Bits and Colors in Swift

May 02, 2016 at 12:00 AM · swift

While perusing Apple’s Swift language guide, I came across an interesting section regarding bit manipulation. Unbeknownst to me, Swift sports the same bitwise and bit shifting operators used by both C and Objective-C. Here’s a recap:

AND

The Bitwise AND & operator combines the bits of two numbers, maintaining the value of 1 for each bit ONLY if they are both 1.

let firstAND: UInt8 = 0b11110000
let secondAND: UInt8 = 0b00001111
let thirdAND: UInt8 = 0b00000001

let firstANDsecond = firstAND & secondAND // 0b00000000
let secondANDthird = secondAND & thirdAND // 0b00000001

The Bitwise OR | operator combines the bits of two numbers, maintaining a value of 1 for each bit that is 1 in EITHER of the pertaining numbers.

let firstOR: UInt8 =  0b01010101
let secondOR: UInt8 = 0b10101010
let thirdOR: UInt8 = 0b00000001

let firstORsecond: UInt8 = firstOR | secondOR // 0b11111111
let secondORthird: UInt8 = secondOR | thirdOR // 0b10101011

XOR

The Bitwise XOR ^ operator (exclusive or) combines the bits of two numbers, maintaining a value of 1 for each bit ONLY if the value is different between each number.

let firstXOR = 0b00000011
let secondXOR = 0b00000001

let firstXORsecond = firstXOR ^ secondXOR // 0b00000010

NOT

The Bitwise NOT ~ operator ‘flips’ each bit of the input number.

let beforeNOT: UInt8 = 0b00000000
let afterNOT = ~beforeNOT // 0b11111111

Left Shift

The Bitwise Left Shift << operator does exactly what the name infers - shifts all bits to the left. Using the operator requires specification to how many spaces the numbers bits should be shifted. A left shift essentially doubles the current integer amount. An important tidbit (bit humor?) is that Swift does not allow an operator to overflow by default, meaning if bits are shifted beyond integer bounds a crash will occur.

let leftShift: UInt8 = 0b00000001

leftShift << 1 // 0b00000010
leftShift << 2 // 0b00000100
leftShift << 3 // 0b00001000
leftShift << 8 // error

Right Shift

The Bitwise Right Shift >> operator is just like its left counterpart, except that the input bits are shifted to the right. An important difference is that a right shifted number will be capped at zero (i.e. if shifted beyond its bounds, the result will default to 0).

let rightShift: UInt8 = 0b00010000

rightShift >> 1 // 0b00001000
rightShift >> 2 // 0b00000100
rightShift >> 3 // 0b00000010
rightShift >> 4 // 0b00000001
rightShift >> 5 // 0b00000000

Hexadecimal Colors

While working with a designer it is not uncommon to receive a color value represented in hexadecimal rather than RGB decimal format. Rather than converting these values using one of the numerous conversion tools available, lets instead utilize what we’ve learned about bit manipulation to create our own handy UIColor extension which converts these values for us.

extension UIColor {
    convenience init(hex: UInt32) {
        let redComponent = CGFloat((hex & 0xFF0000) >> 16)
        let greenComponent = CGFloat((hex & 0x00FF00) >> 8)
        let blueComponent = CGFloat((hex & 0x0000FF))
        self.init(red: redComponent/255.0, green: greenComponent/255.0, blue: blueComponent/255.0, alpha: 1.0)
    }
}

Here’s a quick rundown of how it works. The hexadecimal number system uses 16 distinct symbols to represent the values 0 - 15 (0-9 and A-F respectively). Each hexadecimal digit consists of 8 bits (a byte), which allows the representation of any number from 0 - 255. Using this information, we can extract each red, green and blue value by isolating its corresponding bits.

Starting with red, we isolate the component by ANDing our hex number with the maximum possible number of bits (FF). Next we shift our value for red to the right 16 bits to make sure it resides in the 0-255 bounds. The same process is used for both green and blue, with the exception of the number of bits required in the right shift step (8 and 0 respectively). Each component is casted to a CGFloat to ensure the proper type, then used to initialize a UIColor using each respective RGB value. We can then create UIColor’s like this:

let skyBlue = UIColor(hex: 0x86CAFF)
let grassGreen = UIColor(hex: 0x87DD65)

Another worthy addition to the toolbox!