Bytes collection for NSData

NSData.bytesView.swift

import Foundation

// Why bytesView rather than just extending NSData directly?
// Because this way we can keep our extension internal and not conflict
// with someone who imports us and has also extended NSData.
// If you're top-level code, you can just hoist everyting up to NSData directly.
internal extension NSData {
    var bytesView: BytesView { return BytesView(self) }
}

struct BytesView: CollectionType {
    // The view retains the NSData. That's on purpose. NSData doesn't retain the view, so there's no loop.
    let data: NSData
    init(_ data: NSData) { self.data = data }

    subscript (position: Int) -> UInt8 {
        return UnsafePointer<UInt8>(data.bytes)[position]
    }
    subscript (bounds: Range<Int>) -> NSData {
        return data.subdataWithRange(NSRange(bounds))
    }
    var startIndex: Int = 0
    var endIndex: Int { return data.length }
}

internal extension NSMutableData {
    var mutableBytesView: MutableBytesView { return MutableBytesView(self) }
}

final class MutableBytesView: RangeReplaceableCollectionType {
    let data: NSMutableData
    init() { self.data = NSMutableData() }
    init(_ data: NSMutableData) { self.data = data }

    subscript (position: Int) -> UInt8 {
        get {
            return UnsafePointer<UInt8>(data.bytes)[position]
        }
        set {
            replaceRange(position...position, with: [newValue])
        }
    }
    subscript (bounds: Range<Int>) -> NSData {
        get {
            return data.subdataWithRange(NSRange(bounds))
        }
        set {
            replaceRange(bounds, with: newValue.bytesView)
        }
    }
    var startIndex: Int = 0
    var endIndex: Int { return data.length }

    // You could optimize here by specializing NSData and [UInt8] to avoid copying.
    // But this is the most general implementation.
     func replaceRange<C : CollectionType where C.Generator.Element == UInt8>(subRange: Range<Int>, with newElements: C) {
        let newElements = Array(newElements)
        data.replaceBytesInRange(NSRange(subRange), withBytes: newElements, length: newElements.count)
    }
    
    // And of course you can optimize here by implementing append and the like (though it may not optimize much).
}