BinaryNegativeBase2.playground

//Swift 4+
import Foundation

///given formula for representing bits in in number
/// bits[index]*(-2)^index
func getNumber(from bits:[Int]) -> Int{
    var number = 0
    for (index, value) in bits.enumerated(){
        number = number + value * Int(powf(-2, Float(index)))
    }
    return number
}

///ceil to move forward and abs to get number from bit by reversing the bits representation formula
func getBits(for number:Int) -> [Int]{
    var tempNumber = number
    var bits:[Int] = []
    while tempNumber != 0 {
        let remainder = abs(tempNumber % 2) // remove negative
        bits.append(remainder)
        tempNumber = Int((Float(tempNumber) / -2).rounded(.up)) // ceil(round-upwards)
    }
    return bits
}

///test
print(getNumber(from:[1,0,0,1,1,1]))
print(getNumber(from:[0,0,1]))
print(getNumber(from:[1,0,0,0,1,1]))

print("\n") // :-p
print(getBits(for: 22))
print(getBits(for: 11))
print(getBits(for: -5))
print(getBits(for: -4))