extension String {
    /// The base address of the contiguous buffer containing the String elements
    /// if the String is backed by a contiguous buffer.
    ///
    /// There is no guarantee that the String is backed by a contiguous buffer.
    /// Even if it is, there is no guarantee that the buffer is writable.
    public var baseAddress: UnsafeRawPointer? {
        return UnsafeRawPointer(self._core._baseAddress)
    }

    /// The number of elements stored in the buffer.
    ///
    /// The count must not be increased beyond the buffer size for an immutable buffer
    /// or capacity for a mutable buffer.
    public var elementCount: Int {
        get {
            return self._core.count
        }
        set {
            self._core.count = newValue
        }
    }

    /// The read-only width of the elements stored in the buffer (1 or 2).
    public var elementWidth: Int {
        return self._core.elementWidth
    }

    /// The read-only capacity of the String buffer if the String is backed by a mutable contiguous buffer.
    public var capacity: Int? {
        if let capacity = self._core.nativeBuffer?.capacity {
            return capacity + 2 - self.elementWidth
        } else {
            return nil
        }
    }

    /// Ensures that the String is backed by a uniquely owned mutable contiguous buffer with at least
    /// the requested capacity and returns the base address of the this buffer.
    ///
    /// This call may require copying the String elements into a new buffer and modifying the element size.
    /// The reserved capacity may be greater than the requested capacity.
    /// Repeated calls with the same or lower capacity requirements are no ops.
    ///
    /// - Parameter capacity: the requested String capacity (number of elements).
    /// - Parameter minElementWidth: the requested minimum element width.
    /// - Returns: the base address of the mutable contiguous buffer backing the String.
    @discardableResult public mutating func claimOwnership(withCapacity capacity: Int? = nil, withMinElementWidth minElementWidth: Int = 1) -> UnsafeMutableRawPointer {
        if self.elementWidth < minElementWidth {
            self.append("\u{a3}") // force element width = 2
            if let capacity = capacity {
                self._core.reserveCapacity(capacity)
            }
            self.elementCount -= 1
        } else {
            self._core.reserveCapacity(capacity ?? 0)
        }
        return self._core.nativeBuffer!.start
    }

    /// Executes action on String buffer.
    ///
    /// Assumes String is backed by uniquely owned mutable contiguous buffer already.
    ///
    /// - Parameter action: the action to execute.
    @inline(__always) public mutating func withUnsafeMutablePointer(execute action: (UnsafeMutablePointer<UInt8>) -> Void) {
        action(self._core.nativeBuffer!.start.assumingMemoryBound(to: UInt8.self))
    }
}

import Foundation

var now = mach_absolute_time()
var string = "AAA"
string.claimOwnership()

for _ in 1...3 {
    // mutate string element using withUnsafe...
    now = mach_absolute_time()
    for i in 0..<100000000 {
        string.withUnsafeMutablePointer { $0.pointee = 65 + UInt8(i % 26) }
    }
    print("closure: \(Double(mach_absolute_time() - now) / 1e6)ms")

    // mutate string element using pointer obtained before entering loop
    now = mach_absolute_time()
    let ptr = string.claimOwnership().assumingMemoryBound(to: UInt8.self)
    for i in 0..<100000000 {
        ptr.pointee = 65 + UInt8(i % 26)
    }
    print("pointer: \(Double(mach_absolute_time() - now) / 1e6)ms")
}