Avoid ByteBuffer copies when reading nested MapBuffers (#44436)

Summary:
Pull Request resolved: #44436

The backing buffer behind `ReadableMapBuffer` is effectively immutable, so we can make reads of nested MapBuffers work on an inline view of the same buffer. This book-keeping is kept within ReadableMapBuffer (we can not user `ByteBuffer.wrap()` because the fbjni produces ByteBuffer is not array backed).

The main downside I can think of is that the whole buffer is kept in memory until all children buffers leave, but current use-cases don't involve long-term storage of MapBuffer children, so this is probably a better tradeoff.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D57020759

fbshipit-source-id: d2f5a76561fa4a4219fe5022ba62cc96f56ce022

Author: NickGerleman

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/common/mapbuffer/ReadableMapBuffer.kt

@@ -32,6 +32,8 @@ public class ReadableMapBuffer : MapBuffer {
 
   // Byte data of the mapBuffer
   private val buffer: ByteBuffer
+  // Offset to the start of the MapBuffer
+  private val offsetToMapBuffer: Int
   // Amount of items serialized on the ByteBuffer
   override var count: Int = 0
     private set
@@ -40,12 +42,21 @@ public class ReadableMapBuffer : MapBuffer {
   private constructor(hybridData: HybridData) {
     mHybridData = hybridData
     buffer = importByteBuffer()
+    offsetToMapBuffer = 0
     readHeader()
   }
 
   private constructor(buffer: ByteBuffer) {
     mHybridData = null
     this.buffer = buffer
+    offsetToMapBuffer = 0
+    readHeader()
+  }
+
+  private constructor(buffer: ByteBuffer, offset: Int) {
+    mHybridData = null
+    this.buffer = buffer.duplicate().apply { position(offset) }
+    offsetToMapBuffer = offset
     readHeader()
   }
 
@@ -135,12 +146,7 @@ public class ReadableMapBuffer : MapBuffer {
 
   private fun readMapBufferValue(position: Int): ReadableMapBuffer {
     val offset = offsetForDynamicData + buffer.getInt(position)
-    val sizeMapBuffer = buffer.getInt(offset)
-    val newBuffer = ByteArray(sizeMapBuffer)
-    val bufferOffset = offset + Int.SIZE_BYTES
-    buffer.position(bufferOffset)
-    buffer[newBuffer, 0, sizeMapBuffer]
-    return ReadableMapBuffer(ByteBuffer.wrap(newBuffer))
+    return ReadableMapBuffer(buffer, offset + Int.SIZE_BYTES)
   }
 
   private fun readMapBufferListValue(position: Int): List<ReadableMapBuffer> {
@@ -151,18 +157,15 @@ public class ReadableMapBuffer : MapBuffer {
     var curLen = 0
     while (curLen < sizeMapBufferList) {
       val sizeMapBuffer = buffer.getInt(offset + curLen)
-      val newMapBuffer = ByteArray(sizeMapBuffer)
       curLen = curLen + Int.SIZE_BYTES
-      buffer.position(offset + curLen)
-      buffer[newMapBuffer, 0, sizeMapBuffer]
-      readMapBufferList.add(ReadableMapBuffer(ByteBuffer.wrap(newMapBuffer)))
+      readMapBufferList.add(ReadableMapBuffer(buffer, offset + curLen))
       curLen = curLen + sizeMapBuffer
     }
     return readMapBufferList
   }
 
   private fun getKeyOffsetForBucketIndex(bucketIndex: Int): Int {
-    return HEADER_SIZE + BUCKET_SIZE * bucketIndex
+    return offsetToMapBuffer + HEADER_SIZE + BUCKET_SIZE * bucketIndex
   }
 
   override fun contains(key: Int): Boolean {