7.4.1 (2025-04-11)
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------
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‎extras/tests/Deprecated/BasicJsonDocument.cpp‎

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@@ -54,7 +54,7 @@ TEST_CASE("BasicJsonDocument") {
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doc["hello"] = "world";
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auto copy = doc;
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REQUIRE(copy.as<std::string>() == "{\"hello\":\"world\"}");
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REQUIRE(allocatorLog == "AA");
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REQUIRE(allocatorLog == "AAAA");
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}
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SECTION("capacity") {

‎extras/tests/Helpers/Allocators.hpp‎

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@@ -275,6 +275,12 @@ inline size_t sizeofPool(
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return MemoryPool<VariantData>::slotsToBytes(n);
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}
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inline size_t sizeofStaticStringPool(
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ArduinoJson::detail::SlotCount n = ARDUINOJSON_POOL_CAPACITY) {
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using namespace ArduinoJson::detail;
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return MemoryPool<const char*>::slotsToBytes(n);
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}
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inline size_t sizeofStringBuffer(size_t iteration = 1) {
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// returns 31, 63, 127, 255, etc.
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auto capacity = ArduinoJson::detail::StringBuilder::initialCapacity;

‎extras/tests/JsonArray/add.cpp‎

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@@ -56,6 +56,7 @@ TEST_CASE("JsonArray::add(T)") {
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REQUIRE(array[0].is<int>() == false);
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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‎extras/tests/JsonDeserializer/destination_types.cpp‎

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Original file line numberDiff line numberDiff line change
@@ -104,6 +104,8 @@ TEST_CASE("deserializeJson(MemberProxy)") {
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REQUIRE(err == DeserializationError::Ok);
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REQUIRE(doc.as<std::string>() == "{\"hello\":\"world\",\"value\":[42]}");
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REQUIRE(spy.log() == AllocatorLog{});
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofStaticStringPool()),
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});
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}
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}

‎extras/tests/JsonDeserializer/filter.cpp‎

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@@ -825,7 +825,9 @@ TEST_CASE("shrink filter") {
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deserializeJson(doc, "{}", DeserializationOption::Filter(filter));
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REQUIRE(spy.log() == AllocatorLog{
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Reallocate(sizeofPool(), sizeofObject(1)),
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});
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REQUIRE(spy.log() ==
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AllocatorLog{
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Reallocate(sizeofPool(), sizeofObject(1)),
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Reallocate(sizeofStaticStringPool(), sizeofStaticStringPool(1)),
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});
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}

‎extras/tests/JsonDocument/ElementProxy.cpp‎

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@@ -31,6 +31,7 @@ TEST_CASE("ElementProxy::add()") {
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REQUIRE(doc.as<std::string>() == "[[\"world\"]]");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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‎extras/tests/JsonDocument/MemberProxy.cpp‎

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@@ -25,6 +25,7 @@ TEST_CASE("MemberProxy::add()") {
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REQUIRE(doc.as<std::string>() == "{\"hello\":[42]}");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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@@ -34,6 +35,7 @@ TEST_CASE("MemberProxy::add()") {
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REQUIRE(doc.as<std::string>() == "{\"hello\":[\"world\"]}");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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@@ -44,6 +46,7 @@ TEST_CASE("MemberProxy::add()") {
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REQUIRE(doc.as<std::string>() == "{\"hello\":[\"world\"]}");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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Allocate(sizeofString("world")),
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});
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}
@@ -55,8 +58,8 @@ TEST_CASE("MemberProxy::add()") {
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REQUIRE(doc.as<std::string>() == "{\"hello\":[\"world\"]}");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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Allocate(sizeofString("world")),
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});
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}
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@@ -71,6 +74,7 @@ TEST_CASE("MemberProxy::add()") {
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REQUIRE(doc.as<std::string>() == "{\"hello\":[\"world\"]}");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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Allocate(sizeofString("world")),
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});
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}
@@ -399,7 +403,7 @@ TEST_CASE("MemberProxy under memory constraints") {
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}
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SECTION("value slot allocation fails") {
402-
timebomb.setCountdown(1);
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timebomb.setCountdown(2);
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// fill the pool entirely, but leave one slot for the key
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doc["foo"][ARDUINOJSON_POOL_CAPACITY - 4] = 1;
@@ -412,6 +416,7 @@ TEST_CASE("MemberProxy under memory constraints") {
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REQUIRE(doc.overflowed() == true);
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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AllocateFail(sizeofPool()),
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});
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}

‎extras/tests/JsonDocument/add.cpp‎

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@@ -32,6 +32,7 @@ TEST_CASE("JsonDocument::add(T)") {
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REQUIRE(doc.as<std::string>() == "[\"hello\"]");
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REQUIRE(spy.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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‎extras/tests/JsonDocument/constructor.cpp‎

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@@ -64,6 +64,7 @@ TEST_CASE("JsonDocument constructor") {
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REQUIRE(doc2.as<std::string>() == "{\"hello\":\"world\"}");
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REQUIRE(spyingAllocator.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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@@ -87,6 +88,7 @@ TEST_CASE("JsonDocument constructor") {
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REQUIRE(doc2.as<std::string>() == "[\"hello\"]");
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REQUIRE(spyingAllocator.log() == AllocatorLog{
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Allocate(sizeofPool()),
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Allocate(sizeofStaticStringPool()),
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});
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}
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