Deduplicate static strings

extras/tests/JsonDocument/MemberProxy.cpp

@@ -382,6 +382,24 @@ TEST_CASE("Deduplicate keys") {
                              Allocate(sizeofString("example")),
                          });
   }
+
+  SECTION("string literals") {
+    doc[0]["example"] = 1;
+    doc[1]["example"] = 2;
+    doc.shrinkToFit();
+
+    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);
+
+    REQUIRE(spy.log() ==
+            AllocatorLog{
+                Allocate(sizeofPool()),
+                Allocate(sizeofStaticStringPool()),
+                Reallocate(sizeofPool(), sizeofPool(6)),
+                Reallocate(sizeofStaticStringPool(), sizeofStaticStringPool(1)),
+            });
+  }
 }
 
 TEST_CASE("MemberProxy under memory constraints") {

extras/tests/ResourceManager/saveString.cpp

@@ -19,6 +19,25 @@ static StringNode* saveString(ResourceManager& resources, const char* s,
   return resources.saveString(adaptString(s, n));
 }
 
+TEST_CASE("ResourceManager::saveStaticString()") {
+  SpyingAllocator spy;
+  ResourceManager resources(&spy);
+
+  auto a = resources.saveStaticString("hello");
+  auto b = resources.saveStaticString("world");
+  REQUIRE(a != b);
+
+  auto c = resources.saveStaticString("hello");
+  REQUIRE(a == c);
+
+  resources.shrinkToFit();
+  REQUIRE(spy.log() ==
+          AllocatorLog{
+              Allocate(sizeofStaticStringPool()),
+              Reallocate(sizeofStaticStringPool(), sizeofStaticStringPool(2)),
+          });
+}
+
 TEST_CASE("ResourceManager::saveString()") {
   ResourceManager resources;
 

src/ArduinoJson/Memory/MemoryPool.hpp

@@ -81,6 +81,14 @@ class MemoryPool {
     return slots_ + id;
   }
 
+  SlotId find(const T& value) const {
+    for (SlotId i = 0; i < usage_; i++) {
+      if (slots_[i] == value)
+        return i;
+    }
+    return NULL_SLOT;
+  }
+
   void clear() {
     usage_ = 0;
   }

src/ArduinoJson/Memory/MemoryPoolList.hpp

@@ -114,6 +114,15 @@ class MemoryPoolList {
     return pools_[poolIndex].getSlot(indexInPool);
   }
 
+  SlotId find(const T& value) const {
+    for (PoolCount i = 0; i < count_; i++) {
+      SlotId id = pools_[i].find(value);
+      if (id != NULL_SLOT)
+        return SlotId(i * ARDUINOJSON_POOL_CAPACITY + id);
+    }
+    return NULL_SLOT;
+  }
+
   void clear(Allocator* allocator) {
     for (PoolCount i = 0; i < count_; i++)
       pools_[i].destroy(allocator);

src/ArduinoJson/Memory/ResourceManager.hpp

@@ -114,10 +114,14 @@ class ResourceManager {
   }
 
   SlotId saveStaticString(const char* s) {
+    auto existingSlotId = staticStringsPools_.find(s);
+    if (existingSlotId != NULL_SLOT)
+      return existingSlotId;
+
     auto slot = staticStringsPools_.allocSlot(allocator_);
-    if (!slot)
+    if (slot)
-      return NULL_SLOT;
+      *slot = s;
-    *slot = s;
+
     return slot.id();
   }