DigitShift.swift

import Foundation
import XCTest

infix operator ^^ : AdditionPrecedence
extension Int {

    /// Add `n` to each digit, exclude carry. e.g.
    ///  ~~~
    ///  let shifted = 1945.shiftedUp(by: 2) // 3167
    ///  ~~~
    /// - Parameter n: Shift each digit up by `n`
    /// - Returns: Shifted `Int`
    func shiftedUp(by n: Int) -> Int {
        var workingCopy = self
        var solution = 0
        var digitFactor = 1
        
        while workingCopy > 0 {
            let (quotient, remainder) = workingCopy.quotientAndRemainder(dividingBy: 10)
            workingCopy = quotient
            solution += ((remainder + n) % 10) * digitFactor
            digitFactor *= 10
        }
        
        return solution
    }
    
    /// Add `n` to each digit, exclude carry
    ///  ~~~
    ///  var number = 1945
    ///  number.shiftUp(by: 2) // 3167
    ///  ~~~
    /// - Parameter n: Shift each digit up by `n`
    mutating func shiftUp(by n: Int) {
        self = shiftedUp(by: n)
    }
    
    /// `lhs` shifted each digit up by `rhs`, exclude carry. e.g.
    /// - Parameters:
    ///   - lhs: `Integer` to shift
    ///   - rhs: Shift each digit up by `rhs`
    /// - Returns: Shifted `Int`
    static func ^^ (lhs: Self, rhs: Self) -> Self {
        lhs.shiftedUp(by: rhs)
    }
}

// MARK: Tests
class DigitShiftTests: XCTestCase {
    let number = 19381723
    
    func testShifted() {
        let shiftedByTwo = number.shiftedUp(by: 2)
        XCTAssertEqual(shiftedByTwo, 31503945)
        
        let shiftedByFive = number ^^ 5
        XCTAssertEqual(shiftedByFive, 64836278)
    }
    
    func testShift() {
        var copy = number
        
        copy.shiftUp(by: 2)
        XCTAssertEqual(copy, 31503945)
    }
}

DigitShiftTests.defaultTestSuite.run()

// MARK: Usage
1945.shiftedUp(by: 2) // 3167
1945 ^^ 2 // 3167
123456789123456789.shiftedUp(by: 3) // 456789012456789012
123456789123456789 ^^ 3 // 456789012456789012

var number = 1945
number.shiftUp(by: 2) // 3167