未对齐原始内存的加载和存储操作

let data = Data([0x0, 0x0, 0x0, 0xff, 0xff, 0xff, 0xff, 0x0])

let result = data.dropFirst(3).withUnsafeBytes { $0.load(as: UInt32.self) }

let result = data.dropFirst(3).withUnsafeBytes { buffer -> UInt32 in
  var storage = UInt32.zero
  withUnsafeMutableBytes(of: &storage) {
    $0.copyBytes(from: buffer.prefix(MemoryLayout<UInt32>.size))
  }
  return storage
}

extension UnsafeRawPointer {
  /// Returns a new instance of the given type, constructed from the raw memory
  /// at the specified offset.
  ///
  /// This function only supports loading trivial types,
  /// and will trap if this precondition is not met.
  /// A trivial type does not contain any reference-counted property
  /// within its in-memory representation.
  /// The memory at this pointer plus `offset` must be laid out
  /// identically to the in-memory representation of `T`.
  ///
  /// - Note: A trivial type can be copied with just a bit-for-bit copy without
  ///   any indirection or reference-counting operations. Generally, native
  ///   Swift types that do not contain strong or weak references or other
  ///   forms of indirection are trivial, as are imported C structs and enums.
  ///
  /// - Parameters:
  ///   - offset: The offset from this pointer, in bytes. `offset` must be
  ///     nonnegative. The default is zero.
  ///   - type: The type of the instance to create.
  /// - Returns: A new instance of type `T`, read from the raw bytes at
  ///   `offset`. The returned instance isn't associated
  ///   with the value in the range of memory referenced by this pointer.
  public func loadUnaligned<T>(fromByteOffset offset: Int = 0, as type: T.Type) -> T
}

extension UnsafeMutableRawPointer {
  /// Returns a new instance of the given type, constructed from the raw memory
  /// at the specified offset.
  ///
  /// This function only supports loading trivial types,
  /// and will trap if this precondition is not met.
  /// A trivial type does not contain any reference-counted property
  /// within its in-memory representation.
  /// The memory at this pointer plus `offset` must be laid out
  /// identically to the in-memory representation of `T`.
  ///
  /// - Note: A trivial type can be copied with just a bit-for-bit copy without
  ///   any indirection or reference-counting operations. Generally, native
  ///   Swift types that do not contain strong or weak references or other
  ///   forms of indirection are trivial, as are imported C structs and enums.
  ///
  /// - Parameters:
  ///   - offset: The offset from this pointer, in bytes. `offset` must be
  ///     nonnegative. The default is zero.
  ///   - type: The type of the instance to create.
  /// - Returns: A new instance of type `T`, read from the raw bytes at
  ///   `offset`. The returned instance isn't associated
  ///   with the value in the range of memory referenced by this pointer.
  public func loadUnaligned<T>(fromByteOffset offset: Int = 0, as type: T.Type) -> T

  /// Stores the given value's bytes into raw memory at the specified offset.
  ///
  /// The type `T` to be stored must be a trivial type. The memory
  /// must also be uninitialized, initialized to `T`, or initialized to
  /// another trivial type that is layout compatible with `T`.
  ///
  /// After calling `storeBytes(of:toByteOffset:as:)`, the memory is
  /// initialized to the raw bytes of `value`. If the memory is bound to a
  /// type `U` that is layout compatible with `T`, then it contains a value of
  /// type `U`. Calling `storeBytes(of:toByteOffset:as:)` does not change the
  /// bound type of the memory.
  ///
  /// - Note: A trivial type can be copied with just a bit-for-bit copy without
  ///   any indirection or reference-counting operations. Generally, native
  ///   Swift types that do not contain strong or weak references or other
  ///   forms of indirection are trivial, as are imported C structs and enums.
  ///
  /// If you need to store a copy of a value of a type that isn't trivial into memory,
  /// you cannot use the `storeBytes(of:toByteOffset:as:)` method. Instead, you must know
  /// the type of value previously in memory and initialize or assign the
  /// memory. For example, to replace a value stored in a raw pointer `p`,
  /// where `U` is the current type and `T` is the new type, use a typed
  /// pointer to access and deinitialize the current value before initializing
  /// the memory with a new value.
  ///
  ///     let typedPointer = p.bindMemory(to: U.self, capacity: 1)
  ///     typedPointer.deinitialize(count: 1)
  ///     p.initializeMemory(as: T.self, repeating: newValue, count: 1)
  ///
  /// - Parameters:
  ///   - value: The value to store as raw bytes.
  ///   - offset: The offset from this pointer, in bytes. `offset` must be
  ///     nonnegative. The default is zero.
  ///   - type: The type of `value`.
  public func storeBytes<T>(of value: T, toByteOffset offset: Int = 0, as type: T.Type)
}

extension UnsafeMutableRawBufferPointer {
  /// Returns a new instance of the given type, constructed from the raw memory
  /// at the specified offset.
  ///
  /// This function only supports loading trivial types.
  /// A trivial type does not contain any reference-counted property
  /// within its in-memory stored representation.
  /// The memory at `offset` bytes into the buffer must be laid out
  /// identically to the in-memory representation of `T`.
  ///
  /// - Note: A trivial type can be copied with just a bit-for-bit copy without
  ///   any indirection or reference-counting operations. Generally, native
  ///   Swift types that do not contain strong or weak references or other
  ///   forms of indirection are trivial, as are imported C structs and enums.
  ///
  /// You can use this method to create new values from the buffer pointer's
  /// underlying bytes. The following example creates two new `Int32`
  /// instances from the memory referenced by the buffer pointer `someBytes`.
  /// The bytes for `a` are copied from the first four bytes of `someBytes`,
  /// and the bytes for `b` are copied from the next four bytes.
  ///
  ///     let a = someBytes.load(as: Int32.self)
  ///     let b = someBytes.load(fromByteOffset: 4, as: Int32.self)
  ///
  /// The memory to read for the new instance must not extend beyond the buffer
  /// pointer's memory region---that is, `offset + MemoryLayout<T>.size` must
  /// be less than or equal to the buffer pointer's `count`.
  ///
  /// - Parameters:
  ///   - offset: The offset, in bytes, into the buffer pointer's memory at
  ///     which to begin reading data for the new instance. The buffer pointer
  ///     plus `offset` must be properly aligned for accessing an instance of
  ///     type `T`. The default is zero.
  ///   - type: The type to use for the newly constructed instance. The memory
  ///     must be initialized to a value of a type that is layout compatible
  ///     with `type`.
  /// - Returns: A new instance of type `T`, copied from the buffer pointer's
  ///   memory.
  public func loadUnaligned<T>(fromByteOffset offset: Int = 0, as type: T.Type) -> T
}

extension UnsafeMutableRawBufferPointer {
  /// Stores a value's bytes into the buffer pointer's raw memory at the
  /// specified byte offset.
  ///
  /// The type `T` to be stored must be a trivial type. The memory must also be
  /// uninitialized, initialized to `T`, or initialized to another trivial
  /// type that is layout compatible with `T`.
  ///
  /// The memory written to must not extend beyond the buffer pointer's memory
  /// region---that is, `offset + MemoryLayout<T>.size` must be less than or
  /// equal to the buffer pointer's `count`.
  ///
  /// After calling `storeBytes(of:toByteOffset:as:)`, the memory is
  /// initialized to the raw bytes of `value`. If the memory is bound to a
  /// type `U` that is layout compatible with `T`, then it contains a value of
  /// type `U`. Calling `storeBytes(of:toByteOffset:as:)` does not change the
  /// bound type of the memory.
  ///
  /// - Note: A trivial type can be copied with just a bit-for-bit copy without
  ///   any indirection or reference-counting operations. Generally, native
  ///   Swift types that do not contain strong or weak references or other
  ///   forms of indirection are trivial, as are imported C structs and enums.
  ///
  /// If you need to store a copy of a value of a type that isn't trivial into memory,
  /// you cannot use the `storeBytes(of:toByteOffset:as:)` method. Instead, you must know
  /// the type of value previously in memory and initialize or assign the memory.
  ///
  /// - Parameters:
  ///   - offset: The offset in bytes into the buffer pointer's memory to begin
  ///     reading data for the new instance. The buffer pointer plus `offset`
  ///     must be properly aligned for accessing an instance of type `T`. The
  ///     default is zero.
  ///   - type: The type to use for the newly constructed instance. The memory
  ///     must be initialized to a value of a type that is layout compatible
  ///     with `type`.
  public func storeBytes<T>(of value: T, toByteOffset offset: Int = 0, as: T.Type)
}