[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/ByteBuffer.swift

@@ -40,11 +40,11 @@ public struct ByteBuffer {
 
     /// This storage doesn't own the memory, therefore, we won't deallocate on deinit.
     private let isOwned: Bool
+    /// Capacity of UInt8 the buffer can hold
+    private let capacity: Int
     /// Retained blob of data that requires the storage to retain a pointer to.
     @usableFromInline
     var retainedBlob: Blob
-    /// Capacity of UInt8 the buffer can hold
-    var capacity: Int
 
     @usableFromInline
     init(count: Int) {
@@ -178,12 +178,10 @@ public struct ByteBuffer {
 
   /// The size of the elements written to the buffer + their paddings
   private var _readerIndex: Int = 0
-//  /// Reader is the position of the current Writer Index (capacity - size)
-//  var reader: Int { _storage.capacity &- _readerIndex }
   /// Current size of the buffer
   public var count: Int { _readerIndex }
   /// Current capacity for the buffer including unused space
-  public var capacity: Int { _storage.capacity }
+  public let capacity: Int
 
   /// Constructor that creates a Flatbuffer object from an InternalByteBuffer
   /// - Parameter
@@ -194,6 +192,7 @@ public struct ByteBuffer {
       blob: .byteBuffer(byteBuffer),
       capacity: byteBuffer.capacity)
     _readerIndex = byteBuffer.writerIndex
+    capacity = byteBuffer.capacity
   }
 
   /// Constructor that creates a Flatbuffer from unsafe memory region by copying
@@ -209,7 +208,8 @@ public struct ByteBuffer {
   {
     _storage = Storage(count: capacity)
     _storage.copy(from: memory, count: capacity)
-    _readerIndex = _storage.capacity
+    _readerIndex = capacity
+    self.capacity = capacity
   }
 
   /// Constructor that creates a Flatbuffer object from a UInt8
@@ -218,7 +218,8 @@ public struct ByteBuffer {
   @inline(__always)
   public init(bytes: [UInt8]) {
     _storage = Storage(blob: .array(bytes), capacity: bytes.count)
-    _readerIndex = _storage.capacity
+    _readerIndex = bytes.count
+    capacity = bytes.count
   }
 
   #if !os(WASI)
@@ -228,7 +229,8 @@ public struct ByteBuffer {
   @inline(__always)
   public init(data: Data) {
     _storage = Storage(blob: .data(data), capacity: data.count)
-    _readerIndex = _storage.capacity
+    _readerIndex = data.count
+    capacity = data.count
   }
 
   /// Constructor that creates a Flatbuffer object from a ContiguousBytes
@@ -241,7 +243,8 @@ public struct ByteBuffer {
     count: Int)
   {
     _storage = Storage(blob: .bytes(contiguousBytes), capacity: count)
-    _readerIndex = _storage.capacity
+    _readerIndex = count
+    capacity = count
   }
   #endif
 
@@ -259,7 +262,8 @@ public struct ByteBuffer {
     _storage = Storage(
       blob: .pointer(memory),
       capacity: capacity)
-    _readerIndex = _storage.capacity
+    _readerIndex = capacity
+    self.capacity = capacity
   }
 
   /// Creates a copy of the existing flatbuffer, by copying it to a different memory.
@@ -275,6 +279,7 @@ public struct ByteBuffer {
   {
     _storage = Storage(blob: blob, capacity: count)
     _readerIndex = removeBytes
+    capacity = count
   }
 
   /// Write stores an object into the buffer directly or indirectly.
@@ -289,11 +294,11 @@ public struct ByteBuffer {
   func write<T>(value: T, index: Int, direct: Bool = false) {
     var index = index
     if !direct {
-      index = _storage.capacity &- index
+      index = capacity &- index
     }
-    assert(index < _storage.capacity, "Write index is out of writing bound")
+    assert(index < capacity, "Write index is out of writing bound")
     assert(index >= 0, "Writer index should be above zero")
-    withUnsafePointer(to: value) { ptr in
+    _ = withUnsafePointer(to: value) { ptr in
       _storage.withUnsafeRawPointer {
         memcpy(
           $0.advanced(by: index),
@@ -387,7 +392,7 @@ public struct ByteBuffer {
     count: Int) -> [T]
   {
     assert(
-      index + count <= _storage.capacity,
+      index + count <= capacity,
       "Reading out of bounds is illegal")
 
     return _storage.readWithUnsafeRawPointer(position: index) {
@@ -405,7 +410,7 @@ public struct ByteBuffer {
     type: String.Encoding) -> String?
   {
     assert(
-      index + count <= _storage.capacity,
+      index + count <= capacity,
       "Reading out of bounds is illegal")
     return _storage.readWithUnsafeRawPointer(position: index) {
       let buf = UnsafeBufferPointer(
@@ -427,7 +432,7 @@ public struct ByteBuffer {
     count: Int) -> String?
   {
     assert(
-      index + count <= _storage.capacity,
+      index + count <= capacity,
       "Reading out of bounds is illegal")
     return _storage.readWithUnsafeRawPointer(position: index) {
       String(cString: $0.bindMemory(to: UInt8.self, capacity: count))
@@ -446,7 +451,7 @@ public struct ByteBuffer {
     body: (UnsafeRawBufferPointer) throws -> T) rethrows -> T
   {
     assert(
-      index + count <= _storage.capacity,
+      index + count <= capacity,
       "Reading out of bounds is illegal")
     return try _storage.readWithUnsafeRawPointer(position: index) {
       try body(UnsafeRawBufferPointer(start: $0, count: count))