BitVector.swift

struct Bitfield<Value: FixedWidthInteger> {
    let value : Value
    
    func split(widths : [Int]) -> [Value] {
        var ranges = [Range<Int>]()
        var at = 0
        for width in widths {
            let thisRange = Range(uncheckedBounds: (lower: at, upper: at + width))
            ranges.append(thisRange)
            at += width
        }
        
        return ranges.map { self[$0] }
    }
    
    subscript(bounds : Range<Int>) -> Value {
        let mask = Value( (1 << bounds.count) - 1 )
        return (value >> bounds.startIndex) & mask
    }
    
    subscript(index : Int) -> Value {
        return (value >> index) & 0x1
    }
}