An array subscript extension that returns the element from the positive or negative circular index.

CircularArrayExtension.swift

import Foundation

extension Array {
  /// An array subscript extension that returns the element from the positive or negative circular index.
  public subscript(circular index: Int) -> Element? {
    guard count > 0 else { return nil }
    let mod = index % count
    let offset = index >= 0 ? 0 : count
    let idx = mod == 0 ? 0 : mod + offset
    return self[idx]
  }
}

// Examples
let a = [0, 1, 2, 3, 4, 5]
a[circular: 0] // 0
a[circular: 1] // 1
a[circular: 2] // 2
a[circular: 3] // 3
a[circular: 4] // 4
a[circular: 5] // 5
a[circular: 6] // 0
a[circular: 7] // 1
a[circular: 8] // 2
a[circular: 12] // 0
a[circular: -1] // 5
a[circular: -2] // 4
a[circular: -3] // 3
a[circular: -4] // 2
a[circular: -5] // 1
a[circular: -6] // 0
a[circular: -7] // 5
a[circular: -12] // 0