Swift Index Revolver (requiring Swift 5.6)

IndexRevolver.swift

// (c) 2021 and onwards Shiki Suen (MIT-NTL License).	
// ====================	
// This code is released under the MIT license (SPDX-License-Identifier: MIT)	

import Foundation

extension Collection {
  public func revolvedIndex(_ id: Int, clockwise: Bool = true, steps: Int = 1) -> Int {
    if id < 0 || steps < 1 { return id }
    var result = id
    func revolvedIndexByOneStep(_ id: Int, clockwise: Bool = true) -> Int {
      let newID = clockwise ? id + 1 : id - 1
      if (0..<count).contains(newID) { return newID }
      return clockwise ? 0 : count - 1
    }
    for _ in 0..<steps {
      result = revolvedIndexByOneStep(result, clockwise: clockwise)
    }
    return result
  }
}

extension Int {
  public mutating func revolveAsIndex(with target: any Collection, clockwise: Bool = true, steps: Int = 1) {
    if self < 0 || steps < 1 { return }
    self = target.revolvedIndex(self, clockwise: clockwise, steps: steps)
  }
}

// MARK: - Tests

let theArray: [Int] = [1,2,3,4]
var currentIndex = 0
for _ in 0..<8 {
  currentIndex.revolveAsIndex(with: theArray)
  print(currentIndex)
}

print("")

for _ in 0..<8 {
  currentIndex.revolveAsIndex(with: theArray, clockwise: false)
  print(currentIndex)
}

Swift 5.5:

// (c) 2021 and onwards Shiki Suen (MIT-NTL License).	
// ====================	
// This code is released under the MIT license (SPDX-License-Identifier: MIT)	

import Foundation

extension Collection {
  public func revolvedIndex(_ id: Int, clockwise: Bool = true, steps: Int = 1) -> Int {
    if id < 0 || steps < 1 { return id }
    var result = id
    func revolvedIndexByOneStep(_ id: Int, clockwise: Bool = true) -> Int {
      let newID = clockwise ? id + 1 : id - 1
      if (0..<count).contains(newID) { return newID }
      return clockwise ? 0 : count - 1
    }
    for _ in 0..<steps {
      result = revolvedIndexByOneStep(result, clockwise: clockwise)
    }
    return result
  }
}

extension Int {
  public mutating func revolveAsIndex<T: Collection>(with target: T, clockwise: Bool = true, steps: Int = 1) {
    if self < 0 || steps < 1 { return }
    self = target.revolvedIndex(self, clockwise: clockwise, steps: steps)
  }
}