Swift circular array subscript

circular-array-subscript.swift

extension Array {
	subscript(circular index: Int) -> Element? {
		var i = index
		if i < 0 || isEmpty {
			return nil
		} else if i > count - 1 {
			i = index % count
		}
		return self[i]
	}
}

extension Array {
	subscript(circular index: Int) -> Element? {
		guard index >= 0 && !isEmpty else { return nil }
		guard index >= count else { return self[index] }
		return self[index % count]
	}
}

My attempt:

1. Handles a negative index so you can go circular in either direction.
2. Has a non-optional return so that using it on an empty array will cause a fatal error like the standard subscript.

extension Array {
    subscript( circular index: Int ) -> Element {
        let modIndex = isEmpty ? 0 : index % count
        return self[ modIndex < 0 ? modIndex + count : modIndex ]
    }
}

If you prefer the optional return you can use instead:

extension Array {
    subscript( circular index: Int ) -> Element? {
        guard !isEmpty else { return nil }
        let modIndex = index % count
        return self[ modIndex < 0 ? modIndex + count : modIndex ]
    }
}