[Swift] Moves capacity outside of Storage (#8650)

- Cleans up capacity usage within the lib and moves it outside of the Storage

-  Use overflow operators

swift/Sources/FlexBuffers/_InternalByteBuffer.swift

@@ -32,32 +32,18 @@ struct _InternalByteBuffer {
   final class Storage {
     /// pointer to the start of the buffer object in memory
     var memory: UnsafeMutableRawPointer
-    /// Capacity of UInt8 the buffer can hold
-    var capacity: Int
 
     @usableFromInline
     init(count: Int, alignment: Int) {
       memory = UnsafeMutableRawPointer.allocate(
         byteCount: count,
         alignment: alignment)
-      capacity = count
-    }
-
-    @usableFromInline
-    init(memory: UnsafeMutableRawPointer, capacity: Int, unowned: Bool) {
-      self.memory = memory
-      self.capacity = capacity
     }
 
     deinit {
       memory.deallocate()
     }
 
-    @usableFromInline
-    func copy(from ptr: UnsafeRawPointer, count: Int) {
-      memory.copyMemory(from: ptr, byteCount: count)
-    }
-
     @usableFromInline
     func initialize(for size: Int) {
       memset(memory, 0, size)
@@ -66,14 +52,11 @@ struct _InternalByteBuffer {
     /// Reallocates the buffer incase the object to be written doesnt fit in the current buffer
     /// - Parameter size: Size of the current object
     @usableFromInline
-    func reallocate(_ size: Int, writerSize: Int, alignment: Int) {
-      while capacity <= writerSize &+ size {
-        capacity = capacity << 1
-      }
-
-      /// solution take from Apple-NIO
-      capacity = capacity.convertToPowerofTwo
-
+    func reallocate(
+      capacity: Int,
+      writerSize: Int,
+      alignment: Int
+    ) {
       let newData = UnsafeMutableRawPointer.allocate(
         byteCount: capacity,
         alignment: alignment)
@@ -97,7 +80,7 @@ struct _InternalByteBuffer {
   /// Public Pointer to the buffer object in memory. This should NOT be modified for any reason
   public var memory: UnsafeMutableRawPointer { _storage.memory }
   /// Current capacity for the buffer
-  public var capacity: Int { _storage.capacity }
+  public private(set) var capacity: Int
 
   /// Returns the written bytes into the ``ByteBuffer``
   public var underlyingBytes: [UInt8] {
@@ -113,6 +96,7 @@ struct _InternalByteBuffer {
   ///   - allowReadingUnalignedBuffers: allow reading from unaligned buffer
   init(initialSize size: Int) {
     initialSize = size.convertToPowerofTwo
+    capacity = initialSize
     _storage = Storage(count: initialSize, alignment: alignment)
     _storage.initialize(for: initialSize)
   }
@@ -123,8 +107,9 @@ struct _InternalByteBuffer {
     writerIndex = 0
     alignment = 1
     if keepingCapacity {
-      _storage.initialize(for: _storage.capacity)
+      _storage.initialize(for: capacity)
     } else {
+      capacity = initialSize
       _storage = Storage(count: initialSize, alignment: alignment)
     }
   }
@@ -136,10 +121,22 @@ struct _InternalByteBuffer {
 
   /// Makes sure that buffer has enouch space for each of the objects that will be written into it
   /// - Parameter size: size of object
-  @inline(__always)
+  @usableFromInline
   mutating func ensureSpace(size: Int) {
-    guard size &+ writerIndex > _storage.capacity else { return }
-    _storage.reallocate(size, writerSize: writerIndex, alignment: alignment)
+    guard size &+ writerIndex > capacity else { return }
+    
+    while capacity <= writerIndex &+ size {
+      capacity = capacity << 1
+    }
+
+    /// solution take from Apple-NIO
+    capacity = capacity.convertToPowerofTwo
+
+    _storage.reallocate(
+      capacity: capacity,
+      writerSize: writerIndex,
+      alignment: alignment
+    )
   }
 
   @inline(__always)
@@ -215,7 +212,7 @@ extension _InternalByteBuffer: CustomDebugStringConvertible {
 
   public var debugDescription: String {
     """
-    buffer located at: \(_storage.memory), with capacity of \(_storage.capacity)
+    buffer located at: \(_storage.memory), with capacity of \(capacity)
     { writerIndex: \(writerIndex) }
     """
   }