Playing around with a high-level API for working with bit masks

BitMask.swift

struct BitMask<T: FixedWidthInteger>: RandomAccessCollection {
    typealias SubSequence = BitMask<T>
    
    var rawValue: T {
        var rawValue: T = 0
        for bit in self {
            rawValue |= bit
        }
        return rawValue
    }
    
    private(set) var startIndex = 0
    private(set) var endIndex = T().bitWidth
    
    subscript(i: Int) -> T {
        return 1 << i
    }
    
    subscript<R: RangeExpression>(range: R) -> BitMask<T> where R.Bound == Int {
        let range = range.relative(to: self)
        var mask = self
        mask.startIndex = range.lowerBound
        mask.endIndex = range.upperBound
        return mask
    }
}

extension FixedWidthInteger {
    static var bitMask: BitMask<Self> {
        return BitMask()
    }
    
    subscript<R: RangeExpression>(bitMask range: R) -> Self where R.Bound == BitMask<Self>.Index {
        let mask = Self.bitMask[range]
        return self[bitMask: mask]
    }
    
    subscript(bitMask mask: BitMask<Self>) -> Self {
        return self & mask.rawValue >> mask.startIndex
    }
}

UInt8.max[bitMask: ..<6] // 0b0011_1111 = 63
UInt8.max[bitMask: 6...] // 0b1100_0000 = 192