Fix ReactCommon Break for Windows (#33047)

Summary:
Changes to MapBuffer code in aaff15c...d287598 broke build for Windows. Errors included incompatible type conversions, the use of `__attribute__(__packed__)` which is only supported by GCC and Clang, and the usage of designated initializers which are only supported on C++20.

Changes here restore build on Windows.

## Changelog

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[General] [Fixed] - Fix build break on Windows with ReactCommon

Pull Request resolved: #33047

Test Plan: React Native project built on Windows and passes react-native-windows repository pipeline. These edits are currently merged into the main branch of react-native-windows.

Reviewed By: ShikaSD

Differential Revision: D34101367

Pulled By: philIip

fbshipit-source-id: 1596365c2e92f377c6375805b33de5e1c7b78e66

ReactCommon/react/renderer/mapbuffer/MapBuffer.cpp

@@ -33,11 +33,11 @@ MapBuffer::MapBuffer(std::vector<uint8_t> data) : bytes_(std::move(data)) {
   }
 }
 
-uint32_t MapBuffer::getKeyBucket(Key key) const {
-  uint32_t lo = 0;
-  uint32_t hi = count_ - 1;
+int32_t MapBuffer::getKeyBucket(Key key) const {
+  int32_t lo = 0;
+  int32_t hi = count_ - 1;
   while (lo <= hi) {
-    uint32_t mid = (lo + hi) >> 1;
+    int32_t mid = (lo + hi) >> 1;
 
     Key midVal =
         *reinterpret_cast<Key const *>(bytes_.data() + bucketOffset(mid));
@@ -112,7 +112,7 @@ MapBuffer MapBuffer::getMapBuffer(Key key) const {
   return MapBuffer(std::move(value));
 }
 
-uint32_t MapBuffer::size() const {
+size_t MapBuffer::size() const {
   return bytes_.size();
 }
 

ReactCommon/react/renderer/mapbuffer/MapBuffer.h

@@ -80,14 +80,16 @@ class MapBuffer {
     uint32_t bufferSize; // Amount of bytes used to store the map in memory
   };
 
-  struct __attribute__((__packed__)) Bucket {
+#pragma pack(push, 1)
+  struct Bucket {
     Key key;
     uint16_t type;
     uint64_t data;
 
     Bucket(Key key, uint16_t type, uint64_t data)
         : key(key), type(type), data(data) {}
   };
+#pragma pack(pop)
 
   static_assert(sizeof(Header) == 8, "MapBuffer header size is incorrect.");
   static_assert(sizeof(Bucket) == 12, "MapBuffer bucket size is incorrect.");
@@ -124,7 +126,7 @@ class MapBuffer {
   // TODO T83483191: review this declaration
   MapBuffer getMapBuffer(MapBuffer::Key key) const;
 
-  uint32_t size() const;
+  size_t size() const;
 
   uint8_t const *data() const;
 
@@ -140,7 +142,7 @@ class MapBuffer {
   // returns the relative offset of the first byte of dynamic data
   int32_t getDynamicDataOffset() const;
 
-  uint32_t getKeyBucket(MapBuffer::Key key) const;
+  int32_t getKeyBucket(MapBuffer::Key key) const;
 
   friend ReadableMapBuffer;
 };

ReactCommon/react/renderer/mapbuffer/MapBufferBuilder.cpp

@@ -14,7 +14,7 @@ namespace facebook {
 namespace react {
 
 constexpr uint32_t INT_SIZE = sizeof(uint32_t);
-constexpr double DOUBLE_SIZE = sizeof(double);
+constexpr uint32_t DOUBLE_SIZE = sizeof(double);
 constexpr uint32_t MAX_BUCKET_VALUE_SIZE = sizeof(uint64_t);
 
 MapBuffer MapBufferBuilder::EMPTY() {
@@ -23,6 +23,8 @@ MapBuffer MapBufferBuilder::EMPTY() {
 
 MapBufferBuilder::MapBufferBuilder(uint32_t initialSize) {
   buckets_.reserve(initialSize);
+  header_.count = 0;
+  header_.bufferSize = 0;
 }
 
 void MapBufferBuilder::storeKeyValue(
@@ -76,7 +78,7 @@ void MapBufferBuilder::putInt(MapBuffer::Key key, int32_t value) {
 }
 
 void MapBufferBuilder::putString(MapBuffer::Key key, std::string const &value) {
-  int32_t strSize = value.size();
+  auto strSize = value.size();
   const char *strData = value.data();
 
   // format [length of string (int)] + [Array of Characters in the string]
@@ -94,7 +96,7 @@ void MapBufferBuilder::putString(MapBuffer::Key key, std::string const &value) {
 }
 
 void MapBufferBuilder::putMapBuffer(MapBuffer::Key key, MapBuffer const &map) {
-  int32_t mapBufferSize = map.size();
+  auto mapBufferSize = map.size();
 
   auto offset = dynamicData_.size();
 
@@ -122,9 +124,9 @@ MapBuffer MapBufferBuilder::build() {
   // Create buffer: [header] + [key, values] + [dynamic data]
   auto bucketSize = buckets_.size() * sizeof(MapBuffer::Bucket);
   auto headerSize = sizeof(MapBuffer::Header);
-  uint32_t bufferSize = headerSize + bucketSize + dynamicData_.size();
+  auto bufferSize = headerSize + bucketSize + dynamicData_.size();
 
-  header_.bufferSize = bufferSize;
+  header_.bufferSize = static_cast<uint32_t>(bufferSize);
 
   if (needsSort_) {
     std::sort(buckets_.begin(), buckets_.end(), compareBuckets);

ReactCommon/react/renderer/mapbuffer/MapBufferBuilder.h

@@ -38,7 +38,7 @@ class MapBufferBuilder {
   MapBuffer build();
 
  private:
-  MapBuffer::Header header_ = {.count = 0, .bufferSize = 0};
+  MapBuffer::Header header_;
 
   std::vector<MapBuffer::Bucket> buckets_{};